MPT 1331
Code Of Practice
For Radio Site Engineering
June 2001
Digitised & Reprinted by:
Fylde Microsystems limited
8 Avroe Crescent
Blackpool Business Park
Squires Gate
Blackpool FY3 8HR
The growth of radio services has resulted in an increase in the number of radio sites required and in the number of
users sharing their facilities.
The radio frequency spectrum is a finite natural resource for which there are many competing demands, therefore
radio systems must be designed so that individual systems are very efficient and operate with minimum
interference to other systems.
The aesthetic impact of radio structures provides an increasing constraint on the development of further radio sites.
It is essential therefore to obtain the support of the community with regard to environmental issues, consequently it
is necessary to demonstrate that the optimum use will be made of the proposed installation.
In granting planning permission for a radio structure local authorities expect radio system users to operate the
maximum number of systems from existing structures before giving consideration to an application for another
structure in the same area.
This code of practice has been prepared to assist radio system designers to obtain the optimum use of radio sites
and the radio spectrum.
The engineering problems encountered on sites should be dealt with in relation to the site as a whole and with the
interests of all site users in mind, and not simply in relation to a single user.
Definition of a Communal Site
A communal site is a location at which there is more than one fixed transmitter.
Fixed Site Configurations
1. Single user - single fixed station
Only one radio frequency carrier can be radiated at any one time; the fixed station equipment is only required to
meet the limit specified for intermodulation attenuation. All other limits in the relevant MPT specifications should be
2. Multiple fixed stations
At all communal sites equipment installed on the site must meet the limits as specified in the relevant MPT
1 Scope of the code _________________________________________________________________________________ 7
2 System design objectives___________________________________________________________________________ 8
3 System deficiences ________________________________________________________________________________ 9
Generation of unwanted products ___________________________________________________________________ 9
Intermodulation, cross modulation and blocking effects in receiver systems. _________________________________ 9
Degradation of antenna performance ________________________________________________________________ 9
Radiation pattern_______________________________________________________________________________ 9
Gain ________________________________________________________________________________________ 10
Cross-polar performance _______________________________________________________________________ 10
VSWR_______________________________________________________________________________________ 11
Corrosion and climatic effects _____________________________________________________________________ 11
4 Radio site selection criteria________________________________________________________________________ 12
Location chosen by propagation analysis ____________________________________________________________ 12
Availability of capacity on existing sites _____________________________________________________________ 12
Electromagnetic compatibility with existing installations ________________________________________________ 13
4.3.1 Ambient noise levels _____________________________________________________________________________ 13
4.3.2 Interference generated on site _____________________________________________________________________ 13
4.3.3 Technical responsibility of the site operator __________________________________________________________ 13
Environmental and planning considerations ________________________________________________________ 14
5 Recommendations _______________________________________________________________________________ 14
Control of unwanted products ____________________________________________________________________ 14
5.1.1 The ferrite circulator ____________________________________________________________________________ 15
5.1.2 The cavity resonator _____________________________________________________________________________ 15
5.1.3 The spectrum dividing filter _______________________________________________________________________ 15
Control of intermodulation, cross modulation and blocking in effects receiver systems_______________________ 16
5.2.1 Filter protection ________________________________________________________________________________ 16
5.2.2 Receiver distribution networks_____________________________________________________________________ 16
5.2.3 On site interference _____________________________________________________________________________ 17
Control of antenna system performance ____________________________________________________________ 17
5.3.1 Choice of antenna type___________________________________________________________________________ 17
5.3.2 Antenna specification____________________________________________________________________________ 17
6 Intermodulation performance : The following specifications are desirable: ________________________________ 18
5.3.3 Location of antennas ____________________________________________________________________________ 18
5.3.4 Cables and connectors ___________________________________________________________________________ 18
Control of corrosion and climatic effects ___________________________________________________________ 19
5.4.1 Standards _____________________________________________________________________________________ 19
5.4.2 Dissimilar metals _______________________________________________________________________________ 19
5.4.3 Protective coatings ______________________________________________________________________________ 19
5.4.4 Wind loading __________________________________________________________________________________ 19
5.4.5 Wind vibration _________________________________________________________________________________ 20
5.4.6 Icing _________________________________________________________________________________________ 20
5.4.7 Sealing _______________________________________________________________________________________ 20
5.4.8 Ultraviolet degradation __________________________________________________________________________ 20
Choice of site _________________________________________________________________________________ 20
Installation and maintenance ____________________________________________________________________ 21
5.6.1 Orientation of support structure and antennas ________________________________________________________ 21
5.6.2 Data logging ___________________________________________________________________________________ 21
5.6.3 Feeder identification, terminations, earthing and sealing _______________________________________________ 22
5.6.4 Structural integrity ______________________________________________________________________________ 22
5.6.5 Working arrangements __________________________________________________________________________ 22
5.6.6 Equipment room installation ______________________________________________________________________ 23 Environment _________________________________________________________________________________ 23 Choice of cables_______________________________________________________________________________ 23 Choice of connector ___________________________________________________________________________ 23 Cable routes__________________________________________________________________________________ 23 Earth connections _____________________________________________________________________________ 23 Electrical supplies _____________________________________________________________________________ 24
5.6.7 Antenna feeder systems __________________________________________________________________________ 24 Incoming cables from the mast___________________________________________________________________ 24 Antenna distribution networks ___________________________________________________________________ 24 Use of dissimilar metals ________________________________________________________________________ 24 Inspection for moisture _________________________________________________________________________ 24
Lightning protection____________________________________________________________________________ 25
5.7.1 Effects and responsibilities _______________________________________________________________________ 25
5.7.2 Protection arrangements _________________________________________________________________________ 25
5.7.3 Lightning conductors ____________________________________________________________________________ 26
5.7.4 Earthing of antenna support structures _____________________________________________________________ 26
5.7.5 Earthing of feeders______________________________________________________________________________ 26
5.7.6 Earthing of associated plant ______________________________________________________________________ 27
5.7.7 Earthing of buildings ____________________________________________________________________________ 27
6 Health and Safety _______________________________________________________________________________ 27
APPENDIX 1 _______________________________________________________________________________________ 29
Protection ratios based on internal noise and distortion in the receiver _________________________________ 29
I .2
Man-made noise _______________________________________________________________________________ 30
Noise Amplitude Distribution (NAD)) determination of degradation _____________________________________ 32
Definitions ______________________________________________________________________________ 32
Noise amplitude distribution ___________________________________________________________________ 32
Spectrum amplitude__________________________________________________________________________ 32
Impulse rate _______________________________________________________________________________ 32
Impulsive-noise tolerance ____________________________________________________________________ 33
I .3.2
Determination of degradation __________________________________________________________________ 33
Minimum values of field strength to be protected _____________________________________________________ 33
APPENDIX 2 _____________________________________________________________________________________ 38
SYSTEM AVAILABILITY ____________________________________________________________________________ 38
APPENDIX 3 _____________________________________________________________________________________ 39
Introduction __________________________________________________________________________________ 39
Transmitters ____________________________________________________________________________________ 39
Coupling loss. Aªª ____________________________________________________________________________________ 40
Intermodulation conversion loss. Aªª _____________________________________________________________________ 40
External non-linear elements ______________________________________________________________________ 42
Receivers_______________________________________________________________________________________ 42
Reduction of intermodulation product levels in transmitters __________________________________________________ 43
Intermodulation conversion loss __________________________________________________________________ 43
Coupling loss _________________________________________________________________________________ 43
5.3 Identification of the source of an intermodulation product ________________________________________________ 47
Reduction of intermodulation products in receivers _____________________________________________________ 47
Reduction of intermodulation interference by frequency arrangements _____________________________________ 47
Reduction of intermodulation interference by other arrangements _________________________________________ 48
Introduction ____________________________________________________________________________________ 48
Simple frequency relationships _____________________________________________________________________ 48
Complex frequency relationships ___________________________________________________________________ 49
Generation of intermediate Frequency and/or its derivatives____________________________________________ 49
Generation of transmit/receive (Tx/Rx) difference frequency ___________________________________________ 49
Intermodulation products _________________________________________________________________________ 49
Generated external to the site ____________________________________________________________________ 49
Intermodulation products generated on-site by non-linear junctions on the mast ___________________________ 49
Intermodulation products generated on-Site by non-linearity in components of the system____________________ 49
Transmitter noise ________________________________________________________________________________ 49
External electrical noise___________________________________________________________________________ 51
Summary______________________________________________________________________________________ 51
APPENDIX 5 _____________________________________________________________________________________ 52
INTERMODULATION INTERFERENCE _______________________________________________________________ 52
INTERMODULATION SPECTRUM ____________________________________________________________________ 52
INTERMODULATION PRODUCTS ____________________________________________________________________ 54
APPENDIX 7 _____________________________________________________________________________________ 57
HORIZONTAL AND VERTICAL FRAME OF AXES ______________________________________________________ 57
APPENDIX 9 _____________________________________________________________________________________ 59
APPENDIX 10 ____________________________________________________________________________________ 61
CONTROL OF PRECIPITATION NOISE _______________________________________________________________ 61
APPENDIX 11 ____________________________________________________________________________________ 62
NOISE POWER ON TYPICAL RADIO SITES ____________________________________________________________ 62
APPENDIX 12 ____________________________________________________________________________________ 63
PARAMETERS OF CAVITY RESONATORS _____________________________________________________________ 63
APPENDIX 13 ____________________________________________________________________________________ 64
Typical filter System __________________________________________________________________________________ 64
Spectrum dividing filter response curve __________________________________________________________________ 65
Single Aerial UHF system _____________________________________________________________________________ 66
Typical Sub-band TX/RX system ________________________________________________________________________ 67
APPENDIX 14 ____________________________________________________________________________________ 71
BAND Ill TX/RX TRUNKING COMBINER ______________________________________________________________ 71
BIBLIOGRAPHY _________________________________________________________________________________ 77
Annex I To MPT 1331 Case Studies _____________________________________________________________________ 78
1 Scope of the code
This code provides guidance for engineers concerned with the design, specification, installation, operation and
maintenance of radio systems. It is particularly directed towards systems working in the VHF and UHF bands
where co-sited operation of many different users equipment has become common.
The code examines the objectives of good design and the effects of common deficiencies. It provides
recommendations designed to ensure that users avoid interactions which result in mutual interference,
spectrum contamination, or danger to personnel or equipment. References and appendices are provided for
further reading by engineers who are new to the field or are encountering the problems which are described for the
first time.
This code also includes information relating to the safety precautions required when dealing with non-ionising
A bibliography at the end of this document gives relevant information on:
1 relevant British Standards:
2 health and safety;
3 Department of Trade and Industry radio equipment specifications:
4 CCIR and CCITT Recommendations.
The contents of this document have been arranged to identify the source of the problems found on radio sites and
recommendations are made for the control of these problems.
2 System design objectives
Radio equipment for the mobile and fixed services is built to specifications which are directed to ensure the efficient
use of the radio spectrum. One set of parameters control emitted bandwidth and the level of out of band radiation
which will cause interference to other users; they establish suitable transmitter power or effective radiated power
(erp) limits and will specify the receiver sensitivity and limits to the levels of spurious emission from receivers.
Another set of parameters define conditions which make a system less susceptible to interference by others; they
include receiver selectivity, dynamic range and blocking characteristics. Good installation design ensures that as
far as possible the performance of a complete installation preserves the professional characteristics of the
components, laying down the intended field strength in the designated area, avoiding the radiation of spurious
emissions and preserving the sensitivity of receivers.
The objectives are as follows
to obtain the coverage required from the chosen site in a
precise and well defined manner;
to cause minimal spectrum pollution to other users on
adjacent sites;
to cause minimal interference to other co-sited users;
to operate the system with the erp and optimum spectral
efficiency compatible with providing the required service;
to minimise the effects of lightning.
To fulfil the requirements of all relevant legislation and recommendations, the above criteria should be met for the
whole of the working life of the installation and should allow for future expansion. The quality of service is largely
dependant on the planning of the system and considerable guidance on the topic of protection ratios is given in
Appendix 1.
Preventive maintenance and repairs will be required to ensure that the installation continues to meet the
performance criteria described; good engineering design will allow these activities to be carried out safely
and with minimum loss of service, (see Appendix 2).
3 System deficiences
Generation of unwanted products
There are three main sources of radiated products, and these are defined as follows:
the noise and spurious products generated within transmitters;
these occupy a broad bandwidth on both sides of the carrier
frequency (see Appendices 3 and 4);
intermodulation products caused by mixing of two or more source
frequencies which produce well defined and often high level signals.
These are normally caused by transmitters coupling into an adjacent
transmitter output stage, due to inadequate isolation between the two
transmitters (see Appendices 3, 4 and 5);
intermodulation products caused by non-linear effects on the mast
and antenna hardware (see Appendices 3, 4 and 5).
Intermodulation, cross modulation and blocking effects in receiver systems.
Problems are usually caused by large signals at the input of the receiving system causing non-linearity.
Examples are provided in Appendix 3.
The specifications for receivers are well defined in existing documents; distribution amplifier may be
called upon to operate in a more hostile environment on densely utilised site and require a mandatory
Degradation of antenna performance
It is important to appreciate that the performance of an antenna is very dependent on the environment in
which it is mounted. This is particularly true of many antenna types commonly used in the VHF and UHF
bands. The data quoted by manufacturers will generally relate to parameters measured on a test-range
in which the antenna is erected clear of all obstructions, using the optimum mounting arrangement. Such
an environment will not normally apply at a typical user's installation, and inferior performance may result
unless particular care is taken.
Appendix 6 shows a number of common configurations and indicates their relative merits.
Radiation pattern
As a general rule, the less directional the radiation pattern of an antenna, the greater the influence the
mounting environment has on the pattern.
Highly directional antennas such as paraboloidal dishes and antennas with large mesh reflectors have
high front/back ratio and may be regarded as largely independent of what lies behind them. Antennas of
moderate front/back ratio such as yagis must be mounted with their rear elements at least one
wavelength from the supporting tower if optimum performance is to be achieved.
Nominally omnidirectional antennas (dipoles, stacked dipole and collinear arrays) will only achieve
genuine omnidirectional performance when mounted on top of the supporting structure. When side
mounted, large currents flow in the supporting structure, causing distortion of the omnidirectional pattern
and the probability of intermodulation product radiation due to non-linear joints between structure
members or mast sections. In practice omnidirectional azimuth patterns can be obtained only by side
mounting several antennas (usually 3 or 4) firing in equispaced radial directions; radiation patterns may
be most accurately predicted when the individual units of the antenna have a large front/back ratio. As
such antennas are very expensive, their use is most attractive when a number of services can be
multiplexed onto a single broadband antenna system.
The modification of the radiation pattern of an antenna, referred to above (3.3.1), also implies a change
in its directivity and hence its gain. In general an antenna will lose forward gain when mounted too close
to the supporting tower; side and rear lobes will be increased. These changes result in reduced forward
range and reduced protection against co-channel interference.
Cross-polar performance
The cross-polar performance of base station antennas for the mobile service has in the past been noncritical, as all stations used vertical polarisation. An important change has taken place with the opening
of services in Band Ill (174-225 MHZ) to the mobile radio service in the UK as polarisation protection is
an important parameter in ensuring low interference levels caused to (and by) continuing overseas
television transmissions in that band. Similar consideration will also apply in Band 1(41.5-67 MHz).
In bands used for fixed services the cross-polar protection provided by link antennas is an important
factor in frequency planning and management; the geographical separation between stations using the
same frequency can be reduced when orthogonal polarisation is used. The cross-polar discrimination
(CPD) achieved by a practical antenna under test range conditions will lie between
20 dB, for a simple yagi or dipole of orthodox construction, and 40 dB, for a paraboloidal reflector
illuminated by a well designed feed horn. The significance of the path to CPD must be considered.
Two commonly seen faults degrade the CPD of an installed antenna:
If an antenna is installed in such a position that currents are induced into
members of the supporting structure, these currents, flowing in arbitrary
directions, will couple energy from the plane in which it was radiated into the orthogonal
plane. This is a particular hazard for installations of yagi and similar antennas of
moderate or low directivity.
Failure to erect antennas with the plane of polarisation aligned exactly in the
Required direction produces a field component in the orthogonal plane. The CPD
of an ideal linearly polarised antenna falls as misalignment increases, as shown in
Appendix 7.
There are two major causes of increased VSWR when an antenna system is mounted for use. In the
VHF band, where antennas of low or moderate directivity are used, the proximity of structural or
mounting components will change the antenna VSWR; this change will be of most significance when a
very low antenna VSWR is needed. Above VHF (where more directive antennas are generally used) the
chief cause of degradation is reflection from connectors and from discontinuities in the line itself.
It should be noted that the attenuation of the antenna feeder has the effect of reducing the VSWR seen
by the transmitter, and for critical installations a reflection budget should be drawn up as indicated
typically in Appendix B.
The budget may be used to determine the worst-case VSWR which may occur with specified
components, or to determine the component limits when the overall system performance is already
determined. The cable reflection coefficients quoted are typical; the reflection caused by the terminations
and any adaptors which are used must be included. See Appendix 9 for the method of calculation of the
system reflection performance.
Corrosion and climatic effects
The materials which are used in the construction of antennas and their support structures are prone to
corrosion. The UK environment combines wet and humid conditions with mild temperatures: industrial
pollutants and coastal conditions accelerate corrosion in many locations. With one of the windiest
climates in the world, ice and snow, and the incidence of ultra-violet radiation to degrade paints and
plastics, UK designers of outdoor installations must understand the problems which can arise and
recognise the practices which have proved adequate to overcome them.
Deterioration will take the following forms:
Corrosion of metallic components, causing structural weakening of antenna elements
and mountings. Corrosion will be accelerated at bi-metallic contacts and will give rise to
non-linear conduction with consequent generation of intermodulation products. A rise in
contact resistance at connections will increase ohmic losses and reduce antenna gain.
Water ingress into insulating materials will cause changes in permittivity (giving rise to
VSWR changes) and will increase dielectric losses, especially if the water is polluted
or has run off metallic components.
Water ingress into feeders and connectors produces mismatch and increases loss.
Wind-induced vibration causes antenna elements to break by fatigue failure and
accelerates corrosion at element clamps.
Snow and ice cause temporary increases in VSWR and losses of gain and polarisation
purity. These effects will become permanent if the weight or wind load is large
enough to cause permanent distortion. Freezing splits components into which water
has penetrated. Falling ice can cause damage, distorting elements or even
breaking off complete antennas.
Wind-induced vibration and falling ice cause damage to badly mounted feeders.
Damage is often in the form of complete annular cracks in the outer conductor of
corrugated semi-flexible cables; these produce intermittent faults with high VSWR
and severe non-linearity.
The problems listed above result not only in a degraded service for the user of the antenna and feeder
concerned, but by loss of directivity, polarisation discrimination and linearity may result in problems for
other users, whether co-sited or not.
The management of these problems lies in the care with which an installation is designed, carried out
and maintained.
4 Radio site selection criteria
Performance criteria can be classified as follows:
location chosen by propagation analysis;
availability of capacity on existing sites;
electromagnetic compatibility with existing installations;
environmental and planning considerations.
Location chosen by propagation analysis
A search for existing sites should be undertaken; consultations with site operators should produce
propagation information concerning existing sites.
If a new site is considered necessary, a propagation study based on an initial theoretical analysis backed
up by a physical survey may be required. Computer predictions are available from a number of sources,
and these are based on the Ordnance Survey grid. A number of commercial organisations offer these
services. Propagation predictions are essentially statistical by nature and are subject to wide local
Availability of capacity on existing sites
When a suitable site has been located there are several options available to the new user and these are
as follows:
to share an information channel on an existing system;
to share a frequency division filter system on an existing antenna;
to share the accommodation and install his own antenna on the structure;
to provide his own equipment accommodation and antenna and share space for
the antenna on the structure;
in the eventuality that neither the structure or accommodation are suitable for
the new facility, a complete redevelopment may be required.
There may be structural loading implications introduced by the proposals listed above, and these are
discussed in Section 5.4.4 of this document.
Electromagnetic compatibility with existing installations
It is necessary to establish whether a compatible background noise level is available at the frequencies
under consideration for the proposed installation. The overriding consideration is whether signals
emanating from existing installations will adversely affect the proposed installation. Contributing factors
that will form the basis of a decision are as follows:
4.3.1 Ambient noise levels
It is recognised that any ambient noise measurement is only an approximate indication, since it is
strictly applicable to the antenna employed and the noise conditions at the time. The ambient noise
level particularly at urban sites in the lower VHF bands, has a major influence on system range and
Precipitation static noise, caused by the exchange of static charges between raindrops and the antenna
system, is a significant source of noise at frequencies below 150 MHz. It may be controlled by the fitting
of insulating shrouds to antenna elements (see Appendix 10).
Ambient noise includes atmospheric, sky noise and man made electrical noise. In general this is
beyond control of the site operator (see Appendix 11).
On a remote green field site it may be possible to operate receivers in the
VHF band at levels below - 107 dBm (2 µV emf). However in a more
realistic situation the minimum usable signal will be typically -104 dBm (2.8 µV emf).
4.3.2 Interference generated on site
The examination and control of unwanted products are considered in Section 5 of this code of practice.
4.3.3 Technical responsibility of the site operator
It is essential that the site operator is able to quantify any unwanted products which give rise to
unreasonable degradation of service to co-sited installations.
Some solutions to the problems arising from unwanted products are offered in Section 5. There is a
need for further procedures to control the lack of coordination that often exists between the site
operator, the users and the licensing authority.
Environmental and planning considerations
Radio sites are prominent features of the landscape. It is therefore understandable that many Planning
Authorities are paying particular attention to applications for both new sites and redevelopment of
existing sites. There are also organisations and individuals who will raise objections to any application.
These objections will be more numerous where the site is in a
National Park, Area of Outstanding Natural Beauty or Area of High Landscape Value.
It is important that these aspects are carefully considered at the planning stage of any new site. Whilst
the site should have sufficient capacity for the foreseeable requirements, it should create the minimum
impact on the environment. For example:
relocating the site a small distance without changing its performance may dramatically
reduce its impact;
the careful choice of antennas together with their arrangement in a symmetrical form,
subject to a satisfactory performance, or a reduction in their numbers by the use
of combiners will provide a better appearance;
an alternative type of support structure may present a more acceptable profile;
varying materials, styles and colours for construction of equipment buildings may result in
a more acceptable appearance;
landscaping of the compound with the addition of trees and shrubs will improve the visual
impact of the site.
Helpful advice can usually be obtained from the Local Council's Planning Authority.
Applications prepared without due consideration of the foregoing factors may result in a refusal.
NOTE: An existing mandate DOE Circular 16/85, makes specific recommendations that sites shall be
shared wherever possible, and that new applications must take sharing into consideration.
5 Recommendations
Control of unwanted products
The origins of unwanted products are related to mixing processes that take place in any non-linear
component of the complete system. (See Appendices 3, 4 and 5). The simple guidelines of increasing
the isolation between the components of the mixing process will result in the reduction of the
intermodulation product level.
5.1.1 The ferrite circulator
The ferrite circulator is a practical component which can be utilised to provide directional isolation at the
output stage of the transmitter.
A ferrite circulator has directional properties that typically result in additional isolation of between 20
and 40 dB. The isolation parameter has to be considered in conjunction with the insertion loss and
bandwidth, all parameters being considered in the system design.
For isolation in excess of 40 dB, a dual circulator version may be fitted.
The third port of the ferrite circulator is terminated in a matched load, the power rating of which is
intended to withstand the maximum return power that is envisaged in the worst fault condition that can
arise on the system.
5.1.2 The cavity resonator
The cavity resonator is a bandpass circuit, having a centre resonance frequency related to its physical
dimensions. The unloaded 0 of such a device depends on its physical volume, and at VHF it ranges
between 2000 and 10,000. The loaded Q is normally arranged to be between 500 and 1500 according
to the insertion loss and isolation required (see Appendix
12 for details).
A system using a cavity resonator gives protection to co-sited receivers by reducing the radiation of
wide-band noise.
Cavity resonators may be connected together to provide additional isolation when used to combine
several transmitters to a single antenna.
When used in conjunction with ferrite isolators, cavity resonators provide the necessary isolation to
combine several transmitters into a single antenna configuration. This system employs several cavity
resonators coupled together with a precisely dimensioned cable harness, to allow single antenna
working with insertion losses of typically 2 dB with a relative frequency separation of 1%. With high
performance cavities the separation can be reduced to 0.25%, and still give isolations greater than 20
dB. When used in conjunction with a ferrite isolator it is typically possible to attain 50 dB
isolation between adjacent transmitters when coupled to a single antenna (see Appendix 14 for details).
5.1.3 The spectrum dividing filter
When the outputs of several transmitters are to be considered as a combined signal it is convenient
that each antenna shall have the frequency spectrum coupled to it defined by a filter having a
comparatively broad, flat topped response. This enables any transmitter to be operated within the
specified band without excessive filter insertion loss, and ensures the attenuation of signals outside the
defined frequency band.
This system defines the band edges, controls spurious emissions, and is therefore given the tile
"spectrum dividing filter".
The spectrum dividing filters can be coupled together by means of a precisely dimensioned cable
harness to other similar filters, to provide duplex and combiner facilities for multiple bands to a single
antenna system. (See Appendix 13 for further details).
Control of intermodulation, cross modulation and blocking in effects receiver systems
5.2.1 Filter protection
A common source of receiver problems is that incoming signals outside the band of interest arrive at
the receiver front end at an amplitude which can cause blocking, inter-modulation and distortion of the
wanted signals. This situation is mostly likely to occur when the receiver is connected to an antenna
which may be in close proximity to other antennas on a communal site. An improvement in this situation
can be obtained by positioning the receiver antenna well away from any other installation and in
particular from other transmit antennas. On shared sites this is often not possible.
An alternative procedure is to connect bandpass filters between the antenna and the receiver input.
These filters need to have the necessary shape factor to limit the bandwidth to that which is required for
the receiver system.
A single small cavity resonator providing 20 dB isolation at the offending frequency will often provide a
solution to receiver interference problems but, in those cases where the interfering signal is closer than
1% of the centre frequency of resonance to the wanted signal then multiple section filters or large cavity
resonators may be required. The typical responses for such filters are given in the Appendices (see
Appendix 13).
5.2.2 Receiver distribution networks
There are many instances where many receiver systems in the same frequency band are required to
be installed at the same radio site. It may therefore be appropriate to fit a receiver distribution network
comprising one antenna input feeding a suitable bandpass filter, followed by a low noise amplifier which
then distributes its output, usually by a passive network, to the receivers in that band.
The low noise amplifier is carefully chosen to have a very good signal to noise performance to minimise
degradation of the overall system signal to noise performance whilst feeding up to typically 16 receivers
in the same band. The amplifier needs also to be chosen for large signal handling capability together
with an inherent protection against damage by transient impulse voltages. The use of a high quality
semiconductor device operated at a small fraction of its rated dissipation will meet these requirements
and provide a MTBF typically in excess of 100,000 hours. It is essential to appreciate that the design
objectives for the receiver distribution network will determine the failure rate of the subsequent systems
It should be noted that overloading of a receiver distribution network could affect many other users on
the same system.
The power supply associated with such amplifiers can operate either from mains supply or batteries
which are becoming increasingly common on remote hilltop sites. Again it must be stressed that the
reliability of the supply is essential for the maintenance of the service and it is usual to provide back-up
in the event of mains failure. (See Appendix 15 for further details of distribution amplifiers).
5.2.3 On site interference
A common problem caused by mobile transmitters is being able to come within a very short distance of
the receiving site, e.g. 25 metres, in which case signals typically in excess of +10 dBm can arrive at the
receiver within the wanted frequency band; this is a source of non-linearity. On a communal site where
there is a multiplicity of users, it is likely there will be several potential users in the same band able to
visit the site using their mobile transmitters. This problem can be avoided by strict discipline on this
topic or a complete ban on the use of mobile transmitters on the site.
A further cause of interference on base station receivers can be distant mobiles using their transmitters
when on elevated locations or during conditions of enhanced propagation.
These problems can be minimised by co-operation with the distant user, the use of signalling systems,
the choice of site and assignment of frequencies in association with the appropriate authority.
Control of antenna system performance
5.3.1 Choice of antenna type
The principle which governs the choice and siting of transmitting antennas is that only the minimum
necessary erp must be radiated in each desired azimuth direction.
Omnidirectional antennas should be used only when necessary for the service requirements. The
simplest examples of this class of antenna are top-mounted end-fed and coaxial dipoles, monopoles
and collinear arrays. When omnidirectional characteristics are required of a side mounted array, a
number of antenna elements must be placed around the supporting structures.
There are many satisfactory types of directional antennas; common examples are yagi arrays, corner
reflector antennas and panel antennas.
Many antennas in common use fall between the omnidirectional and directional types described. They
include simple dipoles side mounted from support structures. Many of these antennas have ill-defined
radiation pattern performance and are likely to give rise to radiation of intermodulation product
frequencies originating from currents excited in the supporting structure. Their use is not recommended
for multi-frequency applications.
5.3.2 Antenna specification
The following parameters must all be specified when procuring or selecting antennas.
1 Gain: Specified either in dB relative to an isotropic radiator (dBi) or a half-wave dipole (dBd).
2 VSWR : Specify the maximum value compatible with the system being considered.
3 Radiation pattern: Specify the beamwidth in the azimuth and elevation planes, together with
any necessary restrictions on side or rearlobe levels.
4 Balance ratio: This parameter defines the effectiveness of the balun fitted to balanced driven
elements and consequently the acceptable level of currents on the outside surface of the
feeder cable. A value of 20 dB should be considered as a minimum.
5 Input power: For combined transmitter outputs specify both the mean and effective peak
6 Intermodulation performance : The following specifications are desirable:
for single frequency transmit and receiving applications: - 100 dBc
for multiple frequency transmission: - 130 dBc
for multiple frequency transmission and reception on a single antenna; -143 dBc
the more severe specification will be met using all-welded construction and exceptional care in
the encapsulation of antenna terminal arrangement.
7 Bandwidth: Specify the frequency band over which the antenna is to be used, over which all
the parameters specified must be met. The practice of regarding the VSWR bandwidth as
indicating the usable frequency band is unsound.
1 Structural design of antennas and supports must comply with BS CP3 ChV Pant 2; BS CP1
18 and BS 449.
2 Electrolytic contact potentials between dissimilar metals must be less than 0.25V even for
encapsulated assemblies.
3 Conformity to a chosen environmental test specification (see
5.3.3 Location of antennas
When determining the mounting positions for antennas each antenna must be mounted in a manner
which does not impair its performance (see Appendix 6). The spacing between antennas must be
chosen to provide sufficient isolation to allow system intermodulation product targets to be met.
5.3.4 Cables and connectors
Semi-flexible cables with corrugated copper outer and conductors are in widespread use for long
feeders. Recommended connector interfaces include types N', 'HN', 'C', and 'IEC' flanges. The use of
low-performance connectors such as type 'UHF' is deprecated. All connectors must be fitted in
conformity with manufacturers' instructions to ensure proper sealing and electrical uniformity. A flexible
tail must be used to connect a semi-rigid feeder to an antenna.
Control of corrosion and climatic effects
5.4.1 Standards
It should be recognised that corrosion and climatic effects cannot be eliminated; however, the effects
can normally be contained by careful design and selection of materials, high quality manufacturing, high
standards of installation and a maintenance programme planned for the life of an installation. Detailed
information is found in BS 5493 and
BS PD 5484.
Maintenance on the basis of remedial action only is a dangerous practice. For further guidance see
Section 5.6.
5.4.2 Dissimilar metals
Structure design must take particular account of corrosion between dissimilar metals: electrolytic
contact potentials between metals in contact in outdoor exposure must be less than 0.25 volt, and in
indoor situations should be less than 0.5 volt.
Connections to site earthing systems (where corrosion may be unavoidable) should be made by means
of a sacrificial earth lug of a material compatible with the structure being earthed. Replacement of
sacrificial lugs should be part of the site maintenance programme.
5.4.3 Protective coatings
Steel structures should be protective coated to BS 729, with screwed fasteners spun galvanised to BS
4190. Aluminium structures should be anodised to BS 1615. It should be noted that anodising on
aluminium is likely to insulate the components and thus produce difficulties in terms of earthing and
conductivity of the structure.
The cutting or drilling of protective coated items should not be permitted during installation.
On the occasions when cutting or drilling is unavoidable, consideration should be given to possible
structural weakening, and the affected areas must be treated with a recommended protective coating.
The painting of structures should be considered as an essential part of the post installation programme.
A well defined schedule of time scale and of the exact process should form part of the design of the
structure and must be implemented rigorously.
In the case of a galvanised structure there will be a recommended period after which a paint process
should be applied.
For structures of other materials requiring protective treatment, BS CP 118 indicates the recommended
5.4.4 Wind loading
The design of antenna support structures should be in accordance with 88 CP 3 Ch V Part 2, BS CP
118 and BS 449 and should take into account the wind loading of all the components on the structure,
e.g. antennas, feeders and associated hardware.
Twist and tilt limitations for parabolic antennas may also have a bearing on design or reinforcement.
The design or selection of a suitable support must be by qualified structural engineers.
The design of new structures should where possible take into account the probability of future
5.4.5 Wind vibration
All antennas, mounting steelwork, feeders and ancillary equipment should be securely clamped to
protect feeders and other semi flexible items from damage by vibration throughout the projected life of
the installation.
Manufacturers' recommended feeder clamp spacings should be observed, with particular attention to
exposed areas and transitions from antenna to tower, tower to gantries and gantries into buildings;
feeders should not be laid loose on gantries. Where necessary additional protection should be
5.4.6 Icing
The structure design and site layout should take into account the icing which could reasonably be
expected to occur on structures and antennas in a particular location and the danger of falling ice in
relation to personnel and damage to buildings, equipment, antennas and feeders.
5.4.7 Sealing
Feeder and cable entries, external cable or feeder termination's, and earth connections to feeders on
towers or gantries should be suitably sealed or protected against the ingress of moisture using non
setting pastes, self amalgamating tapes, neoprene paints as appropriate and in accordance with
manufacturers instructions. Particular attention should be paid to shedding of surface water.
The use of PVC boots or drip covers is not recommended; self amalgamating tape, and non setting
sealant methods are preferred. (See Section 5.6.3).
All underground clamps on site earthing arrangements should be suitably protected by the use of non
reactive non setting pastes and tapes.
5.4.8 Ultraviolet degradation
Products liable to degradation by ultraviolet light should not be used in external situations where there
is an acceptable alternative. Where the life of an item is known to be limited, its periodic replacement
should be included in the site maintenance programme. Replacement only on failure is not generally
Choice of site
For a proposed service, the ideal site is one that is located in the service area with a position for
installation of a suitable antenna system. A high building or existing structure may suffice if the final
antenna height that can be provided is above the mean height of physical obstructions in the service
area. In an urban environment it may be extremely difficult to provide a clear path for the required
radiation pattern and usually a compromise has to be reached.
If there are no suitable locations fulfilling the basic parameters, it will then be necessary to construct a
purpose built mast or tower to provide the necessary service. In every case a careful search should be
made for all other existing users who may have an additional interest in extending their own systems.
When a new structure is proposed, a large number of new users may wish to share the facility.
It is essential that all these potential users are taken into account in the initial planning as there have
been many cases in the past where a multitude of structures have been erected in close proximity due
to lack of early consultation.
Wherever possible, the location of a radio structure should be determined by the isolation from any
other radio transmission activity, and should preferably be a minimum distance of 500 metres from a
busy road. This is to minimise the possibility of mobiles operating close to the site, for reasons which
are discussed elsewhere in this document (see section 5.2.3), and to avoid radiated vehicle noise. Care
should be taken to avoid close proximity to sources of industrial and domestic electrical noise.
In the event that no suitable location having sufficient height is available in the service area, it may be
necessary to utilise an existing site on the fringe of the service area, in which case directional antennas
may be required. In these circumstances it is essential from the outset that the user is given the
opportunity to consider the effects on his service that will be caused by the irregularity in the service
There are many existing installations in which the service area is not in accordance with the users
requirements; this may have been caused by lack of appreciation of user requirements and the
propagation parameters or the use of an unsuitable site for economic or planning reasons. The
fundamentals to the choice of radio site are:(a)
research into the service requirements,
careful examination of the service area,
determined attempts to co-locate with existing users.
If when these points have been investigated a new site has to be found, then its location should be
subject to extensive investigation (See section 4).
Installation and maintenance
5.6.1 Orientation of support structure and antennas
Orientation should be based on True North, although it may be preferred that a statement of magnetic
bearing, deviation and date is also kept on records. A clear method for referring to the legs and faces of
the structure should be observed.
5.6.2 Data logging
The efficient administration of complex radio sites relies on precise details of physical facilities, users
and emissions. Information should be kept centrally, and displayed in a useful form on site.
(a) Physical information should include:
antenna types,
feeder lengths and types,
connector types and sex,
distribution harness details,
details of mounting hardware.
A common practice is to maintain a master outline for each structure, referring to detailed drawings of
mounting arrangements, feeder routes and other information.
(b) Electrical information established at systems commissioning provides a useful
reference for the diagnosis of later problems. Data recorded for each antenna
system should include:
VSWR, insulation and attenuation of feeder cables
VSWR measurements on complete antenna Systems, bench measurements of
power division networks.
5.6.3 Feeder identification, terminations, earthing and sealing
Feeder cables should be uniquely and permanently identified at each end, and at the point of exit from
the structure. More frequent identification may be advisable when cables are buried in a duct.
Connectors and earthing kits should be fitted in accordance with manufacturers' instructions. Connector
fitting should be carried out in dry surroundings wherever possible, feeders should be lifted in
accordance with manufacturers' recommendations, with connectors already fitted to their upper ends
and suitably protected from water ingress. Earthing of feeders should follow the recommendations of
section 5.6.7.
On completion, connectors should be wrapped with polyisobutylene (PIB) self-amalgamating tape and
over-wrapped with a carefully applied layer of petroleum jelly impregnated waterproof tape. Where PVC
covers are provided for connectors, they should be removed and the connectors taped as described.
5.6.4 Structural integrity
The structural integrity of the mast or tower must be established by a competent structural engineer, the
analysis must include the loads imposed by each antenna system.
Structural components must be designed to comply with BS 449 (or BS CP118), steel components
being hot-dip galvanised to BS 729 with threads spun galvanised to BS 4190. All nuts should be
provided with spring washers or other means of locking.
5.6.5 Working arrangements
Operations at site must follow safe working practice. Only one user or contractor must work on the
structure at any time and arrangements for lifting equipment past working antennas must be agreed
with the Site Manager. Attention is drawn to section 6 (Health and Safety).
5.6.6 Equipment room installation Environment
Consideration should be given to ensuring that the equipment room should be kept at an ambient
condition which never allows the temperature to fall below the dew point and which keeps within the
specified temperature range of the equipment, or at an acceptable working temperature for personnel.
It may be necessary to provide heating, ventilation or cooling to achieve this condition.
Precautions may be necessary to exclude pests and vermin from the equipment room. Choice of cables
It is recommended that wherever possible, solid, semi-rigid or double-screened cables shall be used for
all radio frequency connections. This is to ensure maximum screening between adjacent cables and
feeders and to reduce coupling between equipments.
The use of single screened cable, e.g. UR67, UR43, RG58 should be avoided wherever possible and in
particular, in cable runs where several of these conductors are brought close together. Choice of connector
Wherever possible, high quality connectors should be used, typically type N, preferably with a silverplated finish. Use of such connectors produces maximum screening effect and gives the best radio
frequency connection between the various components of the system. Cable routes
The direct and shortest route is always the best for minimum radiation and minimum insertion loss.
However it is important that transmitter cables and receiver cables should be installed as far apart as
possible. It is advisable that when they cross they should cross at right angles. Cable trays carrying
transmitter cables should not be directly connected to receiver path cable trays and it is best if cables
are insulated from each other at all times. It is normal to use cables with insulated outer jackets and the
only points at which earth straps and earth bonding should be employed are those specifically chosen
for the purpose. Earth connections
There should be a careful plan designed at the outset of the station design in order to provide the best
earthing procedures and to minimise earth loop currents. This should be achieved by taking the outer
casings of the cables by a large section copper strap to a central earth bonding point and the size of the
conductor should increase as each branch path is added. The final conductor should go directly to the
earth system. For a radio site a single copper spike is insufficient for the station earth as its impedance
is likely to be too high.
23 Electrical supplies
The majority of sites will have AC power provided by the local Electricity Supply Authority. It is essential
to arrange sufficiently large capacity for future expansion, and wherever possible a sub-division of the
input circuits should be provided separately for each user function. This ensures that individual fuses or
trips protecting sub-sections of the site installation cannot interrupt the supply of other users.
In many instances, standby power supplies will be required and this should be based on the
requirement of the service.
There are an increasing number of sites where DC supplies, in the form of large capacity batteries, are
used to power equipment, and these batteries are charged continuously by means of 'float charge"
systems. This has the dual advantage of automatic standby and "no-break" characteristics.
5.6.7 Antenna feeder systems Incoming cables from the mast
It is often convenient to break down very large feeder cables to a more convenient size and there is a
tendency to put connectors just inside the equipment room when reducing the main incoming feeder to
a more manageable size. It is best, however, to take the main feeder as close as possible to the
equipment to which it is to be connected before having a break in its outer conductor.
The only exception to this rule is to provide an earth connection for lightning conductor purposes and
this should be carried out by means of an external clamp on the outer copper conductor and this should
be taken via the most direct route (as outlined in section 5.7.5).
Incoming feeders must not be interconnected by a "patch panel", traditionally used in the past. The
"patch panel" is a source of earth current coupling and intermodulation and should be avoided. Antenna distribution networks
In the case of a filter system, combining network, or receiver distribution network, it is important that the
cables are treated very carefully and that the distribution network should be mounted away from the
transmitters and receivers whenever possible. Ideally the filter or distribution network should be
mounted on the wall adjacent to the antenna incoming feeders and the transmitter section shall be
connected as far away from the receiver section as possible. The interconnecting cables between
various sections of the filter network should also be treated as in section Use of dissimilar metals
It should be carefully noted that wherever possible all metals used in contact with each other shall be in
the yellow metal series, i.e. copper, brass, silver, nickel, or possibly even gold. The iron and steel part
of the metals table should be avoided at all times as their oxides form non-linear junctions and can
cause intermodulation.
The ideal combinations are silver/brass to copper using nickel plated nuts and bolts. Inspection for moisture
In cases where the mast is exposed and there is a possibility of moisture gathering at the outer jacket
of the copper case of the incoming cables, it is wise to remove the outer insulating jacket at a point well
inside the equipment room where it can be inspected for traces of moisture forming.
It is essential that between the incoming cable glands and the earth strap assembly an easily visible
section of the outer copper jacket is available for routine inspection.
Lightning protection
5.7.1 Effects and responsibilities
Radio sites can be particularly prone to lightning strikes by virtue of their normally exposed locations
and the presence of relatively tall antenna support structures.
The effects of strikes on a site could comprise any or all of the following:a
Death or injury to personnel.
Damage to equipment, or loss of service.
Damage to buildings and structures
Loss or corruption of stored data.
It is not possible to provide and guarantee complete protection from these dangers; however they can
be considerably reduced by careful attention to earthing, protection devices and the layout of the site
Understandably site owners and users will be concerned with the protection of equipment to maintain
the integrity of Systems. However this concern must go alongside the prime consideration which is the
safety of personnel.
Site owners and site users have a responsibility for safety under current Health and Safety legislation.
This section (5.7) provides some guidelines for designers of radio systems but is not in itself a complete
Reference should also be made to various relevant publications, some of which are listed in the
Bibliography. Where any site owner or site user is in doubt about the protection requirements for a
particular location, the appropriate authority should be consulted.
5.7.2 Protection arrangements
The aims of any protection arrangements should be to provide a suitable path to earth for the lightning
current, to ensure adequate bonding between structures, all metalwork on the site and the site common
earthing system in order to reduce the side flashing, and to attempt to prevent the entry of flashes or
surges into buildings.
The resistance to eanh should be kept to a minimum and a value of less than 10 ohms is
recommended. The important feature is that the system should ideally be equipotential across the
whole site.
Reference should generally be made to the Code of Practice for the Protection of Structures Against
Lightning BS 6651:1985.
Certain authorities and service providers have their own particular practices which may have to be
followed where applicable.
Arrangements will vary considerably from very simple sites to complicated sites with a multiplicity of
buildings, antenna support structures and associated plant, and may involve integration with existing
systems. Such systems may require upgrading.
5.7.3 Lightning conductors
Down conductors, bonding interconnections, earth rings and radial tapes should be of uninsulated solid
copper tape of minimum cross section 25 x 3 mm with all connection clamps and supports protected by
non reactive paste or tape (aluminium conductors may be acceptable)
Where the tape may be subject to chemical attack, e.g. when in close proximity to concrete, it should
be protected by the use of non reactive paste or similar.
Protected test points should be included if appropriate and sacrificial earth lugs should be clearly
marked and easily accessible for periodical inspection and replacement if necessary.
5.7.4 Earthing of antenna support structures
A structure will generally act as its own lightning conductor and will not therefore require a conducting
tape from the apex to its base. A lightning finial may be required to extend the zone of protection to
protect equipment mounted on the top of the structure. The finial should extend to about 2.5 metres
above the highest equipment.
Ground mounted support structures should be connected at their base to an earth ring arrangement (or
equivalent) via sacrificial earth lugs. Towers may require a connection from each leg.
An earth ring may consist of copper tape with driver earth electrodes or radial tapes round the base of
the structure as close to it as possible, buried to a depth of approximately 0.6 metre where solid
conditions allow.
The earth rings should be connected to the main building earth by the most direct route, buried as
Roof mounted structures should be connected to the main building earth by the most direct route using
sacrificial lugs and copper tapes as appropriate.
Mast guy wires should be directly bonded at their lowest point to a suitable earth electrode or
connected to the site earth by the most direct route.
5.7.5 Earthing of feeders
All antenna feeders should be bonded to the tower at the upper and lower ends and earthed at the
point of entry into the building (see Appendix 18). Weatherproof earthing kits are available from
antenna manufacturers.
Fast acting gas filled surge arrestors can be used on some systems and may provide additional
equipment protection, providing that VSWR degradation is acceptable.
5.7.6 Earthing of associated plant
All gantries, fuel tanks, above and below ground pipes, fences and other metalwork within 3 metres of
the support structure or building should be bonded to the earthing system by the most direct route
using copper tape, buried where appropriate. This should include any reinforcing rods in foundations
which are not already bonded to earth.
5.7.7 Earthing of buildings
An earth ring ideally should surround the building and be connected to the individual earths associated
with the feeder entry, antenna support structure, building lightning conductor, equipment room, mains
supply and other facilities. Each connection should be made by the most direct route to minimise
interaction between the different earthing functions.
The earth ring should consist of copper tape with electrodes or radial tapes buried to a depth of 0.6
metre and at a distance from the building preferably not exceeding 1.0 metre.
Building may require lightning air terminals (finials) where they are not within the zone of another
protected structure.
6 Health and Safety
With the increasing use of single antenna arrays to radiate the combined output of several transmitters,
radio site operators must assess the physiological hazard presented by the radio frequency energy
radiated from their antennas. It is the responsibility of the site operator to ensure that antenna systems
and access arrangements do not expose personnel to hazards. Everyone who is allowed access to the
antenna structure must be properly informed of all necessary precautions.
The current UK limit for continuous exposure is a power density of 10mW/cm2 (100 WIm2). The limit is
currently under review and may well be reduced, especially for frequencies in the range 70 - 250 MHz.
As an example, it is unsafe to work continuously within 1 metre of a dipole radiating 500W. Even at
stations radiating only 250W from a dipole mounted on a small guyed mast, it may be unsafe to climb
past the antenna unless power is reduced or removed.
For further guidance the reader is referred to:
"Safety Precautions Relating to Intense Radio Frequency Radiation". HMSO 1960
(ISBN 011 340576 6). This is the present standard which is currently under review.
"Proposals for the Health Protection of Workers & Members of the Public against the Dangers of
Extra Low Frequency, Radio Frequency and Microwave Radiations: A Consultative Document",
National Radiological Protection 1982, ISBN 085951185 5). This document sets out proposals for
future standards.
Protection ratios and minimum field strengths
System availability
Interference due to intermodulation products
Sources of unwanted signals
Intermodulation interference
Common antenna configurations
Achieved cross polar discrimination
Calculation procedure for a system reflection coefficient budget
Formulae for calculation of system reflections
Control of precipitation noise
Noise power on typical radio sites
Parameters of cavity resonators
Filter systems
Band III trunking combiner
Characteristics of distribution amplifiers
Stacking and baying data
Isolation between antennas
Typical example of good earthing practice
REPORT 358-5
(Question I /8)
VHF and UHF land and maritime mobile services
Protection ratios based on internal noise and distortion in the receiver
The World Administrative Radio Conference, Geneva, 1979, defined the protection ratio as the
minimum value of the wanted-to-unwanted signal ratio, usually expressed in decibels, at the
receiver input determined under specified conditions such that a specified reception quality of the
wanted signal is achieved at the receiver output (RR No. 164). For further information on the
definition see Report 525. This ratio may have different values, according to the type of service
However, in the absence of information submitted to Study Group 8 on subjective measurements
made in the VHF and UHF land and maritime mobile services, several administrations submitted
the results of laboratory measurements, using appropriate test signals. of the degradation of the
signal-to-noise ratio of the wanted test signal. when a co-channel interfering signal is
superimposed on the latter. A degradation of the initial signal-to-noise ratio of 20 dB to a signalto-noise + interference ratio of 14 dB is taken as the criterion. For some systems this grade of
service is acceptable.
In the tests described by the various administrations, the frequency deviations are 70% or 60% of
the maximum specified frequency deviations, and for amplitude modulation the modulation
percentages are 70% or 60%, for both wanted and unwanted signals. From a study of the
documents submitted, it may be deduced that the slight differences in measurement conditions
and in the characteristics of the receivers used in the different tests, may result in differences in
the measured receiver protection ratios, of up to about < 3 dB.
One administration performed tests to determine the protection ratio for the case where the
wanted narrowband G3E signal is interfered by a direct-printing F2B signal (see
Recommendation 476) [CCIR. 1978-82]. The e.m.f. of the wanted signal at the receiver input was
2 µV. In these tests the level of the interfering co-channel F2B signal was so adjusted that the
subjective effect on the wanted signal was the same as that of an interfering co-channel
narrowband G3E signal attenuated by the protection ratio of 8 dB laid down in Table I for this
case. The peak frequency deviations used for the F2B signal were < I. < 3 and < 5 kHz
respectively. The sub-carrier was 1500 Hz and the frequency shift 170 Hz. 12 dB was found to be
a suitable representative value for the protection ratio and is therefore included in Table 1.
Although the ability of the receiver to receive the wanted signal is dependent on the passband
characteristics of the receiver, the frequency difference between the co-channel wanted and
unwanted signals, the frequency deviation, etc., the receiver protection ratios in Table I may be
used as the basis for the calculation of system protection ratios for mobile systems for a minimum
grade of service. Additional protection should be provided to allow for the effects of multipath
propagation, man-made noise, terrain irregularities. and in the case of very closely spaced
assignments, adjacent-channel interference (see Report 319).
When using frequency modulation. "capture effect" is enhanced as the frequency deviation of the
wanted signal is increased: therefore, a wideband F3E, G3 E system requires less protection
than a narrowband F3E, G3F system for the same type of interfering source.
If' a higher grade of service is required. a higher protection ratio should be adopted, particularly in
the case of amplitude-modulated wanted emissions.
I .2
Man-made noise
Man-made noise degrades the performance of a mobile system. To maintain a desired grade of
service in the presence of man-made noise, it is necessary to increase the level of the field
strength of the wanted signal. Motor vehicles have been shown, by measurements [US Advisory
Committee, 1967], to be the primary source of man-made noise for frequencies above 30 MHz.
Other noise sources are fewer in number and usually radiate from fixed locations.
Rep. 358-5
Table 1 - Typical receiver protection ratios, for use in
Calculating system protection ratios
Wanted emission
(Note 1)
Unwanted emission
(Note 1)
Receiver protection ratio
Wideband F3E, G3E
Narrowband F3E, G3E
Wideband F3E, G3E
Narrowband F3E, G3E
Narrowband F3E, G3E
Direct printing F2B
8 - 17 (Note 2)
8 - 17 (Note 2)
F3E, G3E
F3E, G3E
F3E, G3E
F3E, G3E
See Report 319
See Report 319
Note 1. - Wideband F3E, G3E Systems normally employ frequency deviations with a maximum value in the range
< 12 to < 15 kHz.
The narrowband F3E, G3E systems considered here normally employ frequency deviations with maximum values
of either < 4 or < 5 kHz.
The value of the F2B case is with a peak frequency deviation of < 5 kHz. Frequency deviations of < 3 and < I kHz
do not significantly decrease this value.
Note 2. - The receiver protection ratio may vary within the range shown dependent upon the difference in
frequency between the carriers of the wanted and unwanted emissions and the frequency deviation of the unwanted
emission. In general, it will tend towards the higher figure as the frequency deviation of the unwanted emission
For convenience in evaluating the degradation of performance of a base receiver, the following
classifications of noise sources are provided:
high noise locations - traffic density of 100 vehicles/km² at any given instant of time:
moderate noise locations - traffic density of 10 vehicles/km² at any given instant of time:
low noise locations - traffic density of I vehicle/km² at any given instant of time:
concentrated noise sources (hot spots): noise radiated from individual sources or closely spaced
multiple sources which are usually located within 500 m of the receiving antenna, such as a
high concentration of vehicles, manufacturing plants and defective power transmission lines.
Noise data for base stations at high, moderate and low noise locations are presented by a noise amplitude
distribution (NAD) (the number of pulses per second equal to or greater than the value shown as ordinate)
and are illustrated in Fig. 1. The amplitude (A) (in dB(µV/MHz)) of noise pulses at a rate of 10 pps (pulseper-second) is expressed as follows:
A = C + 10 log V - 28 log ƒ
C : constant (tentative value: 106 dBµV / M Hz)
V: traffic density vehicles/km²
ƒ: channel frequency, MHz.
Noise data for hot spots can also he presented in the form of a noise amplitude distribution.
However, due to a wide variety of noise sources, it is not yet practical to provide a classified list.
The constant C is a function of the electrical noise suppression applied to vehicles and may also
vary according to the relative proportion of goods and passenger vehicles if the level of
suppression is not the same for both categories. A tentative value of 106 dB(µV/MHz) is shown
and this may be revised as more information becomes available.
FIGURE 1 - Noise amplitude distribution at base station (150 MHz)
For frequencies other than 150MHz, raise or lower curves H,M and L
In accordance with the formula below
A = C + 10 log V-28 log ƒ
Where A=dB (µV/MHz) at 10pps
Curve H: high noise location (V=100)
Curve M: Moderate noise location (V = 10)
Curve L: low noise location (V=1)
Noise Amplitude Distribution (NAD)) determination of degradation
Noise amplitude distribution
A presentation of impulsive noise data in terms of its basic parameters of spectrum
amplitude and impulse rate.
Spectrum amplitude
The vector sum of the voltages produced by an impulse in a given bandwidth, divided by
the bandwidth.
Impulse rate
The number of impulses that exceed a given spectrum amplitude in a given period of
Impulsive-noise tolerance
The spectrum amplitude of impulses at a given pulse-repetition frequency at which the
receiver, with an input signal applied at specific levels, produces standard signal-to-noise
ratios at the output terminals.
Rep. 3585
I .3.2
Determination of degradation
Degradation of receivers can be determined as follows: measure the impulsive noise tolerance of the receiving equipment in accordance with
applicable IEC standards: measure NAD in accordance with applicable IEC standards: Superimpose the graphs for the receiver impulse noise tolerance and the NAD.
Minimum values of field strength to be protected
The minimum values of field strength to be protected in the land mobile service at frequencies
above 30 MHz are determined by internal noise generated in the receiver, man-made noise
usually in the form of radiation from ignition Systems of motor vehicles and the effects of
multipath propagation to and from moving vehicles. Some information on the effects of traffic
density is now available. In the maritime mobile service, the level of man-made noise depends
on the number and nature of high level sources of noise on the ship.
A convenient measure of the threshold of performance for narrowband receivers is a specified
value of
S+ N+ D
N+ D
ratio, the conventionally accepted value being 12 dB (see Recommendation 331).
This defines the minimum usable field strength for any particular installation, in the absence of
man-made noise.
The sensitivity of typical receivers is such that an input signal of 0.7 µV e.m.f. (assuming a
receiver input impedance of 50 Ω) would result in a 12 dB
S+ N+ D
N+ D
ratio at the output. A mobile service is characterised by large variations of field strength as a function
of location and time. These variations may be represented by a log-normal distribution for which
standard deviations of 8 dB at VHF and 10 dB at UHF are appropriate for terrain irregularities of 50 m
(see Recommendation 370). To determine the minimum value of median field strength to be protected.
it is necessary to specify the percentage of time for which the minimum usable field strength should be
exceeded for different grades of service. For land mobile radiotelephony. a high
grade of service
would require that the value be exceeded for 99% of the time, but, for a lower (or normal) grade of service, for
90% of the time.
The minimum values of field strength to be protected can be determined subjectively, taking into account
man- made noise and multipath propagation. Ignition systems of motor vehicles are usually the most prevalent
source of man-made noise. Field strength cancellations due to multipath propagation produce an annoyance
somewhat similar to that created by ignition systems. When a mobile unit is in motion, both of these annoyances
occur at the same time. Only the effects of receiver noise and man-made noise remain when the mobile unit is
stationary. The separation of motor vehicles is generally less with slow-moving or stationary traffic and under
these circumstances, particularly at the lower frequencies, the degradation experienced in a stationary mobile unit
is greater than when it is in motion.
Figures 3 and 4 can be used to determine the combined degradation effects of man-made noise and multipath
propagation for the case of vehicles in motion. These figures are based on subjective testing under traffic
conditions commonly experienced by most mobile vehicles [FCC, 1973]. Specifically, these traffic conditions
are the following: in motion while in a low noise area, in motion in traffic surrounded by other vehicles and
stationary surrounded by other stationary or moving vehicles.
The tendency for the curves of Figs. 3 and 4 to merge at the higher frequencies is due to the almost constant
multipath degradation effect with frequency and the fact that the degradation effect of man-made noise
decreases with frequency.
Degradation is defined as the increase of level necessary in the desired input signal to maintain the receiving
signal at the degree of quality obtainable when affected by receiver noise only.
Rep. 358-5
Definitions of signal are as follows:
Interfering effect:
Almost Nil
] Speech understandable,
] but with increasing
] effort as the grade
Very Annoying ] decreases
so bad that the presence
of speech is barely
Some information on field strengths can be derived from Recommendation 370. Additional information can be
found in the document of the CCI R, [1966-69], and in the article of Okumura et al. [1968].
Information on protection ratios and minimum field strengths may also be found in the "Special Agreement
between the Administrations of Belgium, the Netherlands, and the Federal Republic of Germany relating to the use
of metric and decimetric waves for fixed and mobile services in border areas, Brussels, 1963", and in the Final Acts
of the Special Regional Conference, Geneva, 1960. Similar information may be found in the Agreement between
the Telecommunications Administrations of Austria, the Federal Republic of Germany, Italy and Switzerland,
Vienna, 1969.
The document of the CCIR [1963-66]. deals with the above questions for signal-to-noise ratios of 30 dB and 40 dB
at the receiver output.
Until values based on man-made noise and multipath effects are available, the calculated values of minimum and
median values of field strength shown in Fig. 2 may he used for hand-portable stations.
FIGURE 2 - Minimum usable and median field strengths for typical hand-portable stations
(based on minimum usable input of 0.7µV e.m.f., in the absence of man-made noise)
{A and C: - 9dB
Characteristics assumed: antenna gain {
{B and D: - 6dB
A, B: median, normal grade
C, D: median, high grade
E: minimum usable field strength (dipole antenna)
Rep. 358-5
Figure 3 - Variation of degradation of mobile reception and minimum values of field strength
To be protected for signal quality grade 4 and receiver sensitivity of 0.7 µVe.m.f.
Field Strength + -41 +d+20 log f
A: mobile vehicle stationary within a high noise area
B: mobile vehicle in motion within a high noise area
C: mobile vehicle in motion within a low noise area
Rep. 358-5
FIGURE 4 - Variation of degradation of mobile reception and minimum values of field strength
To be protected for signal quality grade 3 and receiver sensitivity
Of 0.7 µV e.m.f.
Field strength =-41+d+20 log f
A: mobile vehicle stationary within a high noise area
B: mobile vehicle in motion within a high noise area
C: mobile vehicle in motion within a low noise area
This information is abstracted from CCIR Report 358-5
The cost and practicability of any communications system depend on the proportion of time for which the
communications channel must be available, ie deliver the required signal with at least the minimum
specified ratio of signal to noise. Factors which contribute to communications channel failure include:
Propagation Variability
Co-channel interference
Icing or wind deflection of antennas
Radio equipment failure
Down time required for equipment servicing
Loss of supply
Failure of signal input equipment
Unavailability of a multi-access channel
The reduction of down time due to each of these causes usually costs money and the most economic
system requires a careful balance of these factors. Where long breaks in service (typical of site
inaccessibility and antenna icing or power loss in winter) are not acceptable, reserve equipment,
alternative routing or other costly measures may be needed.
The service life over which the system availability is required to meet its objective must be defined.
System Type
Private mobile radio
Private multi user mobile
Trunked mobile
Fixed links
Median Field Strength
% time
Define as required by
system and grade of
service necessary
Site operators should agree basic rules for loss-of-service with their site users, for example:
1 Longest permissible outage without notice
2 Longest permissible outage with 7 days notice
3 Signal reduction (dB) acceptable (with 7 days notice) which allows minimum essential facility.
This parameter may also be subdivided by time of day or week.
REPORT 739-1
(Study Programme 7C/8)
(1978 - 1986)
7Intermodulation causes a degradation to radio services when:
Unwanted emissions are generated in transmitters:
Unwanted emissions are generated in non-linear elements external to the transmitters:
G In-band intermodulation products are generated in the radio-frequency stages of
These cases occur with varying probability and varying severity. They may be reduced by
equipment design or careful choice of channels, but solutions of the latter type to one case
intermodulation may increase another.
The last active stage of a transmitter is usually an amplifier. The current in this stage will
be repeatedly swept from zero amplitude to a maximum and the impedance of the output
active device is liable to contain a small amount of non-linearity.
If any other signal from another emission is also present at the output of this stage the nonlinearity will give rise to a number of products having frequencies with specific frequency
relationships to the frequency of both the wanted and unwanted signals. These products
are called intermodulation products, and their frequencies may be expressed as
ƒª = Cª ·ƒª + C² · ƒ² + … + Cⁿ · ƒⁿ
where the sum │ Cª │+ C² │+…+│ Cⁿ│ is the order of the product.
The odd-order intermodulation products may be relatively close in frequency to the wanted
signal frequency and thus coupled via the output circuit to the antenna with minimal
In order to be able to calculate the effects of these products, it is necessary to establish
certain terms.
Coupling loss. Aª
The coupling loss, Aª, in dB, is the ratio of the power emitted from one transmitter to
the power level of that emission at the output of another transmitter which may
produce the unwanted intermodulation product.
Typical values for the coupling loss on a common site are of the order of 30 dB.
Intermodulation conversion loss. Aª
The intermodulation conversion loss Aª, in dB is the ratio of power levels of the
interfering signal from an external source and the intermodulation product, both
measured at the output of the transmitter.
Without any special precautions, typical values for semi-conductor transmitters are to
be found in the range of 5 to 20 dB and for the value transmitters, in the range of 10
to 30dB, in respect of the 3rd order product (2ƒª - ƒ²)
The overall loss between a transmitter providing the unwanted emission giving rise
to the intermodulation product and a receiver operating at the frequency of the
product is:
A = .4, + Aª + AF
Where AF, in dB, is the propagation loss of the intermodulation product between the
relevant transmitter output and the receiver input.
* this information is abstracted from CCIR Report 739
Rep. 739-I
Note that the power level of the transmitter in which the intermodulation is produced is not
included in the formula hut this level may have an effect on the value of the intermodulation
conversion loss Aª.
Signal frequency of transmitter producing intermodulation product:
Signal frequency of transmitter whose emission is coupled into transmitter (ƒª): ƒ²
Power level of transmitter (ƒ²):
-10 dl3W
Assumed coupling loss Aª
30 dB
Assumed conversion loss A,:
Assumed receiver threshold signal level:
Overall path loss is equal to 10 dBW - ( - 150 dBW) = 160 dB.
If Aª,. + Aª, = 45dB. then the required value of AF is 115dB
Figure 1 gives an example of propagation path losses at 100 MHz and, under free space
conditions' a very large distance is required between the "product producing" transmitter and
the receiver. If the receiver is a mobile station, this distance is considerably reduced. It may be
concluded therefore that 2-frequency operation provides - better conditions for the reduction of
the effects of inter-transmitter intermodulation if the base receive frequency band is remote
from the transmit frequency band.
FIGURE 1- Short range path loss at 100 MHz ( dipoles assumed)
Curves A: free space
B: Recommendation 370-3:hª=37.5m,h²=2 m
The intermodulation caused by two or more mobile transmitters will be worse when the mobiles are
closest together and when the desired signal originates at a mobile at the edge of the service area,
an event which is associated with some (perhaps small) probability. The mobile being interfered with
will be received at its base as a signal of widely varying level (due to fades and shadows) which will
be independent of the 1M interference. These wide and independent variations can allow the 1M to
reach harmful values for periods of time, even when its average value is much less than that of the
Rep. 739-I
External non-linear elements
On most sites. external non-linear elements will be at junctions in masts, feeders, and
other antenna which are closely coupled to the radiating elements of nearby
It would be useful to determine conversion losses for masts etc., of various qualities in
terms of the isotropic loss between transmitters and the masts. etc. It would then be
possible to establish specific values a good engineering practice.
.An intermodulation response is a response at the output of a receiver from an in-band
signal generated in the RF stages of the receiver. This in-band signal is generated by
the presence of two (or more) high-level signal in a non-linear section of the RF stages.
As with transmitters, the two (or more) unwanted signals must have specific frequencies
such that the intermodulation product lies within the frequency band accepted by the
This receiver characteristic is normally recorded as a single measurement with the level
of the unwanted signals equal and is given as a single ratio which is:
the ratio of the level of these two equal signals
the apparent level of the intermodulation product at the input to the receiver.
It is possible, however, to cause a similar product level when the unwanted signals are
not equal.
Figure 2 gives examples (3 theoretical and I measured) of the overall third order
characteristic of receivers. It shows that intermodulation may easily be a problem
when one of the unwanted signals is not excessively high. Such curves can he used
to calculate other intermodulation product levels when the unwanted signals do not
have values equal to those plotted.
For a product with a frequency relationship of the form (2ƒ1 - ƒ2), the level will be
proportional to the level of the signal at frequency ƒ2, but will vary as the square of the
level of the signal ƒ1 i.e. the product will have an amplitude of the form k. VB V2, where
Vª. V2 are the amplitudes of the signals at frequencies ƒ1 and ƒ2 respectively.
When a mobile receiver is used in a multi-channel system it will be subject to an
intermodulation response due to many equally spaced high level signals. The following
relationship has been suggested by the People' Republic of China to relate the
maximum permissible signal level with the intermodulation response rejection ratio of the
receiver [CCIR, 1982-86a]:
Es, + 3EM≥ 3Eªmax + B + k(n,p)
Es :
wanted signal level (dB) above sensitivity:
E1max: maximum interference signal level (dB) above sensitivity:
receiver's third-order intermodulation rejection ratio (d B) (for two signals):
RF protection ratio (dB)
k(n.p) : a constant dependent on the number of channels n and channel sequence p.
The derivation of this formula and the calculation of k(n.p) are given in Annex 1
Reduction of intermodulation product levels in transmitters
Intermodulation conversion loss
It is obvious that a reduction of the non-linearity, particularly of the odd-numbered
orders, will improve the overall performance and increase the value of the
intermodulation conversion loss A1.
From the example in ş 2. it is evident that a considerable improvement is necessary
before the relevant path loss reduces to manageable values.
Coupling loss
The coupling loss can obviously be increased by increasing the distance between the
relevant transmitter but it may not always be possible to do so effectively at a particular
Ferrite isolators could be used in the output circuits of the transmitter in which the
product is generated but present production units do not provide much more than 25 dB
additional loss and the use of multiple units is. inhibited by the inherent non-linearity of
the isolators themselves. To suppress undesirable products, filters may bc required after
such isolators. These isolators are equally effective irrespective of the frequency spacing
between ƒ1 and ƒ2.
FIGURE 2 - Receiver intermodulation characteristic
Levels of unwanted input signals which together produce a constant product level.
Curves A,B and C: derived characteristics based on a single recorded value of the
receiver's third order intermodulation characteristic, i.e. for (2ƒ1 - ƒ2).
Curves A: based on a single value, with both input levels at a level of 60dB(µV) (e.m.f. to 50 ohms).
B: based on a single value, with both input levels at a level of 70dB(µV) (e.m.f. to 50 ohms).
C: based on a single value, with both input levels at a level of 80dB(µV) (e.m.f. to 50 ohms).
D: measured values for a receiver for which the specified criterion is achieved with equal input
signal levels of 65.5 dB(µV) (e.m.f. to 50 ohms).
Rep. 739-1
Cavity Filters can also be used and examples of their theoretical responses are given in Fig. 3.
They may be used in cascade or in more complex series-parallel combinations but in all cases,
their performance is dependent on the frequency spacing between ƒ1 and ƒ2. They have the
advantage that they will also attenuate the product level at the input to the antenna or
transmission line and thus increase A1.
FIGURE 3 - Theoretical response of cavity band-pass filters
For values of loaded Q of 250-2500.
Note. - The unloaded Q should be at least 5 times the loaded Q and preferably 10 times
An economic and efficient filter is the coaxial cavity resonator, either in its pure quarterwavelength form or with varying degrees of modification to reduce the overall tenth and improve
the value of the loaded Q. The resonator should be robust, simple to tune. highly efficient in terms
of transmission loss, and provide a high degree of isolation at the required frequencies.
Resonators for use with transmitters should have a low temperature coefficient and good thermal
conductivity. So that their performance is not affected by changes in ambient temperature or
through being heated by transmission losses. Temperature compensation can be employed to
maintain the length of the centre conductors. Physical robustness is necessary to avoid changes
in technical parameters from being caused by mechanical shock or defo mation. The physical
and mechanical design should also prevent the formation of electrical discharges or corona
Adjustable telescopic centre conductor assemblies permit a variation of resonant frequency of,
typically, ± I 5% of the centre frequency.
Reliable and economical resonators can be manufactured from high-conductivity aluminium for
the larger units, and silver-plated copper or brass for smaller units. Practical limitations of
mechanical engineering govern the upper limits of Q obtainable with a cavity resonator. As the
diameter is increased. the value of the unloaded Q is increased, but the sensitivity of tuning and
the temperature coefficient become more critical. Practical and satisfactory resonators with a
power handling capacity of up to 250 W can, however, be made for the band 150-170MHz, for
example. having an unloaded Q as large as I 8 000, with a diameter of 0.58 m. and length
0.63m, giving 35 dB discrimination at a frequency 1% removed from the resonant frequency.
Rep 739-I
It is not usual to employ cavity resonators for values of Qo below about 1000, since there are
more satisfactory techniques, e.g. helical resonators, which can be coupled together to form
smaller but relatively efficient filter units. Tables I and II give the choice of types of filter and their
relative costs.
TABLE I Relative sizes and costs of resonators (150-174 MHz)
1% Fo (dB)
cost of
TABLE II - Relative costs of practical resonators for other frequencies
50- 60
60- 80
95- 110
120- 150
150- 174
160- 180
400- 500
Unloaded Q
Note. - Items not tabulated are identified as follows:
*Helical resonator superior
+ Single cavity large and somewhat uneconomic.
Compared with the total cost of the radio equipment at a base station, cavity resonator filters are
an economical and efficient means of reducing spurious emissions and preventing or minimizing
5.3 Identification of the source of an intermodulation product
The frequency of the third order intermodulation resulting from the interaction of two transmitters
may be expressed as either 2ƒ1 - ƒ2 or 2ƒ2 - ƒ1.
If the product is 2ƒ1 - ƒ2, the mixing is occurring within or close to the transmitter operating
on ƒ1.
Conversely, if the product is 2ƒ2 - ƒ1, the mixing is occurring within or close to the transmitter
operating on ƒ2
In the case of FM or PM emissions, the deviation caused by modulation is doubled when a
second harmonic is generated. So if the modulation on one of the intermodulation products
appears to be excessive, this modulation is probably transferred from the ƒ1 signal of a
2ƒ1 - ƒ2 mixing.
Rep. 739~I
Reduction of intermodulation products in receivers
As with transmitters, a reduction in the non-linearity of a receiver will improve the
Attenuation at the input of the receiver may be used to reduce the level of an intermodulition
product. The levels of these products are related to the levels of the signals that produce them,
in such a way that the attenuation (in dB) of each "nΤΗ" order product will, in most cases, be ,n
times the attenuation (in dB) of the wanted signal.
For example, a 3 dB attenuator will reduce a third order product by 9 dB while reducing the
wanted signal by 3 dB. This may also he used as a test device to prove that the intermodulation
product is being generated in the receiver.
Cavity filters can be used, either as rejection filters to ƒ1 and/or ƒ2 or as band~pass filters to
the wanted signal. Again the effectiveness of these filters depends on the frequency spacings
Reduction of intermodulation interference by frequency arrangements
The frequencies to be used can be arranged so that no receiver on the product frequency is
required to operate in an area where the unwanted signals may produce an intermodulation
product of sufficient level to disturb the service. If this level is at the maximum sensitivity level
of the receiver; it will mean receivers cannot be used for distances up to 2 km from the sites of
the base station operating at ƒ1 and ƒ2. This applies even when the ƒ1 and ƒ2 stations are
separated by several kilometres and thus implies that the base station on the product channel
must be sited outside the service area of stations operating on ƒ1 and ƒ2. This leads to very
poor use of the frequency spectrum.
In systems that operate a number of frequency channels, most cases of harmful base
transmitter and mobile receiver intermodulation within the system can be alleviated by the
choice of even channel sets at the base stations. This means that the channels of each base
station are evenly distributed at a constant frequency separation. In a service area the
intermodulation products within the band used will in that case coincide with channels of the
set, and the ratio of the desired signal to the intermodulation product in a mobile receiver is
independent of the distance and propagation characteristics.
Reduction of intermodulation interference by other arrangements
If continuous tone signalling is used. the receiver will operate only in the presence of this
signalling tone and it is then necessary only to ensure that the wanted signal on the product
channel exceeds the level of an unwanted product off ƒ1 and ƒ2 by an amount in excess of the
required protection ratio. This can be best assured by siting the product channel base
transmitter at the same, or near to, the site of stations operating on ƒ1 and ƒ2.
Under these conditions, the need for filters or other devices in the transmitter or receiver
Is reduced.
Rep. 1019
(Question 7-2/8)
The greatly increased use of land mobile services has resulted in a dramatic increase in the
number of base stations on any one site, particularly on those sites strategically placed to serve
large built-up areas. This has led to instances of severe interference due to unwanted signals
being generated at the site. This Report is not intended to examine every possible type of
interference but rather to indicate the more commonly occurring sources. It should be
particularly noted that transmitters of other services may be involved.
Simple frequency relationships
As land mobile frequency bands are used throughout the VHF/UHF spectrum there may be
harmonic relationships between frequencies in the various bands. The equipment cabinet, the
power supply cabling and land-line cabling can contribute to the level of these unwanted
harmonic signals.
Other interfering signals can be caused by simple mixes either in transmitter output stages or at
the antenna mast. As an example, if the signal from a VHF broadcasting transmitter at 93 MHz
mixes with a signal of the mobile service at 170.5 MHz. a difference signal of 77.5 MHz can be
produced. This can cause a problem if -it is a receive frequency of the mobile service.
Complex frequency relationships
Generation of intermediate Frequency and/or its derivatives
Interference can be caused in a receiver where signals are received from two transmitters
whose frequencies are separated by an amount equal to the IF, or a submultiple of the IF, of the
Generation of transmit/receive (Tx/Rx) difference frequency
This problem arises on Sites where there are several base stations having "repeater" or "talkthrough" facilities, i.e. the transmitters and receivers are in use simultaneously. If the Tx/Rx
spacing is constant (D). an incoming signal from a mobile station will produce in the base
station transmitter output stage a difference frequency, D. Any other base station transmitter
may now mix with D to produce its own receiver frequency in the same band.
Intermodulation products
Generated external to the site
Under this heading. products arise from stations on adjacent sites, and, in particular, the third
order product i.e. 2fj - 12. which is prevalent in large built-up areas. In some instances
significant intermodulation products up to and including the seventh order have been noted and
in exceptional cases the interference has been traced to the nineteenth order.
Intermodulation products generated on-site by non-linear junctions on the mast
More study is required to verify the mechanisms and levels of such interference, which certainly
exists in the land mobile bands. However, at lower radiated powers. the significance of these
products is reduced. compared with other forms of non-linearity, e.g. § 4.1 and 4.3.
Intermodulation products generated on-Site by non-linearity in components of the system
Junctions between dissimilar metals cause non-linearity, and therefore intermodulation
products, when subjected to radio frequency currents, and recent work has highlighted such
products up to the eleventh order at VHF caused by connectors, cables and dissimilar junctions
in what might be regarded as otherwise innocuous components.
For the long-term development of the land mobile radio industry, it may be necessary to define
the non-linearity of passive components in the system.
Transmitter noise
Until quite recently. most transmitters on base station sites had valve output stages, which
fortuitously were not a major contributor to the noise spectrum compared with the more modern
solid-state output stages.
With a valve output stage, the unwanted noise is generally narrow-band, having frequencies
which are multiples of the crystal oscillator frequency or a combination derived from the
multipliers. However, in the case of solid-state output stages the noise is generally wideband
and higher in level.
Figures I, 2 and 3 give the graphical results of measurements made in the United Kingdom of
noise from VHF transmitters with thermionic valve output stages and with solid-state output
stages for the VHF "high band" (150-I 70 MHz) and VHF "low band" (71.5-87.9 MHz).
Rep. 1019
External electrical noise
Apart from ignition noise, there are the well-known sources of radio interference, which continue to
proliferate, particularly from industrial users, i.e. RF heating, microwave ovens, X-ray and medical
equipments. These normally provide a broad spectrum of noise which tends to vary in frequency.
Screening or suppression of the interfering equipment normally reduces the problem to an
acceptable level.
There is however, a new family of sources, namely computers and computer peripherals, which
are currently' causing problems with broadband noise over the VHF spectrum.
There are instances where the present engineering practices in multiple transmitter sites have
allowed the generation of excessive unwanted signals. With the increased use of land mobile radio
it is desirable to perfect techniques to reduce interference effects in the future. There is a need for
better site engineering in order to establish "quiet" base station sites for trunking networks and
cellular radio.
The following should be considered:
spurious emissions from transmitters;
filtering of transmitter outputs to reduce spurious emissions and noise at frequencies near the
use of directive isolators in transmitter output stages;
additional filtering to provide protection at adjacent frequency bands:
non-linear effects at all points in the system.
At the Output of a transmitter of frequency B, the level of the interfering signal due to a transmitter on
frequency A will be attenuated by the isolation between the transmitters In this case the amplitudes of
the intermodulation products of the same order will not be equal.
(Appencix 3 refers, CIIR REPORT 739-l).
(at the output of transmitter frequency B, interfering signal frequency A)
Products Combinations of two frequencies
A and B excluding pure Harmonics.
A=fa+ ~∆f
where fa = 2
and ∆f =
2A+B=3fa+ ∆f
2A+2B =4fa
2B-2A= -4∆f
2nd Order
lOty 2)
3rd Order
(Oty 4)
4th Order
(Oty 7)
3A+2B=4fa+ ∆f
3B+2A=4fa- ∆f
5th Order (Qty 8)
6TH Order
(Qty 11)
4A+3B=7fa+ ∆f
4B+3A = 7fa - ∆f
7th Order
(Oty 12)
5A+4B=9fa+ ∆f
5B+4A=9fa+ ∆f
*5A-4B=fa+9∆f 9th Order
6A+3B=9fa+3∆f (Oty 16)
8th Order
(Otv 15)
* Inband
Intermodulation between frequencies of channels allocated in a bandwidth B Hz will be spread over the
spectrum from DC to n times the highest frequency used (where n is the order of the non-linearity
producing the intermodulation). Of particular interest are those products which fall back within and
around the band B. This group will extend over a range of n x B Hz. the distribution within this being
dependent on the initial distribution within B. In addition the modulation of the generating carriers will
cause each individual product to be spread over n times the occupied bandwidth.
For example if the band B were 2MHz wide from 154 to 156 MHz then 9th order intermodulation
products would extend from 146 to 164 MHz and each product would cover a band of 72 KHz if the
occupied bandwidth is taken as ± 4 KHz.
The number of such products is given in table 5.1
Table 5.1 - Number of Intermodulation Products
Order number
Number of
Angle of plane of
polarisation from
nominal (degrees)
Achieved CPD for various
test range values (dB)
0 (Correctly aligned)
The table indicates the CPD which is achieved between an antenna mounted with its polarisation plane
exactly horizontal and vertical, and an antenna mounted with a small angular error. As an example two
antennas providing 40 dB CPD when correctly mounted provide a CPD of 27 dB if mounted with an
alignment error of 20.
The table emphasises the importance of accurate mounting; the achievement of a CPD of 30dB of more
requires especial care at frequencies at which the antenna elements are small or inaccessible. In such
cases it is necessary to optimise cross polar performance by electrical measurements.
A (from manufacturer's data)
B (from cable data)
C (= A + B)
D (from C)
E (from cable data)
F (= D + 2E)
G (from F)
H (from cable data)
Of System
Flexible link feeders
Reflection coefficient
At top of main feeder
Return loss at top of main feeder
Main feeder attenuation
Return loss at bottom of main
Reflection coefficient at bottom of
main feeder
Link to transmitter
Condition at transmitter
Typical Figures
The cable reflection coefficients quoted are typical; the reflection caused by the terminations and any
adaptors which are used must be included.
See Appendix 9 for formulae and method of calculation
When several components are connected together in series, the reflected waves caused by each
discontinuity travel back from the discontinuity towards the system input. In general the reflection from
each discontinuity will arrive at the system input with a phase which has a random relationship to other
reflections, but in the worst case all these reflections will add in phase. This will almost certainly occur in
a broadband system over some part of the operating band.
To convert from VSWR (u) to voltage reflection coefficient (p v),
pv = σ - 1
and conversely
Note that p v is always less than 1 for a passive system; when p v is expressed as a percentage it must
first be rewritten in standard form, eg 7% = 0.07.
Return loss is expressed in dB and is given by
LR = -20 log10 (p v)
or p V = 10
The return loss measured at the input end of a cable is always greater than that at the load end owing to
the attenuation of the cable. The input VSWR is correspondingly lower than that of the load alone. For a
cable with an attenuation of 2dB, the input return loss (Lin) is related to the return loss at the load (Le) by
Lin = Le + 2
It should be noted that cable manufacturers generally guarantee the input VSWR which will be achieved
from an installed cable with both connectors fitted. Now allowance can be made in calculation for the
attenuation of the cable in respect of this figure, as it is only guaranteed as the figure which will be
measured at the cable input. In practice longer cables are more likely to suffer discontinuities along their
length, but the increased attenuation inherent in longer cables limits the degradation of input VSWR
caused by distributed discontinuities.
For this reason, very long large diameter cables with very low loss present the greatest VSWR problems
to the cable manufacturer and to their installers. The discontinuity represented by any defect becomes
more significant with increasing frequency and great care must be taken when handling, bending,
clamping and terminating cables for UHF and SHF use.
When choosing cables for the UHF band (900 MHz, 1.5 GHz etc) it should be remembered that coaxial
cables have an upper frequency limit above which operation becomes uncertain due to overmoding. This
may be approximately found by the relationship.
λc = πvr (d0 + di)/2
Where d0 is the diameter of the outer conductor
di is the diameter of the inner conductor
vr is the velocity ratio
For these frequency bands the intended operating frequency should always be quoted when ordering
cables or connectors.
Raindrops which fall in highly convective conditions (not only during thunderstorms) often carry
electrostatic charges which inject noise impulses when the drops which fall on a shrounded element will
exchange charge with the film of water wetting the shroud; this process radiates some noise energy but
the noise power coupled into the antenna is much reduced.
Shrouds typically provide between 25mm and 5Omm radial clearance between the antenna element and
the wetted surface. They are often fitted only to the dipole element of Yagi antennas.
Mean values of man-made noise for a short vertical
lossless grounded monopole antenna
Environmental category:
quiet rural
The above graphs have been taken from CCIR Report 258-4, and have been expanded from 25 to
1000MHz for ease of reference to the Land Mobile Services.
The resonant frequency is normally proportional to the length of the centre conductor, which
approximates to a quarter wavelength.
The insertion loss is determined by the coupling factor between the input and output coupling
structures, and is related to Q.
The bandwidth of the cavity is directly related to Q as shown below.
Qo is the unloaded Q of the resonator.
Q L is the loaded 0 of the resonator.
The relationships between these parameters are as follows:
Where F = Centre frequency of resonance
∆F = 3 dB bandwidth
Insertion Loss: (dB) = 20 log 10 (1 + ~ Q 0/QL)
Typical filter System
Spectrum dividing filter response curve
Single Aerial UHF system
Typical Sub-band TX/RX system
This system is designed to allow single antenna working for a multi-channel trunking installation
The equipment is configured such that it can operate over various bandwidths (1-10 MHz range)
The system can accommodate up to 16 channels in VHF Band Ill or UHF. The following specification
relates to an 8 channel combiner.
Insertion Loss:
(from input to antenna port):
Typically 2.8dB
TX to TX Isolation:
Minimum FrequencySpacing
between Transmitters:
Input VSWR:
Better than 1.1:1 (Return Loss 26dB)
Output VSWR:
Better than 1.2:1 (Return Loss 21dB)
Power Handling (each channel):
60 watts (17dBW)(100 watt version available)
5th Order Intermodulation
(For 17 dBW Inputs):
Better than - 154dBBW
TX to RX Isolation:
Number of channels;
5, 8, 10, or 16
Overall Gain:
+dB (can be adjusted up to 6dB)
Input VSWR:
Better than 1.2:1 (Return Loss 21dB)
Output VSWR:
better than 1.2:1 (Return Loss 21dB)
RX to TX Isolation:
Noise Figure:
5dB typical
Third Order Intercept:
+25dBm typical
RX to RX Isolation:
25dB typical
BS CP3 Ch. V Part 2: 1972
Wind loads
BS CP118: 1969
The structural use of aluminium
BS 449:
The use of structural steel in buildings
BS 729:
Hot dip galvanised coatings on iron and steel articles
BS PD 6484: 1974 (1984)
Commentary on corrosion at bimetallic contacts and its
BS 5293: 1977
Code of practice for protective coating of iron and steel
structures against corrosion
BS 1615: 1972
Anodic oxidation coatings on aluminium
BS 4190: 1967
ISO metric black hexagon bolts, screws and nuts
BS 2011:
Basic environmental testing procedures
BS 6651: 1985
Code of practice for the protection of structures against
Earthing of Telecommunications Installations - International Telecommunication Union, General ISBN
92- 61 - 0031 -x CCITT, 1976
MPT 1326
Performance specification for angle modulated VHF and
UHF equipment for use at fixed and mobile stations in the
Private Mobile Radio Service
CCIR Report 358 - 5
Protection ratios and minimum field strengths required in
the Mobile Services
CCIR Report 1019
Sources of unwanted signals in multiple base station sites
in the Land Mobile Service
CCIR Report 258 - 4
man made radio noise
Safety Precautions Relating to
Intense Radio Frequency Radiation
HMSO 1960 (ISBN 0 11 340576 6)
Proposals for the health Protection
Of workers & Members of the
Public against the Dangers of
Extra Low Frequency Radio Frequency
And Microwave Radiations:
A constructive Document
National Radiological Protection 1982
(ISBN 085951 185 5)
Annex I To MPT 1331 Case Studies
The following cases contained in this section are typical examples of interference cases which could
have been prevented if the guidelines contained in this code had been followed.
If interference occurs, then a logical sequence of steps should be followed to identify the cause of the
Check that the receiver front end is not being overloaded; a notch filter tuned to the interfering
signal installed at the antenna input to the receiver will normally solve an overload problem.
Check that mixing is not taking place in the front end of the receiver; if the unwanted received
signals are within 1% of the wanted frequency then follow step "a", however, normally a bandpass
filter installed at the antenna input to the receiver will solve this problem.
If the interference is not generated in the receiver, then the direction of the interference can be
traced by using an antenna, with directional properties, connected to a receiver with signal level
When interference has been traced back to a site, and the signals causing the intermodulation
component have been identified, it will be necessary to determine where the mix is occurring.
DO NOT tamper with any equipment on site unless the owners prior permission has been
obtained. If it is necessary to disconnect the antenna feeder from the transmitter output, ensure
that the transmitter cannot be keyed.
It should be noted that when making measurements with sensitive measuring instruments,
particularly when tracing intermodulation products (IMP) in the transmitter output, that appropriate
precautions are taken. [Such as stop or notch filters, attenuators or directional couplers to protect
the input stages of sensitive equipment.]
Mixing can occur in a transmitter output stage, due to the carrier frequency mixing with another
signal being fed back via the antenna feeder. This problem can normally be solved by fitting an
isolator or bandpass filter in the antenna feeder close to the transmitter output.
If interference is still present then mixing is most likely taking place either on the transmitter or
receiver mast structure. It may be possible to overcome the problem by increasing the horizontal
or preferably the vertical separation between the antenna on the existing mast.
Alternatively if the radiating source on the mast can be located, it may be possible to carry out the
necessary maintenance.
In extreme cases it may be necessary for one of the users to change sites, to overcome the
interference problems.
Case 18 is included as a reminder, that the cause of the problem may be in the users own equipment,
therefore it should be ensured that the equipment is regularly maintained.
Case 1
A high power 150 MHz band transmitter was causing interference to a Cositedl6O MHz marine band
receiver by desensitising the receiver and causing blocking. The transmitter already had an isolator and
bandpass filter fitted to the output antenna feeder. The problem was solved by inserting a bandpass
cavity resonator into the antenna feeder to the marine receiver.
Case 2
Interference was being received on a Police VHF base station receiver. It was suspected that a high
power BT Radiophone transmitter, located approximately 500 metres away, was overloading the front
end of the receiver. A bandpass filter was inserted into the antenna feeder of the receiver, and although
the level of interference was reduced, a significant signal level was still present.
Further tests showed that the RF signal from the BT transmitter was getting into the output stage of the
Police transmitter, Co sited with the receiver. The mix between the two carriers produced a resultant
product on the Police receive frequency. An additional band pass filter was inserted in the output feeder
of the Police transmitter. The interference problem was solved by placing bandpass filters in both the
Police transmitter and receiver antenna feeders.
Case 3
Interference was being received on several base station receivers, located on a communal site. During
tests at the site, it was noted that the interference ceased when one particular co-sited transmitter was
keyed. The transmitter used a semiconductor output stage, which was still active even when the drive
had been removed, ie the transmitter was in the standby mode. The problem was caused when cosited transmitters were keyed, causing the output stage of the offending transmitter to go unstable and
radiate spurious noise.
By installing a bandpass filter and isolator at the output of the transmitter the problem was solved.
Case 4
Interference was being caused to a 141 MHz fm base station receiver from a co-sited 138 MHz am
base station transmitter.
It was suspected that the 138 MHz transmitter was overloading the fm receiver. An additional bandpass
cavity filter was installed in series with the existing high-Q bandpass filter which is normally fitted to the
receiver input, but produced no improvement. A Notch filter tuned to the interfering carrier was then
tried in the receiver, again no improvement was noted.
A spectrum analyser was used to observe the transmitted spectrum, which showed a number of low
level spurious signals, one of them falling directly onto the fm channel. By fitting a bandpass filter at the
transmitter output and retaining the single bandpass cavity at the receiver, the interference was cured.
Case 5
Co-channel interference was being caused to a Police base station receiver which was located near to
a communal base station site approximately 500 metres away, with some 10 pmr transmitters on site.
None of the transmitters had filters or isolators fitted to their outputs, therefore a large number of
intermodulation products were being generated over a wide band. The problem was solved by fitting
isolators and filters to the outputs of the offending transmitters.
Case 6
Co-channel interference was received on a 141 MHz band base station receiver.
The source of the interference was traced to a spurious signal being radiated from a 145 MHz amateur
band repeater. The repeater transmitter used a local oscillator with a times 36 multiplying stage to
obtain the required carrier frequency. A spurious signal. which was due to the 35th harmonic of the
local oscillator, was being radiated.
The problem was solved by inserting an additional bandpass filter at the transmitter output.
Case 7
A high band VHF transmitter was set up at the repair depot before being reinstalled on site. The
transmitter when installed on the communal site was connected to the antenna via a high-Q filter. When
the transmitter was keyed high level spurious signals were emitted causing interference to other cosited systems. The problem was solved by retuning the transmitter rf stages to match the filter system.
NOTE: A transmitter should be set up in the workshop into an accurate 50 ohm dummy load.
Case 8
Radio 3 programmes from a VHF FNI Broadcast transmitter were received on a Police base station
Inter modulation products were generated in the structure of the mast on which the Police antennas
were mounted, due to the high powered broadcast transmitter.
This problem was solved by removing the receiving antenna to a position above the mast structure
using a vertically stacked dipole on a 3 metre pole. This succeeded in removing the receiving antenna
away from the field of interference caused by the mast structure.
Case 9
Interference was received on a British Gas base station receiver operating on 106 MHz. Co-sited was a
4 kilowatt Local radio broadcast transmitter, on g6 MHz and also an AA transmitter on 86.525 MHz
band. No 3rd order intermodulation products were detected from the Broadcast transmitter, however,
by using a loop antenna connected to an analyser the intermodulation product was traced to the
Broadcast antenna. The antenna was checked but no fault could be found.
Some reduction in level was obtained by changing the B.Gas Colinear antenna for two dipoles.
Additional reduction was obtained by siting the antennas further down the mast, ie increasing the
vertical separation.
Case 10
An on air third order intermodulation product was traced to a communal site. The two transmitters were
identified, which already had isolators and filters fitted to their outputs. On further investigation the
interference was traced to a receiver antenna which was radiating the antenna intermodulation product.
When the antenna feeder was disconnected from the associated receiver, the interference
disappeared. To solve the problem a band pass filter was installed at the input to the receiver.
Case 11
British Telecom were experiencing on site problems from their Radiophone and paging transmitters
which were causing breakthrough on the radiophone receivers. After a considerable amount of work on
the site, breakthrough was still being experienced. The problem was eventually solved by replacing the
existing feeders with double screen cables and using high quality connectors.
Case 12
An on air interference signal was traced to a communal site. The actual source, however could not
easily be identified, therefore with the owners permission equipment was switched off to try and
eliminate the source. With all known equipment off, the interference was still present. The source was
traced to the system master crystal oscillator, which was still running.
Following modification to the equipment by the manufacturer, the interference was eliminated.
Case 13
A high band user was complaining of interference on his channel: the system (system A) was using
free running talk through. The frequency separation between the transmit and receive on a high band
channel is 4.8 MHz. A nearby transmitter (system B) operating on a channel 2.4 MHz above system A
produced an on air third order 1PM which fell on the receive frequency of system A. Due to the 1PM
system A would remain on whilst system B was transmitting even when the mobiles of system A
ceased transmitting.
Case 14
An on air IMP was traced to a communal site. Work had recently taken place at the site on the mast
and in the equipment room. The problem was traced to an intermodulation product which was being
picked up by a feeder cable and radiated from the attached antenna. The feeder cable and antenna
were no longer being used at the site. When the feeder cable and antenna were removed the problem
was resolved. It should be noted that all unwanted equipment should be removed from mast structures
and equipment rooms.
Case 15
A fire brigade base station receiver was receiving Radio 3 programmes. The co-channel interference
was traced to a nearby communal base station site. The medium wave transmission was being picked
up on the antenna feeder, which provided a path to the power amplifier stage of a pmr transmitter
where the mixing occurred. The broadcast transmitter was several kilometres from the communal site.
The problem was solved by earthing the antenna feeder, the cable ducts and equipment racks.
Case 16
Use of multiple common base stations on a communal site exhibits the same problem as any talk
through stations, that is the generation of transmitter/receiver difference frequencies.
A classic case is that of two VHF high band talk through base stations having standard Tx/Rx spacing
(ie 4.8 MHz) in an urban environment where the mobiles of one system have regular access within 500
metres of the communal site, the transmitter not being fitted with an isolator. The level of the received
mobile frequency can be in the order of several millivolts, and mixing with its own transmitter produces
a different frequency equal to the Tx/Rx spacing. If now the second base station is keyed, its own
receiver will be disturbed by the resultant of the mix from the first base station, and so will any
subsequent VHF high band transmitters keyed on the site.
In many cases of this type the disturbance to the mobiles of the second system is that of an extremely
over-modulated signal carrying the modulation of the first base station.
The solution to this problem would be the fitting of suitable ferrite isolators and/or bandpass filters to
the offending transmitters.
Case 17
There have been numerous cases of moisture within antennas and feeder cables causing a
deterioration of transmission characteristics. In extreme cases severe corrosion takes place at the
lower connector of the feeder run, where the moisture gathers behind the connector. The source of the
problems are numerous, the main points of entry being:
a the drain holes in antennas and antenna structures
b the connectors and interfaces
c cracks and orifices in the outer casing of the antenna feeder cables
Due to atmospheric changes in temperature and pressure there is a tendency for feeder systems to
"breathe" and in extreme wind conditions, "VENTURI effect" may be produced sucking moisture into the
The solution to these problems are mainly inspection and maintenance, since complete abandonment of
drain holes in antennas can lead to other problems due to ingress of water in the upper part of the
antenna, and excessive use of sealants and wrapping of connector assemblies can produce adverse
effects of a secondary nature.
It is possible to provide "dnp loops" and examination points within the equipment room by removing a
short section of the outer casing of the feeder cable. This provides a means of detecting any moisture
that may be trapped between the outer coaxial tube and the casing.
The real solution is to provide annual inspection and regular maintenance of the system.
Case 18
Interference was being caused to a 141 MHz base station receiver whenever a co-sited 139 MHz base
station transmitter was keyed.
A bandpass filter was fitted to the receiver, which reduced the level of interference. However, the wanted
signal was still unworkable, even at this reduced level of interference. As a result of using a spectrum
analyser it was found that the receiver front end was oscillating at around 140 MHz. When the 139 MHz
transmitter was keyed, mixing occurred to produce a 3rd order IMP which fell on to the receiver channel.
The receiver was modified to prevent the front end oscillation and the interference problem was cured.
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