P.845-3 - HF field-strength measurement

P.845-3 - HF field-strength measurement
Rec. ITU-R P.845-3
1
RECOMMENDATION ITU-R P.845-3
HF FIELD-STRENGTH MEASUREMENT
(Question ITU-R 223/3)
(1992-1994-1995-1997)
Rec. ITU-R P.845-3
The ITU Radiocommunication Assembly,
considering
a)
that determination of the accuracy of HF field-strength prediction methods requires comparison of predicted
field strengths against measured field-strength data of sufficient accuracy;
b)
that accurate HF field-strength measurements are therefore indispensable for the effective use of the HF
spectrum,
recommends
1
that HF field-strength measurements conforming to Annex 1 should be continued systematically at various
locations in the world;
2
that, where possible, the standard measurement method described in Annex 2 should be applied to the
measurements;
3
that the field-strength data obtained from such measurements should be forwarded to the Director,
Radiocommunication Bureau (BR) to permit the development of a data base containing uniformly consistent
field-strength data.
ANNEX 1
Measurement of sky-wave signal intensities
at frequencies above 1.6 MHz
1
Introduction
Measurements of sky-wave signal intensities, if undertaken in a carefully controlled manner, are of value in assessing
the accuracy of methods for estimating field strength and transmission loss. Such measurements may also yield an
indication of sources of error in existing prediction methods and may be used either to improve these methods or as a
basis for developing new methods. Ideally, the requirements are for measurements to be carried out systematically over
as wide a range of conditions as possible at a series of frequencies over paths of different lengths in all regions of the
world. Measurements are needed at each hour of the day in the separate seasons and for different solar epochs.
While it is recognized that opportunities to make measurements for particular circuits often arise only incidentally, with
transmission schedules and system parameters such as the choice of antennas being determined by operational
considerations, nonetheless useful results can be obtained in such cases. However, it is evident that data have their
greatest value when measurements are carried out under standardized conditions and when uniform analysis and
tabulation procedures are followed. This Annex presents the desirable criteria to be adopted to the extent that other
constraints permit.
2
2
Rec. ITU-R P.845-3
Choice of circuits and periods of operation
Signal-intensity data are required from circuits of different ranges in all geographical regions. Recordings of a given
transmission should be made for as many hours as possible every day. The objective should be to derive the median and
other percentile values of the day-to-day distribution of signal intensity over all days of the month. Where it is not
feasible to carry out measurements every day, uncertainties arise in estimates of these values. Assuming a log-normal
law of variation with decile deviation from the median of D (dB), the standard error E in the median based on a sample
of N days within a month of 30 days (see Fig. 1) is:
1
1
N – 30
D
E = 1.28
dB
(1)
FIGURE 1
Standard error in monthly median (E)
as a function of number of days sampled (N) for different
decile deviations from the median (D)
15
E (dB)
10
5
D = 20 dB
10
15
5
0
1
10
20
N
30
0845-01
FIGURE 1/PI.845-1 [D01] = 10 cm
Clearly the standard error increases as the number of days of recording decreases. While there is no limiting sample size
giving an abrupt increase in error, as a general rule 10 or more measurements are required for the calculation of the
medians, 14 for the quartiles and 18 for the deciles.
It is seldom feasible to embark on a measurement programme extending over a significant part of a solar cycle but to
ease data interpretation and to be statistically meaningful, measurements should cover a minimum period of one year at a
given fixed frequency. There are particular advantages in attempting to record signals simultaneously over a path at a
series of different frequencies, both to aid the understanding of propagation effects and to permit quantitative data to be
obtained by night when maximum usable frequencies are low, as well as by day when there is much absorption at the
lower frequencies so that signals are masked by background noise.
3
Transmitter and transmitting antenna
The transmitter should be unambiguously identifiable so as to be sure that what is recorded is the wanted transmission
and not co-channel signals, adjacent channel signals, or interfering noise. It is useful if the signals are interrupted at
some periodic rate, say for 5 min once every hour, both as an aid to transmitter identification and to determine received
background levels as confirmation that there is no significant signal contamination. The transmitter should operate
Rec. ITU-R P.845-3
3
preferably for 24 h per day. It must be stable in both frequency and radiated power, and these two parameters must be
known accurately. For reception over short paths it should desirably have a radiated power in excess of 1 kW and over
medium distance and long paths a power of 10 kW or more. Where a special transmitter is operated, this would normally
radiate continuous waves, although other waveforms may be used to study the characteristics of individual propagation
modes. If use is made of commercial transmitters carrying modulated signals, it is important that the type of modulation
should be constant and the mean percentage modulation should not vary. Narrow-band transmissions (approximately
1 kHz bandwidth or less) or a narrow-band component of a composite signal are most appropriate to record. Wider
bandwidth signals are liable to interference contamination. Standard-frequency transmissions have been employed in the
past, but in many receiver locations there is now serious interference between signals from different transmitters
operating this service and sharing the same frequency. Nevertheless, interference can be avoided to some extent by
means of a narrow-band receiver capable of resolving the different audio modulation frequencies of each co-channel
transmitter. Transmitters for point-to-point telephony or telegraphy services offer the advantages of providing channels
which are relatively free from interference, and a detailed log of transmission schedules is usually available. On the other
hand, these transmitters often employ high-gain antennas, which tends to be a disadvantage.
A suitable category of transmitters meeting nearly all of the above criteria is weather-chart (FAX) transmitters using
frequency shift-keying (± 400 Hz). As there are numbers of receivers (ships) with unknown position, these transmitters
use omnidirectional antennas and transmit mostly for 24 h per day. Receiving systems should be very sensitive
especially when recordings are made for very long paths.
Inspection of the International Frequency List maintained by the BR is of value in the selection of suitable transmitters to
monitor. In particular, this usually gives information concerning transmitter radiated power, form of modulation and
period of the day of operation. Sometimes it also yields details of the antenna type and orientation. The International
Frequency List is useful additionally in providing a list of co-channel and adjacent-channel transmitters which should be
considered further to assess the likelihood of possible interference. However, before embarking on a programme of
systematic measurements it is recommended that after selecting a potentially suitable transmitter in this way, firstly a
series of monitoring measurements should be carried out at various times over a period of about a month to determine
the order of signal intensities encountered, the time coverage over which such signals can be detected, and the amounts
of interference experienced. Then, a direct approach should be made to the organization operating the transmitter to
verify the International Frequency List entries, to supply such additional details as are required – for example,
concerning the type of antenna used and the associated ground properties. In particular, it should be checked that the
radiated power is maintained constant, that different antennas are not used by night and day, and that the transmitter is
not part of a network of transmitters operating at the same frequency from geographically separated sites – a procedure
adopted in the HF broadcasting services in some countries. It is important to confirm also that it is proposed that the
transmitter will remain operational throughout the whole period for which it is intended to make measurements. Only
then should a decision be reached to carry out systematic recordings of the transmissions. Whilst ideally it would be
desirable to receive details of the transmitter log, noting in particular any malfunctions or temporary changes in technical
characteristics which might influence the measurements, it is rarely feasible to obtain such data and to apply variable
corrections to results in retrospect. Instead, every effort should be taken at the outset to avoid the monitoring of
transmitters whose characteristics are known to fluctuate.
For a particular transmitter to be suitable for signal-intensity measurements, the performance of its transmitting antenna
needs to be known accurately. Transmitters coupled to antennas with little directivity have advantages over those with
highly-directional antennas because radiation patterns usually approximate more closely to theory, because the relative
strengths at the receiving site of signals travelling via different modes are then determined mainly by propagation
effects, and because valid deductions may be made with a single allowance for transmitting-antenna gain in the absence
of a knowledge of wave launch directions. Unfortunately though, low-gain transmitting antennas are seldom used for
other applications. Most point-to-point HF land-fixed communication circuits employ high-gain rhombic or log-periodic
antennas; for sky-wave broadcasting, arrays of horizontal dipoles, also with significant directivity, are popular. The
exception is with standard-time transmitters which aim to provide all-round azimuthal coverage by means of vertical
half-wavelength dipoles. These transmissions are particularly suitable for monitoring purposes. Radiation patterns for a
vertical-dipole antenna may be estimated fairly accurately, except at low elevation angles where the particular ground
constants control signal intensities. However, even at low angles the performance is known more accurately than for
most other types of antenna. If no such transmitter is conveniently positioned for use, then before monitoring
transmissions from a directional antenna it should be checked that the great-circle path to the receiver does not involve
4
Rec. ITU-R P.845-3
reception of side-lobe signals. If propagation is over medium or long distances, ideally the antenna vertical polar
diagram for elevation angles less than 20° should approximate that of a reference short vertical radiator sited over
average ground (see Fig. 2a)).
Where a special transmitter is operated, a short vertical antenna is to be preferred. Alternatively, for short paths a
horizontal dipole aligned for broadside radiation along the great-circle direction may be used. For greater ranges
corresponding to low elevation angles, the direct and ground-reflected components of the sky wave nearly cancel one
another so that a horizontal antenna is very inefficient unless elevated to a great height and should be avoided.
Transmitting-antenna gain (like receiving-antenna gain) is best determined from near-site measurements in the far-field
region, but it is recognized that these rarely form part of the normal programme of work at a transmitting installation and
that it is not generally possible to be able to arrange for such measurements to be carried out at a remote location, not
under the control of the receiving organization. Accordingly, transmitting-antenna gain must usually be calculated from
theoretical relationships in terms of the known antenna geometry, and by making certain assumptions concerning the
type of ground involved.
4
Receiving antenna, receiver and recording techniques
Since existing methods of prediction of signal intensities do not take account of field distortion effects due to local
features at the receiving site such as undulating ground, obstacles like buildings and foliage and adjacent antennas which
act as re-radiating structures, it is important to site the receiving antenna so that these effects are kept to a minimum. The
ground should have a slope not exceeding 2° out to a distance of five wavelengths and no obstacles should subtend an
angle from the horizontal at the centre of the antenna in excess of 5°. The separation from other antennas should be not
less than ten times the antenna length.
It is more important that the receiving-antenna performance should be known accurately than that it should have high
gain. Except at the lower frequencies during the daytime when there is much ionospheric absorption, threshold levels for
signal detection will normally be determined by external noise intensities whatever receiving antenna is used. In general,
the greater the antenna gain, the more likely the possibility of error in assessing its performance. Accordingly, a short
vertical active antenna or a grounded vertical monopole antenna not exceeding a quarter wavelength high or a small loop
antenna are most appropriate to employ. The loop antenna would normally be aligned in a vertical plane containing the
great-circle direction to the transmitter. For long-distance paths where off-great circle propagation is likely to be
important, the vertical-monopole antenna is preferable since this provides omnidirectional azimuthal pick-up. If several
transmissions from different azimuths are recorded with one antenna, only a vertical antenna should be used. Some
organizations use vertical monopoles for signal measurements but standardize results by means of calibration data
involving comparisons for selected sample signals with the pick-up indicated by a portable “field-strength” meter
incorporating an integral loop-receiving antenna.
Figure 2a) shows the variation with elevation angle of the term E0 – E (a measure of the signal pick-up resulting from a
downcoming sky-wave of constant intensity and its associated ground-reflected wave, defined in § 6.2) for a short
vertical grounded monopole and a loop antenna, both situated over average ground. For elevation angles below
about 30° the monopole and loop antennas have very similar polar diagrams but at higher elevation angles the
loop-antenna pattern is preferable since the pick-up is relatively insensitive to angle. Figure 2b) shows the effect of
antenna siting over ground of different properties. Signal pick-up for wet ground exceeds that for very dry ground by
some 2-6 dB with the largest differences occurring at low elevation angles. The marked dependence of the pick-up on
the ground constants and on the elevation angle when this is low, which has been discussed already with regard to the
transmitting antenna, leads to particular data interpretation difficulties for long paths where elevation angles are not
known accurately. In principle, the use of an artificial ground screen would lead to a receiving system performance less
dependent on weather conditions which affect ground water content. The screen would improve the ground constants
and so increase the signal pick-up, but to be effective in this role it would need to have dimensions of the order of tens of
wavelengths and this is rarely practicable. On the other hand, short screens of length up to about five wavelengths can be
implemented and are of value in stabilizing antenna impedances to improve circuit matching. If a screen is used, it is
desirable to assess its effect by carrying out near-site calibration measurements with signals radiated in the far-field
region from an airborne transmitter.
Rec. ITU-R P.845-3
5
FIGURE 2
Difference between r.m.s. equivalent-incident field strength, E0, and r.m.s.
sky-wave field strength, E, for short vertical monopole
and for loop antenna at a height of 1 m. Frequency: 15 MHz
5
E 0 – E (dB)
0
–5
– 10
– 15
0
10
20
30
40
50
60
Elevation angle (degrees)
a) Monopole and loop sited over average ground of conductivity σ = 0.01 S/m,
relative dielectric constant ε = 15
monopole
loop
5
A
B
0
E 0 – E (dB)
C
–5
– 10
– 15
0
10
20
30
40
Elevation angle (degrees)
b) Loop sited over wet, average and very dry ground
Curves A: wet ground σ = 0.02 S/m, ε = 20
B: average ground σ = 0.01 S/m, ε = 15
C: very dry ground σ = 0.002 S/m, ε = 5
50
60
0845-02
FIGURE 2/PI.845...[D02] = 23.5 CM PAGE PLEINE
Horizontal half-wave dipoles for single-frequency operation or terminated dipoles for multiple-frequency measurements
are sometimes suitable for reception of signals on short paths. In particular, pick-up is not strongly dependent on the
ground constants. However, for medium distance and long paths when elevation angles are low, these antennas provide
6
Rec. ITU-R P.845-3
only limited pick-up, again markedly dependent on elevation angle, unless they are elevated to great heights. They
should not be used for these paths because of calibration difficulties.
Some organizations are equipped to make measurements using special antenna systems, such as rhombic arrays,
designed for specific circuits to improve signal/noise ratios and to enable measurements to be made under conditions
where a simple antenna would be unusable. It is difficult to interpret the results obtained on an extended antenna system
in the presence of a complex field built up of several waves incident at different angles, but measurements made with
such antennas may be acceptable for the purpose in hand, if they can be related consistently to those that would be
obtained at the same time on a standard antenna. In making a choice between antennas responding either to vertical or
horizontal polarization, it is prudent to check that, if propagation paths involve waves with markedly non-circular
polarization, reception (or transmission) is predominantly that of the stronger ordinary wave.
The receiving antenna should be connected to the receiver via a buried coaxial cable and appropriate matching circuitry.
This latter may take the form of a transformer or a wideband pre-amplifier. The receiver bandwidth should be as narrow
as possible consistent with the bandwidth of the transmitted signals, in order to optimize the received signal/noise ratio.
For continuous-wave signals and for the monitoring of the steady tone sideband signals of standard-time transmissions,
bandwidths of the order of 100 Hz or less are suggested.
Received signal intensity depends on radiated power within the receiver bandwidth. This is a function of the carrier,
modulation and recording arrangement. For a receiver bandwidth which encompasses the carrier and all sidebands, the
operative radiated power is equal to the sum of that of the carrier and all other components. Figures for different types of
modulation are given in Recommendation ITU-R SM.326. In the case of narrowband reception of a single sideband of a
standard time transmission of carrier power P where the amplitude modulation depth is m, the sideband power is m2P/4.
Signals should be detected, applied to appropriate integration-smoothing circuitry, and then recorded in suitable form.
Some organizations monitor signals over oblique paths in order to note the occurrence of events like sudden ionospheric
disturbances (SIDs) and magnetic storms, or to study fading statistics. In these cases, special recording procedures may
be necessary. Where, however, the prime requirements are to collect representative hourly signal-intensity data,
measurements are best made using a pen-chart recorder with a logarithmic amplitude scale (i.e. linear in decibels) and a
chart speed of about 2 cm per hour. The integration time constant should be about 20 s. This arrangement provides a
convenient length of record for manual smoothing whilst at the same time permitting the rejection of sections shown to
be contaminated by interfering signals or strong atmospherics. It is often simpler to record the automatic gain-control
voltage from a commercial receiver after modifications to equate and lengthen the rise and decay time constants to the
20 s noted above. However, this approach may lead to unacceptable errors under some conditions, even after
continuous-wave calibration of the response. Output voltage is usually approximately proportional to the logarithm of
the input voltage, but since this non-linearity is associated with the detection process and occurs prior to integration,
recordings give the mean logarithm of the signal intensity and not the mean in logarithmic units as required. These
quantities differ when there is signal fading present. An alternative acceptable form of recording involves digital
quantization of instantaneous amplitudes at a convenient sampling rate so as to cover the known periodicity of typical
fading components (with fading durations up to about 20 min). Representative values may then be determined by
computer processing. Apart from identification problems, the use of a computer to control the measuring receiver can
greatly accelerate and simplify both measurements and statistical analysis. It cannot be emphasized too strongly though
that with these techniques some form of regular check must be introduced to ensure that what is measured is the wanted
transmission.
Hourly figures each day are best expressed in the form of median values. With chart recording it is preferable to derive
the median directly as that amplitude which is exceeded for a total of half the recording duration (i.e. 30 min for hourly
medians). This procedure is independent of the chart amplitude scale. When a precise logarithmic amplitude scale is
used for recording, the median may alternatively be given approximately by two-thirds the chart deflection between the
quasi-minimum value (exceeded for Q% of the time, say, where Q ≥ 90%) and the quasi-maximum value (exceeded for
(100 – Q)% of the time), assuming that fading follows a Rayleigh distribution. With computer recording and processing
it is suggested in Annex 2 that a minimum of 12 independent samples are needed to produce representative hourly
median values. The samples should ideally be distributed uniformly throughout the hour, but if switched recording of
signals from several transmitters is required, groups of 4 samples within 4 min, repeated three times during the hour, are
acceptable.
Rec. ITU-R P.845-3
5
7
Calibration measurements
Pen-chart recorder deflections or computer-recorded data should be related to the associated voltages injected directly
into the receiver from a signal generator. Periodic calibration measurements are needed to express r.m.s. signal-generator
voltage readings in terms of the corresponding amplitudes of the recorded sky waves. Two approaches are possible. In
the one, cable, mis-match and coupling losses, together with antenna impedance measurements, are needed so that signal
data may be expressed as available receiver powers and associated field strengths. In the other, appropriate only to
reception of vertically-polarized wave components, a direct comparison is made with meter values indicated by a
portable “field-strength” measuring system incorporating a vertical loop antenna. In this case, it is important to be certain
what assumptions have been made in calibrating the meter and what field strengths are quoted (see § 6.2).
6
Conversion of measured data to mean available receiver power and r.m.s. sky-wave
field strength
The existing method of Recommendation ITU-R P.533 for estimating sky-wave signal intensities gives values of mean
available receiver power in the absence of receiving-system losses and r.m.s. sky-wave field strength. Hence, conversion
relationships are needed between the measured voltages developed across the receiver input terminals and these
quantities.
6.1
Mean available receiver power
The relationship between measured receiver input voltage when fed from a practical antenna and the available power
from an idealized lossless receiving antenna coupled to a matched load depends on the receiving-system losses and the
impedances of the antenna and receiver. In general, the receiving-system losses and the antenna impedance are
frequency-dependent factors. In particular, the relationship is not a function of the wave-arrival directions or
polarizations.
Consider first the idealized case of a lossless receiving antenna feeding a matched load.
Let
Pa : available power from receiving antenna (dBW)
V0 : r.m.s. voltage developed across matched load (dB(1 µV))
r:
antenna load resistance (Ω).
Then
Pa = V0 – 10 log r – 120
In particular for r = 50 Ω:
Pa = V0 – 137
dB
(2)
Now consider the practical case of an antenna coupled to a receiver via a feeder cable and a transformer or other
matching circuitry, but where some matching losses arise, r is then the load resistance presented by the receiver.
Let
Vr : r.m.s. voltage developed across receiver input terminals (dB(1 µV))
L:
cable loss (dB)
T:
mis-match and coupling losses (dB).
In general, the evaluation of T involves a knowledge of the antenna impedance. T includes losses in transformers and
other antenna-matching circuitry, and losses associated with the matching of the feeder cable to the receiver. Then
V0 = Vr + L + T
so that, from equation (2):
Pa = Vr + L + T – 137
dB
(3)
8
Rec. ITU-R P.845-3
Now for reception of fading sky-wave signals where Pa represents the mean available power (dBW) and Vm is the
hourly median receiver input voltage (dB(1 µV)), a fading allowance must be included in equation (3). Assuming
Rayleigh fading, the r.m.s. voltage is 1.6 dB greater than the median, so that:
Pa = Vm + L + T – 135.4
dB
(4)
Equation (4) may be used to relate measured values of Vm to Pa provided the various system losses are known. If it is not
possible to determine L and T as, for example, where calibration is by standardization with a portable field-strength
meter, then alternatively Pa may be given in terms of E, the r.m.s. sky-wave field strength (dB(µV/m)), when this is
known (see § 6.2), by
 λ2 

480 π2
Pa = E + Gr – 120 + 10 log 
= E + Gr – 20 log ƒ – 107.2
(5)
dB
λ is the wavelength (m) and f the frequency (MHz). Gr is the receiving antenna gain (expressed in decibels relative to an
isotropic radiator in free space) which, in particular, depends on wave-arrival direction. This direction is not usually
measured but must be predicted. Hence, this means of deriving Pa is less appropriate, since it does not lead to
independent data to test the accuracy of the predictions.
6.2
R.m.s. sky-wave field strength
Measured receiver input voltages may be expressed in terms of the corresponding voltages induced in the receiving
antenna, and thence as associated field strengths. In the case of simple configurations such as a vertical monopole
antenna and a broadside or end-on dipole or loop antenna responding to waves with a single (horizontal or vertical)
polarization, it is convenient to introduce the concept of an equivalent-incident field strength. This refers to a resultant
field with the same polarization as that to which the antenna responds. It may be regarded as the sum of a downcoming
sky wave and a ground-reflected wave. Portable commercial field-strength meters are usually calibrated to indicate
equivalent-incident field strength. On the other hand, for extended antennas composed of separate limbs with different
orientations, such as the horizontal rhombic antenna, the term “equivalent-incident field strength” has no physical
significance. The signal pick-up and the resultant field vary for the different limbs. In the case of the off-axis pick-up on
a simple antenna like a dipole or a loop antenna, the equivalent-incident field concept also is not particularly useful. The
antenna then responds to waves of elliptical polarization, and induced voltages depend not only on the wave strengths,
but also on the match between the wave polarizations and the polarizations to which the antenna responds for the
particular directions of incidence. Waves of different polarization and intensity incident from the same direction may
then produce the same induced voltage.
The relationship between equivalent-incident field strength and voltage induced in the receiving antenna is a function of
frequency, but unlike the corresponding relationship for sky-wave field strength, it is independent of wave-arrival
direction and ground constants. In both cases the conversion factor has the dimensions of length, so that where the
concept of an equivalent-incident field strength is meaningful, it is convenient to refer to two different antenna effective
lengths. Let lei be the effective length relating equivalent-incident field strength to antenna induced e.m.f., and les be the
effective length relating sky-wave field strength to antenna induced e.m.f. To compare equivalent-incident field strength
and sky-wave field strength, which is the same as to relate lei to an appropriate les, is usually complicated and involves
assumptions about the prevailing wave-field component amplitudes, polarizations and arrival angles; also a knowledge
of the antenna polar diagram is required.
Let
E0 : r.m.s. equivalent-incident field strength (dB(µV/m))
Vm : median voltage developed across receiver input terminals (dB(1 µV))
Rec. ITU-R P.845-3
9
and let lei be expressed in metres. Then again assuming Rayleigh fading:
E0 = Vm + L + T – 20 log (lei) + 7.6
dB
(6)
For a vertical monopole of physical length l (m)
lei =
λ
πl
tan
λ
2π
(7)
l
and in particular for l << λ, lei ∼
– 2.
For a small n-turn loop antenna of area A aligned in the plane of incidence:
lei =
2π n A
λ
(8)
Now if E is the r.m.s. sky-wave field strength (dB(µV/m)), and les is expressed in metres, then comparison with
equation (6) leads to:
E = Vm + L + T – 20 log (les) + 7.6
dB
(9)
But les is given in terms of the antenna gain relative to an isotropic antenna in free space gr and radiation resistance
ra (Ω) by:
2
les
=
gr λ2 ra
(10)
120 π2
so that with Gr = 10 log gr and f (MHz), E may be expressed alternatively as:
E = Vm + L + T – Gr – 10 log ra + 20 log ƒ – 11.2
dB
(11)
The difference between equivalent-incident field strength and sky-wave field strength is given from equations (6) to (11)
by equating values of Vm. This is shown in Fig. 2 for the cases of a short vertical grounded monopole and a loop
antenna.
6.3
Preferred signal-intensity parameter for comparison of measured data with predictions
It has been seen in § 6.1 and 6.2 that whereas measured data are related to available receiver power in terms of the
receiving-system parameters only, to express the measurements as corresponding sky-wave field strengths involves a
knowledge of wave-arrival directions and polarizations. Since these directions are not normally measured but must be
predicted, it follows that, where possible, the preferred signal-intensity parameter for the purpose of comparison
exercises with predictions is the available receiver power. Therefore, it is recommended that when a new HF field
strength measurement is made, the available receiver power Pa is derived directly from measured values of Vm by the
use of equation (4) in § 6.1. On the other hand, in order to derive Pa from the monthly median value of the r.m.s. field
strength E in the Data Bank D1, the following equation derived from both equations (4) and (11) should be applied:
Pa = E + Gr + 10 log ra – 20 log f – 124.2
7
dBW
(12)
Data-tabulation procedures
Four types of data-tabulation sheet may be prepared, depending on whether it is the wish of the organization undertaking
measurements to quote values in:
–
received power (dBW) (without correcting for receiving antenna effects);
–
receiver input voltage (dB(µV)) (without correcting for receiving antenna effects);
–
incident field strength (dB(µV/m)) (receiving antenna allowance ignores reflection of the downcoming sky-wave
near the receiving antenna);
–
sky-wave field strength (dB(µV/m)).
10
Rec. ITU-R P.845-3
Although received power would be the most suitable parameter, presently it is desirable to use incident field strength
because the receiving antenna allowance is probably better known at the receiving site.
7.1
Circuit and operating details
A sample sheet for the technical information needed is contained in Table 1. Tables 2 and 3 provide a brief amplification
of what is asked for in Table 1. The purpose of these three tables is that they be used by organizations undertaking HF
signal-amplitude measurements.
TABLE 1
Technical information sheet
ITU-R data bank of HF signal-amplitude measurements
This technical information sheet must accompany the hourly data sheets
Kind of information
Transmitter
Receiver
N/S
E/W
N/S
E/W
S/m
S/m
degrees
degrees
dBi
m
m
degrees
degrees
dBi
m
m
Location
Geographical latitude
Geographical longitude
Name and address of station and/or telephone, telex and telefax numbers
Ground conductivity
Ground permittivity
Antenna: type
polarization
horizontal beam
vertical beam
gain
length
height
groundscreen
number of elements
Frequency
Call sign
Class of modulation
Carrier power
Power at antenna feed point
Hours of operation
kHz
kW
kW
UTC
Source of information
Reliability of information
Receiver type
Recording time
Receiver bandwidth
min/h
kHz
median
mean r.m.s.
Hourly parameter reported (please tick which)
Units reported (please tick)
Received power
Receiver input voltage
Incident field strength
Sky-wave field strength
Data reduced to 1 kW
TX-antenna reduced to isotropic
Assumed elevation angle
dBW
dB(µV)
dB(µV/m)
dB(µV/m)
(yes/no)
(yes/no)
degrees
degrees
Rec. ITU-R P.845-3
7.2
11
Monthly tabulation sheets of signal-amplitude measurements
Table 4 contains a sample sheet for recording the hourly and monthly values of signal-amplitude measurements. Table 5
explains Table 4. Table 4 is required for each frequency and month, whereas Table 1 is only needed once for each
frequency.
The evaluation of count, upper and lower deciles, upper and lower quartiles and median is done by a computer or, if not
available, by means of the procedure described in Table 6.
TABLE 2
Explanation of the technical information sheet (Part 1)
ITU-R data bank of HF signal-amplitude measurements
Explanations of the technical information sheet
Location
Name of transmitter/receiver site
Geographical latitude
Latitude of transmitter/receiver (degrees and min)
Geographical longitude
Longitude of transmitter/receiver (degrees and min)
Name and address of station
Name of the organization that is responsible for the transmitter/receiver and
mailing address and/or telephone/telex/telefax number
Ground conductivity
At the transmitter/receiver site
Ground permittivity
At the transmitter/receiver site
Antenna
Type (dipole, log-periodic, curtain ...)
Horizontal or vertical polarization
Horizontal main beam direction (degrees)
Vertical main beam direction (degrees)
Gain (dBi) in the main lobe
Length (m)
Height (m)
Groundscreen, if any, provide details
Number of elements
Frequency
(kHz)
Call sign
If any
Class of modulation
Broadcast, rtty, fax ...
Carrier power
Nominal power of the transmitter (kW)
Power at antenna feed point
Carrier power minus cable and matching loss between transmitter and antenna
(kW)
Hours of operation
In UTC
Hourly median values should be expressed to the nearest 1 dB. Where, for some reason, values are believed only correct
within 2 to 4 dB, they should be accompanied by an appropriate descriptive letter and qualified as uncertain. If an
estimate to within 4 dB cannot be made, then only a descriptive letter and no numerical value should be quoted. The
following letters are to be used:
–
qualifying letters (preceding numerical values)
D : to indicate that the numerical value is a lower-limit value;
E : to indicate that the numerical value is an upper-limit value;
U : to indicate that the numerical value is uncertain;
–
descriptive letters (following numerical values or alone)
C : no measurement was carried out or was possible because of technical trouble;
S : measurements influenced or impossible because of interference or atmospherics.
12
Rec. ITU-R P.845-3
TABLE 3
Explanation of the technical information sheet (Part 2)
ITU-R data bank of HF signal-amplitude measurements
Explanations of the technical information sheet
Source of information
Where is the information about the transmitter from?
Reliability of information
Subjective judge of the reliability of the above information. Most reliable is direct
information from the transmitting station. Rather unreliable e.g. is information from
the International Frequency List
Receiver type
Manufacturer, type of receiver
Recording time
How many minutes per hour are recorded to derive the hourly value?
Receiver bandwidth
(kHz)
Hourly parameter recorded
Are the measured data, median, mean or r.m.s. values?
Units reported
Are the values measured received power (dBW) (without correction for receiving
antenna effects) or receiver input voltage (dB(µV)) (without correction for receiving
antenna effects) or incident field strength (dB(µV/m)) (receiving antenna allowance
ignores reflection of the downcoming sky-wave near the receiving antenna) or
sky-wave field strength (dB(µV/m))?
Data reduced to 1 kW
Are the measured data reduced to 1 kW radiated power?
TX-antenna reduced to isotropic
Are the measured values reduced to an isotropic transmitting antenna?
Assumed elevation angle
How was the elevation angle of the incoming wave calculated or assumed, when the
transmitting antenna gain was deduced or when the measured data were converted
to sky-wave field strength?
Medians, quartiles and deciles should not be quoted if these figures are less than 2 dB above the received background
noise intensity.
If more than half of all possible daily numerical values within a month are missing or qualified for any reason, then the
median, the quartile and the decile values should be qualified by U and be accompanied by the relevant descriptive letter
which is that appropriate to the majority of the missing and qualified data. When the median, quartile or decile is only
between 2 and 4 dB greater than the noise background, it should also be qualified by U and described by S.
7.3
Tabulation of data for computer processing
If signal-intensity measurements are made with the aid of a computerized system, the data exchange should be in
accordance with the procedure described in Table 7.
TABLE 4
Sample sheet for hourly and monthly signal-amplitude measurements
ITU-R data bank of HF signal-amplitude measurements
Day
Circuit:
Frequency:
kHz
Month:
Year:
Time UTC 0030- 0130- 0230- 0330- 0430- 0530- 0630- 0730- 0830- 0930- 1030- 1130- 1230- 1330- 1430- 1530- 1630- 1730- 1830- 1930- 2030- 2130- 2230- 23300130 0230 0330 0430 0530 0630 0730 0830 0930 1030 1130 1230 1330 1430 1530 1630 1730 1830 1930 2030 2130 2230 2330 0030
E: receiver input below recording threshold
C: no measurement carried out or technical trouble
D: receiver input above recording maximum
S: interference
U: uncertain value
13
Qualifying letters:
Descriptive letters:
Rec. ITU-R P.845-3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Count
Upper decile
Upper quartile
Median
Lower quartile
Lower decile
14
Rec. ITU-R P.845-3
TABLE 5
Explanations for the hourly and monthly signal-amplitude measurements
ITU-R data bank of HF signal-amplitude measurements
Explanation of the tabular sheet for the hourly measurements
Circuit
Name of transmitting/receiving station
Hourly values
For continuous transmissions, the average signal from 30 min before the hour to 30 min after the
hour is to be taken. If a transmission lasts 60 min, from one hour to the next full hour, it will be
split into two 30-min periods and two hourly values shall be entered into the table
For hours where there was no measurement the letter C shall be entered into the table
For hours where there was a measurement but this is judged to be influenced by interference or
atmospherics the letter S shall be entered into the table
For hours where there was a measurement but the wanted signal was below the recording
threshold, that threshold and the letter E shall be entered into the table
For hours where there was a measurement but the wanted signal was above the recording
maximum, that maximum and the letter D shall be entered into the table
For hours where there was a measurement but the wanted signal was uncertain due to any reason,
the measured value and the letter U shall be entered into the table
All daily and monthly summary amplitudes shall be quoted only to the nearest integer decibel
Count
The number of hours with numerical values, including those qualified by D, E and U, is the
“count”
Deciles, quartiles and
medians
All measured values should be sorted in order of reducing amplitude. D-values have to appear at
the upper end and E-values at the lower end. Deciles, quartiles and medians are yielded according
to Table 6
Rec. ITU-R P.845-3
15
TABLE 6
Table for deriving deciles, quartiles and medians
ITU-R data bank of HF signal-amplitude measurements
Table for deriving deciles, quartiles and medians
In the table below, the procedure how to calculate deciles, quartiles and medians is given.
If the sorted table contains 31 values, the 4th from the top is the upper decile, half of the sum of the 8th and 9th value is the upper
quartile and the 16th value is the median.
If the sorted table contains 30 values, the upper decile is taken as 9 times the 4th value plus the 3rd value divided by ten etc.
Upper deciles and quartiles are counted by starting at the upper end of the table. Lower deciles and quartiles are counted by starting
at the lower end of the table.
Values to be used in the calculations
Count
Decile
Quartile
Median
31
4th
(8th + 9th)/2
16th
30
(9*4th + 3rd)/10
(3*8th + 9th)/4
(16th + 15th)/2
29
(4*4th + 3rd)/5
8th
15th
28
(7*4th + 3*3rd)/10
(3*8th + 7th)/4
(15th + 14th)/2
27
(3*4th + 2*3rd)/5
(7th + 8th)/2
14th
26
(3rd + 4th)/2
(3*7th + 8th)/4
(14th + 13th)/2
25
(2*4th + 3*3rd)/5
7th
13th
24
(3*4th + 7*3rd)/10
(3*7th + 6th)/4
(13th + 12th)/2
23
(4th + 4*3rd)/5
(6th + 7th)/2
12th
22
(4th + 9*3rd)/10
(3*6th + 7th)/4
(12th + 11th)/2
21
3rd
6th
11th
20
(9*3rd + 2nd)/10
(3*6th + 5th)/4
(11th + 10th)/2
19
(4*3rd + 2nd)/5
(5th + 6th)/2
10th
18
(7*3rd + 3*2nd)/10
(3*5th + 6th)/4
(10th + 9th)/2
17
5th
9th
16
(3*5th + 4th)/4
(9th + 8th)/2
15
(4th + 5th)/2
8th
14
(3*4th + 5th)/4
(8th + 7th)/2
13
7th
12
(7th + 6th)/2
11
6th
10
(6th + 5th)/2
16
Rec. ITU-R P.845-3
TABLE 7
Explanation of the computerized data-exchange sheet
ITU-R data bank of HF signal-amplitude measurements
Explanations of computerized data exchange
Technical information must also accompany the measurements if the data exchange is by
computer. The daily and monthly measured values, however, will have the following format:
Columns 1-3
Circuit and frequency code
Column 5
1:
2:
3:
4:
5:
6:
daily values
upper deciles
upper quartiles
medians
lower quartiles
lower deciles
Column 7
1:
2:
3:
median
mean
r.m.s.
Column 8
1:
2:
3:
4:
available receiver power (dBW)
receiver input voltage (dB(µV))
incident field strength (dB(µV/m))
sky-wave field strength (dB(µV/m))
Columns 10-11
Year (two digits)
Columns 13-14
Month (two digits)
Column 16
1 = 01-12 UTC
2 = 13-24 UTC
Columns 18-19
Day of month
Remaining columns from 21 to 80: 5 columns each for 12 h
Column 1:
Column 2:
Column 3:
Column 4:
Column 5:
qualifying letter (or space if none)
data value – hundreds
data value – tens
data value – units
descriptive letter (or space if none)
Data should be written on diskettes (3½ in MS-DOS format).
ANNEX 2
HF field-strength measurement technique – Specifications
for a field-strength measurement campaign intended
for future improvements in prediction methods
1
Introduction
Major difficulties arise in HF field-strength measurements in the positive identification of transmissions and in the
characteristics of equipment and antennas available for those measurements. Thus it is recommended that a network of
new computer-controlled transmitters and receivers be installed with the characteristics described in this Annex. These
arrangements will minimize the uncertainties, ensuring data of high quality, and should permit a large number of
moderately priced receiving installations to be provided around the world.
Rec. ITU-R P.845-3
2
17
The HF field-strength measurement campaign
The required data cannot be obtained unless new transmitters and receivers are installed. It is recommended that these
transmitters should be frequency agile, each radiating on up to five frequencies sequentially according to a
predetermined schedule. In each case there should be a period of steady signal emission for manual measurements and
coded sequences permitting aural identification of the source, computer identification of the source and computer
evaluation of the intensities of both the signal and the background noise or interference.
To minimize the possibility of interference, the transmissions should be on assigned frequencies probably in the fixed
service bands.
A number of receiver implementations are possible, meeting the specification given below, but a single standard for the
receiving antenna is recommended. Each receiver should operate on a rapid measuring sequence, dwelling on each
frequency for up to 12 s, and should be able to measure signal levels on many of the transmitted frequencies during each
hour. At least one model of receiver, including the interface, computer and software, is available and it is hoped that
some administrations will provide some equipments on loan to those developing countries where there is a great need for
new measurement data.
3
Transmitter
3.1
Transmitter locations
In order to collect data representative of HF propagation conditions on a worldwide scale, there should preferably be at
least nine transmitters. These should be located in the Northern and Southern Hemispheres at middle latitudes and in the
tropics in Regions 1, 2 and 3 (see Table 8). It should be noted however that useful results can be obtained from a more
limited number of transmitters should the distribution proposed in Table 8 not become available.
TABLE 8
Region 1
Region 2
Region 3
Northern Hemisphere
North/Central Europe
North America
Asia
Tropics
Tropical Africa
Caribbean Area
South-East Asia
Southern Hemisphere
Southern Africa
South America
Australasia
3.2
Transmitter power
Transmitter power should be in the range of 5-10 kW.
3.3
Transmitting antennas
Transmitting antennas should be omnidirectional in azimuth and should have a broad elevation pattern. In order to avoid
the necessity for frequent switching of high RF powers, a single broadband monopole or conical structure is preferable.
However, such antennas usually have a changing vertical radiation pattern (vrp) for frequencies greater than about three
times the lowest design frequency, which is when the antenna height is approximately λ /4, and the characteristics of
particular proposed antennas should be studied carefully before implementation. The criterion for the vrp is that at the
higher operating frequencies the pattern should not depart by more than 3 dB from the vrp of a λ /2 monopole (the
mid-band case) over the range of elevation angles up to 65°. A possibility is to select a broadband antenna which is
operating at about three times its lowest design frequency at the highest operating frequency and that operation on the
lowest operating frequency is permitted by the use of a frequency selective matching network.
18
Rec. ITU-R P.845-3
The antenna should be installed on flat ground of no more than 2° slope and should be located on an unobstructed site
with obstacles of not higher than 4° elevation.
3.4
Frequencies and transmitter schedules
Transmissions from one location should take place in five frequency bands on frequencies, preferably in the fixed
service bands, near 5.5, 8, 11, 15 and 20 MHz. The emissions on each frequency should be radiated sequentially
commencing at 0, 4, 8, 12 . . . etc. minutes after each hour. The frequency schedule should be communicated to the
Director, BR, for dissemination to all participants in the measurement campaign.
Should more than five transmitters operate simultaneously, more than one frequency will be needed in all bands if
mutual interference is to be avoided. Six to 10 transmitters require two frequencies in each band, 11 to 14 require three
per band, etc.; the number required for n transmitters is | (n – 1)/5 | + 1. Table 9 shows a schedule for nine transmitters
that use two frequencies, 1 and 2, in each band A to E.
TABLE 9
Frequency schedule for nine transmitters
Min
0

T
r
a
n
s
m
i
t
t
e
r
3.5
4

8

12

16

20

1
A1
B1
C1
D1
E1
2
B1
C1
D1
E1
A2
3
C1
D1
E1
A2
B2
4
D1
E1
A2
B2
C2
5
E1
A2
B2
C2
D2
6
A2
B2
C2
D2
E2
7
B2
C2
D2
E2
A1
8
C2
D2
E2
A1
B1
9
D2
E2
A1
B1
C1
The transmitted signal
3.5.1
The emission should be of class F1B with a frequency shift of 850 Hz. In accordance with
Recommendation ITU-R F.246 the “mark” should be on the lower frequency i.e. 425 Hz below the assigned frequency.
NOTE 1 – If the emission is generated using suppressed carrier single-sideband techniques then it may be convenient to
use, for example, as a reference frequency, a suppressed carrier frequency 1 225 Hz below the assigned frequency, in
conjunction with upper sideband modulation frequencies of 800 Hz for the “mark” frequency and 1 650 Hz for the
“space” frequency.
3.5.2
The transmitted sequence on each frequency should be as described below having a duration of 12 s. The
sequence should be repeated for 4 min, curtailing the final repetitions as necessary so as to allow time for the transmitter
frequency change:
–
FSK preamble at 100 bit/s for 1 s, comprising signal reversals commencing on the mark frequency;
–
a pause of 50 ms;
Rec. ITU-R P.845-3
–
19
an identification signal in Morse code on the higher frequency (see Recommendation ITU-R F.246) contained
within a period not exceeding 3.3 s:
–
the ratios of lengths within the Morse code sequence shall be in the following ratios:
–
dot element length
1 unit
–
dash element length
3 units
–
space between elements within a character
1 unit
–
space between characters of the call sign
3 units
–
the length of the unit for the dot element length, etc., shall be a greatest multiple of 100 ms, so that, to the
greatest extent possible, the call sign fills the 3.3 s time available; the call sign shall commence at the
beginning of the available period, with any unused time at the end of the period;
–
if permissible this identification signal could with advantage be based on the international locator code used by
radio amateurs, provided that the signal can be contained within the period specified;
–
a pause of 50 ms;
–
a 256-bit complementary sequence transmitted at 1 200 bit/s as follows:
Sequence 1:
1110010000101000000101001101100000011011001010001110101111011000000110
1111010111000101001101100011100100110101111110101111011000000110111101
0111111010110010011100011011001010001110101111011000111001000010100011
1010110010011111100100110101111110101111011000
–
a pause of 50 ms;
–
a second 256-bit complementary sequence transmitted at 1 200 bit/s, as follows:
Sequence 2:
000110111101011111101011001001111110010011010 1110001010000100111000110
1111010111000101001101100011100100110101111110101111011000111001000010
1000000101001101100011100100110101110001010000100111111001000010100011
1010110010011111100100110101111110101111011000
–
a pause of 50 ms;
–
a series of FSK reversals, comprising 273 bits at 100 bit/s, commencing on the lower frequency;
–
a 127 bit Gold code identifier sequence, at 100 bit/s, commencing on the lower frequency. The sequences to be used
by each transmitting station taking part in the campaign may be obtained from the BR. The Bureau will also advise
receiving stations of the sequences in use;
–
a constant signal on the higher frequency for a duration of at least 3 s, which should continue until the total time for
the sequence is 12 s.
NOTE 2 – The automatic identifier takes the form of a Gold code, length 127 bits, obtained using the following 7 stage
shift-register generator algorithm:
s1 = x7 + x3 + x
s2 = x7 + x3 + x2 + x + 1
the sequences successively assigned to transmitter stations should be in the order:
s1 ; s2 ; (s1 + s2); (s1 + τ · s2); ......(s1 + τ126 · s2)
where τn represents a cyclic shift of n places.
3.6
Log-keeping
Detailed logs containing information on the status of the transmitter and particularly on the transmitter power should be
completed at all sites and should be forwarded periodically to the Director, BR.
20
Rec. ITU-R P.845-3
4
Receiver
4.1
Receiver locations
There should be as many receiving stations for the recording of the nine proposed transmitters as possible. Due to the
null in the elevation pattern of vertical transmitting antennas, measurements should not be made at ranges shorter than
about 500 km.
4.2
Antennas and siting
Short vertical active antennas are recommended for use with field-strength measurements according to the specifications
given in Appendix 1. The antennas should be installed on flat ground of no more than 2° slope and should be located on
an unobstructed site of at least 25 m radius, with more distant obstacles not higher than 4° elevation, measured at ground
level. The antenna should be situated away from electric power lines and other metallic structures. The antenna may be
protected against damage by a wooden fence or stockade not higher than 2 m at a distance of at least 3 m. It is preferable
that the site should be in an area of good and homogeneous ground conductivity; in any case the conductivity needs to be
known for use in the analysis of results.
4.3
Receiver specifications and calibration
The field-strength measuring receiver should meet the following minimum performance requirements:
–
synthesizer control (10 Hz step);
–
external bus available for computer control;
–
frequency accuracy ±1 part in 106;
–
synthesizer noise sidebands: reciprocal mixing performance better than 70 dB in a bandwidth of 3 kHz at 20 kHz
offset;
–
sensitivity SSB 1.0 µV terminated for at least 10 dB (S + N ) / N for a bandwidth of 3 kHz;
–
unwanted spurious responses (e.g. image, IF) better than 70 dB;
–
selectivity: approximately 3 kHz bandwidth, shape factor (– 60 dB to – 6 dB) 2:1;
–
linearity: 3rd-order intercept, 20 kHz spacing, +10 dBm;
–
true average measuring capability within 4 s;
–
timing accuracy within the receiver system should be maintained within 1 s.
4.4
Receiver measurement sequence
Within the 12 s code sequence transmitted, only 4 s are required for computer measurement, allowing sufficient
tolerance for timing differences.
Section 5 indicates that 12 samples are required to establish a median signal level within a 1 h interval. Thus it is
sufficient to measure 4 samples within 4 min, repeating three times in the hour using the 20 min cycle of transmission. In
this way measurements can be accommodated on 5 frequencies within 4 min and on 25 frequencies within 1 h. The
sequence is illustrated in Fig. 3.
Organizations operating only one receiver may select up to 25 transmissions for measurement from the potentially
available 45 transmissions in order to meet the above requirement. Alternatively, two receivers can be operated in
parallel, to monitor more than 25 transmissions. Advice on the selection of suitable transmissions should be available
from the “international group” as recommended in § 6.
4.5
Recording units and calibration
Use of a standardized receiving antenna and receiving system permits a common conversion to field strengths to be
applied for all sites during data analysis. Measured amplitudes should be expressed in terms of corresponding receiver
input voltages by injecting a reference calibration signal ideally once each day. This should be of fixed amplitude giving
Rec. ITU-R P.845-3
21
approximately full-scale receiver output. The strength of the calibration signal should be noted in the station log together
with a record for any input attenuation used and the receiver output level should be recorded on floppy disk.
FIGURE 3
Schedules and formats
Transmitter schedule
f1
f2
f3
f4
f5
0
f1
f2
f3
f4
f5
20
f1
60 min
40
Receiver schedule
T1
T2
T3
T4
0
T5
T1
T2
T3
T4
T5
1
f ’s 1
0
T2
T3
T4
T5
2
f ’s 2
4
T1
T2
T3
T4
f ’s 4
12
T5
T1
4 min
3
f ’s 3
8
T1
f ’s 5
f ’s 1
16
20
Gold code
Steady tone
24 min
Signal format
1.2 kbit/s sequences
FSK
Morse code
identification
0
FSK
5
10
s
0845-03
FIGURE 3/PI.845 [D03] = 16,5 cm
5
Signal and background noise measurement and data processing
This section describes the basic procedures to be applied to yield signal amplitudes for inclusion in a future ITU-R data
bank. The transmitter signal format (§ 3.5.2) is designed to also permit additional measurements to be carried out for
other types of propagation studies if desired.
22
5.1
Rec. ITU-R P.845-3
Sampling
For each selected circuit (transmitter/receiver/frequency combination) the receiver records sufficient amplitude samples
within each hour to permit statistically meaningful estimates of the hourly medians of both the signal and noise
intensities to be determined in relation to the likely within-an-hour fading. Using either analogue or digital signal
processing, average voltage amplitudes over a period of 4 s within each 12 s transmission cycle should first be
determined. A minimum of 12 such samples within a given hour on a given day should be taken, these to be distributed
approximately uniformly over the period that the transmitter is radiating. This results in a standard error due to sampling
of less than 2 dB.
5.2
Input attenuation
To allow for the varying signal amplitude on different circuits and at different times of day, it is suggested that a
switched attenuator under computer control may be inserted between antenna and receiver, values of attenuation to be
recorded on floppy disk along with the signal and noise data.
5.3
Data recording
Data should be dumped to floppy disk in an agreed format (Appendix 2). Hourly medians are to be calculated centrally.
Rules for dealing with samples in which the signal is not discernable from noise and interference are given in
Appendix 3.
5.4
Determination of monthly medians and deciles
Monthly medians and upper and lower decile values of signal amplitude should be calculated by combining together the
hourly median signal-amplitude data at a given hour for the different days. Rules for dealing with daily samples in which
the signal is below the noise are given in Annex 1.
5.5
Data normalization
Data provided from the separate receiving sites should be accompanied by corresponding receiver and antenna system
calibration factors for each circuit. Using this information together with that from the transmitter logs, centralized data
normalization will be applied to the different daily/hourly values for incorporation in the data bank.
6
Data handling, quality assurance and training
The scheme envisages a large number of receiving locations (some operating in areas where there is a severe shortage of
skilled engineers), with a large quantity of data, probably in floppy-disk format, being sent to a central agency for
compilation and processing. Such a scheme will work only if adequate attention is given to organization and training.
It is recommended that an international group should be set up to oversee the measurement programme and to provide
guidance and assistance to participants. It is further recommended that ITU support should be provided to receive and
process the data in consultation with the group.
The necessary tasks are to:
–
collect and validate measured data provided from the different receiving sites;
–
create and maintain summary files of daily hourly median field strengths;
–
normalize these results in accordance with the known transmitting and receiving system performance, generate a
definitive data bank, and
–
arrange training workshops in various parts of the world so as to ensure the satisfactory operation of receiving
systems.
It is recommended that all data recorded by such equipment be forwarded to the Director, BR, to facilitate the
development of a new ITU-R data base of field-strength measurements.
Rec. ITU-R P.845-3
23
APPENDIX 1
Specifications for a vertical antenna as a field probe
for field-strength measurements
1
General outline
FIGURE 4a
General outline of active antenna
Rod
Amplifier
Metallic support mast
Non-conducting tube
Ferrite
}
see § 4, Appendix 1
50 Ω cable
Ground ( σ, ε r )
0845-04a
FIGURE 4a/PI.845 [D04] = 8,5 cm
2
Geometrical structure
FIGURE 4b
Geometrical structure of active antenna
hr
dr
p
hm
dm
hr = 1 m (rod)
dr = 5 to 10 mm
dm = about 5 cm (tube)
hm = 1 m (tube)
p: plastic housing for shielded amplifier
o: outlet for 50 Ω cable
g: non-conducting tube
o
g
0845-04b
FIGURE 4b/PI.845 [D05] = 8,5 cm
24
Rec. ITU-R P.845-3
3
Amplifier of active antenna
3.1
Basic schematic
For example an FET source follower and bipolar emitter follower could be used. The d.c. power could be fed through
the inner conductor of the coaxial cable.
FIGURE 5
Basic schematic of active antenna
Rod
Rod
Vi
50 Ω
50 Ω
Ci
50 Ω
Vi
Metallic support mast
R out
Vout
Non-conducting tube
Vout
0845-05
FIGURE 5/PI.845 [D06] = 9.5 cm
3.2
Electric characteristics of active antenna
–
Amplification:
^ – 6 dB
Vout / Vi ≈ 1/2 =
–
Input impedance:
Ci = 15 pF
–
Output resistance:
Rout = 50 Ω, VSWR ≤ 2
–
Noise figure measured at output:
^ 11.8 dB
Fout ≤ 15 =
(Available noise power measured at the output of the shielded antenna or at the output of the amplifier when loaded
with CA = 10 pF at the input)
–
Linearity range (signal level for 1 dB compression):
Vcomp, out ≥ 1,2 V (at a load of 50 Ω)
measured with dummy antenna
–
Intermodulation characteristics:
Intercept point 2nd order, measured at the output:
IPOP2 ≥ 50 V (ou 47 dBm) (with dummy antenna)
Intercept point 3rd order, measured at the output:
IPOP3 ≥ 5 V (or 27 dBm) (with dummy antenna)
The intercept points to be measured by the method of two signal generators.
–
Dummy antenna for measurements on the amplifier
Rec. ITU-R P.845-3
25
FIGURE 6
CA = 10 pF
50 Ω
Amplifier
0845-06
FIGURE 6/PI.845 [D07] = 5 cm
4
Antenna cable
–
Characteristic impedance: Zc = 50
–
Suppression of currents on the outer conductor by ferrite rings
FIGURE 7
Plastic surface
of cable
10 mm
5.6 mm
df ~ 0.5 m
6 mm
Ferrite
df
0845-07
FIGURE 7/PI.845 [D08] = 5 cm
–
Type of ferrite: A1 = 3 000 to 5 000 nH/turn2
NOTE 1 – The arrangement shown above (ferrite beads spaced along the full length of the coaxial feeder) is convenient
for temporary installations (e.g. site selection). For permanent installation fit 50 type-73 beads (do not use
type-77 beads) on the antenna end of feeder coax, and use low loss buried cable to route the signal to the signal strength
measuring instrument. The equivalent circuit of 50 beads on type-RG 58 coaxial cable (27 cm of cable fitted with
50 ferrite beads) is an unbalanced-to-unbalanced isolation transformer.
5
Lightning protection
The amplifier should be protected against discharge by means of diodes and a spark gap at the input.
The antenna should withstand a transient electric field of:
dE
dt ≥ 500
kV/(m · s)
26
Rec. ITU-R P.845-3
APPENDIX 2
Recording of signal and noise amplitudes
and related information
1
Introduction
Individual 4 s average signal and noise amplitudes are to be recorded on diskettes. Whilst this will lead to the need for
greater subsequent data processing by the international team than if hourly median amplitudes were recorded it will
avoid the requirement for off-line data analysis at remote sites and will permit changes in analysis procedures to be
introduced if subsequently found necessary.
2
General data organization
The signal and noise amplitudes will be stored in individual files, each covering a period of one hour.
Calibration information will constitute a separate file.
There will also be a general description file that contains information like disk titles, etc.
Data having predetermined values will be coded and organized into tables in order to save space and facilitate some
verification throughout the process.
3
Tables
3.1
Receiver table
Each record contains:
–
–
Receiver numeric code
Maximum value 65 536 (binary)
Receiver name
20 characters
Expected number of records:
3.2
2 bytes
20 bytes
-------
Transmitter table
Each record contains:
–
–
–
Transmitter numeric code
Maximum value 65 536 (binary)
2 bytes
Transmitter call sign
5 characters
5 bytes
Transmitter name
20 characters
20 bytes
Expected number of records:
-------
3.3
Frequency table
Each record contains:
–
Frequency (binary)
centre frequency quoted to the nearest 100 Hz
Expected number of records:
2 bytes
25
Rec. ITU-R P.845-3
4
File formats
4.1
General description file
27
Only one record that contains:
–
Title
containing the string
32 characters
“SG 3 HF measurements”
–
Remarks
(for incorporation of general pertinent information)
4.2
50 characters
Calibration file
Each record contains:
–
Date and time of calibration (binary)
–
One calibration signal for all 25 frequencies
4 bytes
1 byte for each frequency
25 bytes
Total
29 bytes
The number of records depends on the number of calibrations that take place during the weekly period (there is one
record per calibration).
4.3
Measurements file (for each hour and circuit)
–
Date and time (binary)
4 bytes
–
Transmitter code (binary)
2 bytes
–
Frequency identification
1 byte
(index of the frequency tables)
–
Input attenuation (dB)
–
Signal and noise values 12 times (binary)
–
Signal amplitude
1 byte
–
Signal qualifying or descriptive letter
1 byte
–
Noise amplitude
1 byte
Total
5
1 byte
44 bytes
Diskettes organization
Each diskette will contain data for a period of one week (7 days) consisting of the following:
–
1 general description file
–
1 calibration file
–
168 hourly signal and noise files.
6
Storage considerations
3 ½ in diskettes should be used to collect the data. The data corresponding to one week of recording can be
accommodated on one diskette.
28
7
Rec. ITU-R P.845-3
Remarks
–
All receiving stations should register their activity with the international coordination group and be given an
identification code.
–
Date and time – a binary number (4 bytes) that contains the elapsed seconds since 00:00:00 UT, 1 January 1970,
according to the system clock and permits calculation of the actual date and time.
–
Qualifying letters are given in Annex 1.
–
Rules for qualifying numerical values as uncertain are given in Annex 1.
–
Signal and noise amplitudes will be recorded as integer values.
APPENDIX 3
Rules for determining hourly median signal amplitudes
For a particular circuit there will normally be 12 signal amplitude samples within a given hour on a given day. If all
these samples have unqualified numerical values the median is readily determined by ranking them in ascending order
and taking the average of the two middle numbers.
However, complications arise when some sample values are qualified or are accompanied or replaced by descriptive
letters. The following qualifying and descriptive letters are proposed (see also Annex 1):
D : to indicate that the numerical value is lower than or equal to the true value (as for example when exceeding
receiver full scale)
E : to indicate that the numerical value is greater than or equal to the true value (below the composite background
of receiver noise, atmospheric noise and man-made noise and interference)
U : uncertain. A numerical value is classified as uncertain when it is not believed accurate to within ± (2 to 4 dB)
C : no measurement was possible because of technical trouble
S : measurement influenced by or impossible because of interference or atmospherics.
Every measured signal amplitude is accompanied by an associated amplitude of the composite background with which it
is compared in selecting letters E and U.
Cases of C or S do not contribute to the signal sample count but numerical values accompanied by qualifying letters do.
When there are some data values accompanied by qualifying letters a first trial median is determined ignoring all
qualifying letters. If then it is found that all values accompanied by D are greater than this median and all accompanied
by E are less than this median, then the first trial median is the final median. Otherwise a second trial median is produced
by moving all E values to the bottom of the list and all D values to the top. The final median is then the average of the
first and second trial medians. If these differ by more than 2 dB then the final median is qualified by U.
If more than half of the signal samples are qualified by E (or D) the median is determined in the normal way and is also
qualified by E (or D).
_________________
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