Manual MLA-T Loop Antenna

Manual MLA-T Loop Antenna
MLA-T
Magnetic Loop Antenna - Top Bands
Instruction Manual
Thank you for purchasing this new product small Magnetic Loop
Antenna Top Bands. Manual contains important information.
Please read all instructions carefully before operating the antenna.
B PLUS TV a.s.
MLA-T
MLA-T
page 1
Description
The MLA-T Magnetic Loop Antenna for Top Bands is a "Plug & Play" product. It is primarily destined for use at
portable QTHs and can be operated with up to 100 W input *. The sophisticated design of the MLA-T offers
relatively high efficiency even with a relatively small loop diameter ( at 160 m band, the d/ l ratio is only
0.5%) while full-size magnetic loop antennas for 160 m band use a diameter around 4 meters.
By using several turns of a larger-diameter copper pipe, an extremely high Q was achieved; this allows a
high equivalent radiated power (related to antenna size) which is
a product of antenna size and loop current. A perfect impedance
matching of this antenna over all specified bands is achieved by a
user-adjustable “gamma match”, see Fig.1.
Contrary to other commercial MLAs which cannot vary
antenna input impedance, the tunable “gamma match” in the
MLA- allows to optimize the SWR also with respect to the ambient
situation of antenna location.
The MLA-T extends the selection of magnetic loop antennas
for the radio-amateur bands 1.8, 3.5 and 7 MHz and may offer a
solution for mobile hobbyists who want to transmit from a
portable QTH. In many locations where installing a long-wire
antenna is not allowed, like protected town sites, senior homes,
house boats and campings, the use of MLA-T may be the only
available option.
We would recommend to use the MLA/T in a digital-mode
operation, where even signals one cannot hear can be processed.
Against other phenomena of wave propagation, like ionospheric
attenuation, MLA-T loss is lower by several orders of magnitude.
In a practical on-band operation, the MLA-T is excellent
mainly in 80-m band. While the tuning is done remotely,
switching to another band must be done manually. The use of the
MLA-T in rain is limited. To prevent corrosion, the copper pipe is
fig. 1
protected with a special “Komaxite” varnish.
Technical design
Over a selected band, the remote tuning of MLA-T is done remotely by a 12VDC motor through a 1:600
gear, turning a variable capacitor. As the high Q causes an extremely narrow antenna selectivity (several kHz
typically), even this gear is too rough. To reduce the RPM, the pulse-width control(PWM) is used to achieve
the fine tuning with the full motor drive. Each motor start has a 3-second slow drive against the full-speed, so
the tuning is fast and precise. The up/down drive is controlled by two push-buttons, all other operations are
controlled by a uP and firmware in the control electronics. Three color LEDs indicate the tuning procedure.
The tuning motor is fed by the RF coaxial cable using DC bias tees on each end, so no other wire connections
are needed. To power the electronics, a standard wall-plug AC/DC adapter, 220V AC/ 12 VDC, 1A, is included.
The antenna is connected to the control box by one RF coaxial cable, 50 Ohms. The outdoor cable
connector is type N, sealed, while its indoor end uses a common type PL connector. Another (supplied) cable,
2m long, with PL connectors, is used to connect a transceiver to the control box.
While all components of the MLA-T meet the IP53 standard for environmental effects, it was observed
that during a heavy rain the efficiency is degraded. Only under a roof or covered with a plastic bag, MLA-T can
be used in rain under a full power and with a good efficiency.
The band switching of the MLA-T is done by mechanical jumpers located on antenna box, see Figs.2 and 3.
The complete manual band switching takes only a couple of seconds. Tuning over a selected band is then done
remotely by the described motor-driven variable capacitor.
We must emphasize that the precision MLA-T tuning is only possible with a SWR meter which is a standard
component of all modern transceivers. A precision tuning to resonance at a desired frequency is the important
physical condition of an efficient MLA operation. The MLA-T really is extremely selective an offset of several
kHz from the resonance point requires re-tuning, as otherwise a loss of more than 2S-units is to be expected.
The big advantage is that the antennas acts as very hi-Q preselector, highly attenuating out-of-band and even
in-band unwanted signals. Thus, RX intermodulation is dramatically reduced, and receiving performance is
greatly enhanced. Due to the varying L/C ratio over band, and the fact that at 7 MHz some of loop turns are
shorted, the Q values are not constant. The same fact also causes air breakdown in the HV capacitors, see *).
B PLUS TV a.s.
B PLUS TV a. s., Požárnická 140, 742 83 Klimkovice, Czech Republic
phone: +420 556 420 360, fax: +420 556 420 301
e-mail: [email protected], http://www.btv.cz
MLA-T
MLA-T
page 2
Band Switching
There are three manual switch settings:
1). 1.8 MHz Band The external jumper adds one parallel capacitor
to the circuit as shown in Fig.2. The jumper on loop turn is removed,
Fig.3. The “gamma match” has the longest length.
2) 3.5 MHz Band The external jumper, Fig.2, is off. The jumper on
loop turn, Fig.3, is off. The “gamma match” at its shortest length.
3) 7 MHz Band The external jumper, Fig.2, is off, the jumper on
loop turn Fig.3, is on. The “gamma match” is at its shortest length.
fig. 3
fig. 2
Operation
First interconnect the transceiver, the control box with its power supply, and the remote MLA-T. Upon
setting the desired band, see Band Switching, Figs. 2 and 3, and setting the transceiver to the same band,
adjust receiver gains so that a noise can be heard. While in reception, push UP or DOWN push-buttons on the
Control box, and wait till you hear a noise peak or some useful signal. The noise burst is typically quite short;
you can return the variable capacitor, or wait till it turns by 180 degrees. Therefore, either push the other
button or keep pushing the same. After several trials you can hear the strongest band noise; then stop tuning.
After this adjustment in reception, you can continue with transmission. Adjust TX output of ~10 W and try to
improve the tuning by the SWR meter. The goal should be as close as possible to the ideal of 1:1. Due to the
high loop Q, mainly at 80-m band, the training will take some time. When the best tuning by SWR is finished,
increase the TX power to ~100W. Caution: with high humidity, the
antenna may not be able to handle full 100 W on 80 meters in all cases.
In this case internal discharges (arcing over) could occur within the
capacitor. This is indicated by jumping SWR while the power is
increased, but does not damage the antenna. Back off to keep the
SWR low.
To those not familiar with magnetic loop antennas, here are some
important points:
The horizontal radiation pattern of the MLA-T is shaped like number
eight, with a wide maximum and a sharp minimum. This is only valid
for the antenna in a vertical position (its plane normal to the earth's
surface). The depth of the minimum is very much affected by the
ambient environment (conductors around, even within walls), type of
wave propagation, the state of the ionosphere, i.e. the angle of wave
incidence on the antenna, etc., etc.
The vertical radiation pattern, with the loop plan is normal to
ground, can be seen in Fig.4. Magnetic loop antennas located low
above ground are ideal for NVIS ("Near Vertical Incidence Skywave",
for short range HF communication) wave propagation. This particular
feature of a MLA allows to effectively use ionospheric reflections over
short distances. Mostly useful in mountains.
B PLUS TV a.s.
fig. 4
B PLUS TV a. s., Požárnická 140, 742 83 Klimkovice, Czech Republic
phone: +420 556 420 360, fax: +420 556 420 301
e-mail: [email protected], http://www.btv.cz
MLA-T
MLA-T
page 3
An important practical note:
The MLA-T antenna was adjusted as an indoor antenna in a real environment and its location in another
environment may change its parameters (Ra + jX). An optimum position of Gamma-Match bridge made by
the manufacturer is marked: the top (red) mark is for 160-meter band, the down mark (blue) is for 80m
(40m) band. It is recommended to readjust those positions if SWR does not go close to 1:1.
If we do not use to feed MLA-T by a coaxial cable of an exact electrical length of l/2, then MLA-T matching by
the variable Gamma-Match must be done quite carefully. We must keep in mind that a good SWR value at the
feeder end of an unknown length may not guarantee a good performance of the magnetic loop antenna (valid
generally, not only for the MLA-T). To get the best out of the antenna, in a particular environment the GammaMatch must be adjusted for an input impedance of (50 + j0) Ohms. Having fixed the mechanical position of the
antenna it is recommended to measure both impedance components (Ra + jX) directly on the antenna with a
suitable test instrument, and mark the optimum bridge position.
If the output impedance on MLA-T connector is not exactly 50 Ohms and without the imaginary component,
and if the feeder electrical length is not l/2, the ERP of the magnetic loop antenna may decrease substantially.
The important physical principle is to achieve that a maximum real current flows through the resonant LC
loop. This is only met with a well-matched feeder. In a mismatched condition the feeder cable becomes a part
of the antenna LC circuit, damaging antenna performance. This damage is quite significant!
If the user has no suitable test instrument to optimize the first MLA adjustment, then it is recommended to
run the first test with a reduced power (less than 10W), and locate a SWR meter as close to the antenna as
possible. (feeder ~1 meter long with MLA-T). If antenna impedance in resonance is adjusted to (50+j0)
Ohms, then cable length is no more critical. Only cable loss may count.
The BTV-made MLA-T has the advantage in that antenna impedance can be optimized in a real location that
may differ from the company environment where it was adjusted. Most of other commercial models have no
such option, and this fact may explain why user opinions about MLA use differ so widely. Good magnetic loop
antennas that respect the physics principles of their function are really unbelievably good. We at BTV needed
several years to understand the physics of MLAs; some other manufacturers may still need their time to
understand it.
CB4M, the remote tuning control box for magnetic loop antennas, MLA
CB4M is an electronic unit designed to remotely tune magnetic loop antennas. Pulse-width modulation
(PWM) is utilized to control rotation speed of a DC motor with a gear which drives the main tuning capacitor,
the part of L-C circuit in a MLA. The microprocessor-controlled electronic unit was optimized specifically for
the purpose of tuning the MLA. The process is sometimes named “spread tuning”.
Using a mechanical-only tuning system by designing special tuning capacitors is not economical nor
practical. When a MLA using a remote tuning was being designed, this electronic solution was found optimal
and economical, too.
When using the CB4M to tune magnetic loop antennas made by
BTV, one important advantage is that no extra control cable is neededthe PWM control signal travels along the common RF feeder cable.
CB4M is mounted in a plastic case of …cm dimensions. On the rear
panel there are two PL239 coaxial connectors, and one power jack, see
Fig.1. The electronics is powered from an external AC/DC power
adapter, 12V/1 A DC. (Included as accessory with MLA-T, MLA-B, MLAC, MLA-X all made by BTV Plus). The center pin in the jack connector is
positive.
The front panel carries two push-buttons, for left/right drive of the
DC motor. Motor (capacitor) speed and rotation sense are indicated by
fig. 5
two LEDs:. The center LED indicates the ON state and the correct
system function is indicated by its regular blinking.
CB4M is connected with the TRX by a coaxial cable with two PL
connectors. The other PL connector on CB4M is then connected to MLA feeder cable. If one uses more MLAs
switched by an antenna switch, then one CB4M can be used to tune all of them.
CB4M allows after a short training to tune fast and exactly a MLA to a desired frequency. If DC-only tuning
is used, and speed controlled by DC voltage, tuning would never be so easy and exact. Thanks to PWM and
mainly the sophisticated software, motor speed is gradually adjusted to four stages over time, in both
rotation senses.. As the tuning capacitor used in all ML-A-T, MLA-B, MLA-C, MLA-X has no mechanical stops,
then rotation sense is not important to tune in one direction; pushing button 1 does not mention the
frequency should be rising and button 2- frequency decreasing.
B PLUS TV a.s.
B PLUS TV a. s., Požárnická 140, 742 83 Klimkovice, Czech Republic
phone: +420 556 420 360, fax: +420 556 420 301
e-mail: [email protected], http://www.btv.cz
MLA-T
MLA-T
page 4
First speed stage: generates one <200 ms pulse upon pushing a button.
Second speed stage: generates a pulse series with X2 width upon pushing a button < 5 sec.
Third speed stage: generates a pulse series with X3 width upon pushing a button >5 and <10 sec.
Fourth speed range: generates a pulse series with X4 width upon holding a button >10 sec.
After CB4M is connected to a TRX and a MLA, and upon connecting
12 V C power, the center LED blinks with a ~1 Hz rate. (ready). Upon
pushing the left button, the left LED starts blinking, and vice versa. If
all is OK, then motor stepping can be heard from MLA box. Blinking rate
I only informative and does not correspond to rotation speed.
Then adjust RF Gain and AF Gain on TRX to hear noise in the audio
output. When the MLA tuning capacitor is turned by CB4M, noise
increase can be heard at some point. This noise peak may only be
fig. 6
heard for a short time, so returning back is advised, maybe repeatedly.
With more experience this process is easy and fast. As a good MLA is
only several kHz wide, tuning range can cover hundreds of kHz up to
several MHz.
After such pre-tuning with the RX, tuning with the TX is next . For an exact tuning use the SWR meter.
High-quality MLAs are so “sharp” that tuning 10 kHz off-resonance causes a 1-2 S drop.
The CB4M designed to tune MLAs does not need to be used only as an accessory to MLA-T, MLA-B MLA-C, or
MLA-X. It can be used anywhere (but possibly not in an optimum condition) a small DC motor should be
remotely controlled by PWM..
Important caution
The MLA-T can be used indoors with a maximum input of 10 W. With higher power, never use MLA-T as a
room antenna, with which you would want to run a round-the-clock contest! Limit your exposure to the RF
magnetic field to a necessary minimum. With more than 10W input, operator shouldn't be located near the
loop! The RF magnetic field passes also through walls. While the side effects of RF magnetic field component
have not been proven by science, stay safe and keep also others in a safe distance.
Do not touch the antenna loop under RF power, it can cause RF skin burns or even death. Expect EMC
problems to surrounding electronics as the usual screens are not efficient for high-intensity magnetic field
component. Individuals with pacemakers and similar implants should never approach a running MLA-T.
Technical parameters
Frequency range
1.8 to 1.95 MHz, and 3.3 to 4.0 MHz (7 MHz)
Input impedance
50 Ohm
Maximum RF Input
100W
SWR after Tuned
RF Connector at Control Box
1:1.1 max.
2x PL
RF Connector at MLA
N
Maximum Antenna Size
82 cm W, 105 cm H, 22 cm D
Antenna Weight
10 kg
Loop Diameter
800 mm
Conclusion
The MLA-T Magnetic Loop Antenna was developed following a marketing evaluation of the indoor MLA-M
type which was designed or QRP operation. Repeated requests and interest in a remotely tuned higher-power
antenna for Top Band operation stimulated the development of MLA-T. A reserved view of a MLA-T located in a
concrete-building basement indicates that such antenna cannot compete with a dipole stretched between
two such buildings high above ground. Nevertheless we believe there are many situations where the new
MLA-T can find its customers.
B PLUS TV a.s.
B PLUS TV a. s., Požárnická 140, 742 83 Klimkovice, Czech Republic
phone: +420 556 420 360, fax: +420 556 420 301
e-mail: [email protected], http://www.btv.cz
MLA-T
MLA-T
page 5
Measurement protocols
fig. 7
fig. 8
B PLUS TV a.s.
B PLUS TV a. s., Požárnická 140, 742 83 Klimkovice, Czech Republic
phone: +420 556 420 360, fax: +420 556 420 301
e-mail: [email protected], http://www.btv.cz
MLA-T
MLA-T
page 6
Measurement protocols
fig. 9
B PLUS TV a.s.
B PLUS TV a. s., Požárnická 140, 742 83 Klimkovice, Czech Republic
phone: +420 556 420 360, fax: +420 556 420 301
e-mail: [email protected], http://www.btv.cz
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