Description
Technical design
B P L U S T V a . s .
B PLUS TV a. s., Po
ž
árnická 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
page 1
MLA-T
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/ 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
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 *).
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
protected with a special “Komaxite” varnish.
fig.
1
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