MLA-ER/30-10
A Short-Wave Magnetic Loop Antenna for portable and SOTA activities

MLA-ER

MLA-ER (L)

Frequency range 10, 14, 18, 21, 24, 28 MHz
Input impedance 50 Ohms
Maximum Input Power 100 W
SWR under a correct tuning 1:1.0
Input Connector PL
Cable Impedance 50 Ohms
Antenna Diameter 120 cm
Antenna Weight 1 kg
Antenna Weight with a Pole 2,x kg

MLA-ER (H)

Frequency range 14, 18, 21, 24, 28 MHz
Input impedance 50 Ohms
Maximum Input Power 100 W
SWR under a correct tuning 1:1.0
Input Connector PL
Cable Impedance 50 Ohms
Antenna Diameter 80 cm
Antenna Weight 1.x kg
Antenna Weight with a Pole 2,x kg

This product should be understood as a kit designed for introductory experiments, and to be used for a hands-on introduction to function and efficiency of magnetic loop antennas.

MLA-ER is special among other BTV products by that it is an extremely simplified light-weight antenna. This kit is not designed to be used as a permanent heavy-duty outdoor antenna. Practical experience has shown that surprising results could be achieved with MLA-ER on HF bands. Its advantage is the light-weight design, still usable with a larger than QRP RF input. (100W max.) MLA-ER is ideal for digi-mode operations where it is not necessary to tune across a wider frequency spectrum.

The MLA-ER is a small-size compromise design of a short-wave magnetic loop antenna (by OK2ER), suitable mostly for portable activities such as an urban QTH. It can be used in locations where other gain antennas can not be used, such as a balcony, indoors, or from a loft.

(The un-protected MLA-ER cannot be used in rain. Water drops or water that has leaked into the loop cause a significant mis-tuning at a chosen frequency.)

This design is lightweight, is easily transportable, and (contrary to other antennas) it needs no ground connection or counterweight. In its price, frame the MLA-ER is probably the unique commercial MLA that can operate under a full 100 W power input available from commercial transceivers. The MLA-ER is also an ideal antenna for vacations as it can be run from a hotel room with a reduced input power.

Long-time evaluation by skimmers allows to claim that the MLA-ER achieved results comparable to antennas that need much more space to operate. These results prove that the MLA-ER is no longer a “compromise” design. The only addition can be seen in a more demanding operation.

Magnetic loop antennas, contrary to most other designs, are extremely narrow-band, and without precise tuning they cannot cover the full ham-band short-wave frequency range. Their high Q and selectivity, however, are mostly welcome in reception. The MLAs significantly reduce unwanted signals in the spectrum that otherwise cause intermodulation (interference due to virtual “noise”). The high directivity of a MLA allows the reduction of signals carried by surface waves, and a lower directivity for signals reflected by the ionosphere under larger angles. By this, the MLA can effectively reject local QRM which often limits HAM activities in urban QTHs.

The critical parameter, in addition to a precise tuning, is a good impedance matching. Other short-wave antennas allow for a sloppy matching, but with MLAs this is really critical. The MLA-ER therefore utilizes the FCC coupling loop (Faraday Coupling Coil), see Figs. 1a, 1b, which could move in several planes to achieve a perfect SWR (matching to 50 Ohms (jX = 0) in any QTH. This device allows The MLA-ER to be superior to any competing MLAs.

Band selection with the MLA-ER is made by preselected inserts (capacitance sections), see Fig.2, by which the loop is tuned just above a selected band. A capacitive disk on a screw then adjusts the loop capacitance to a lower frequency in the band. For details, see the user manual for the MLA-ER.

IMPORTANT NOTICE

The MLA-ER may be used indoors only with extreme caution. With input power exceeding 10 W, the MLA-ER is not recommended for indoor use: the first caution is that so far, no scientific study has been undertaken for health effects of strong RF magnetic fields (these fields have been used in medical practice). Long term exposure to the MLA magnetic field would not be good to one's health. Under a 100 W input, an operator should stay at least 5 meters from a MLA. Another problem is that an RF magnetic field passes freely through walls.
Common screening materials for LAN cabling (without using ferromagnetic materials) are not efficient enough for RF magnetic field, therefore, it is uncertain which electronics could be affected by it. An indoor antenna with 100 W input can block, reset or destroy electronics located nearby.
Do not touch the MLA under RF input power! It is not lethal, but it can leave unpleasant burns. Operators with implanted pacemakers should never operate magnetic loop antennas.

Making MLA-ER operational

Step 1 Frequency Tuning

To tune the MLA-ER to a desired frequency, Antenna Analyzer (AA) is best to use. With an AA the procedure can také a few minutes. Without an AA, the path to get an optimum adjustment of a MLA-ER more complex but not impossible. After some training with a MLA-ER, the time spent for an optimization will get shorter.

In the first step the MLA-ER should be tuned to close to the desired frequency without achieving the impedance matching. The selected capacitive pad should be inserted into the end of the resonant loop till the distance ring stop. The loop so becomes tuned to the highest possible frequency of the selected band defined by the size of the inserted capacitive pad. Utilizing the external movable capacitor, the user can fine-tune the resonant loop frequency towards a lower frequency.

Tune carefully, the response is sharp: within a selected band the external capacitor moves by millimeters only.

MLA-ER/30-10

To indicate the loop resonant frequency one can use a receiver (tuned to that frequency) connected to PL connector on the cable on the feed loop of the MLA. The proper tuning to a selected frequency is indicated by peaking noise in the band. After such pretuning the precise tuning can be done by following the SWR meter on the TRX. By fine tuning, a significant dip can be seen on SWR meter. Use a maximum RF power of less than 5W when tuning a MLA: the reason is not only a health concern but also reduced in-band interference .

Caution: the SWR meter must be connected to PL connector at the feed loop of the antenna, NOT at the feed cable end next to the transmitter!

If the dip (resonance) appears close to the desired frequency, go to the second step. If the resonance occurs far from the desired frequency, however, repeat the Step 1 till the close point is achieved.

Step 2 Impedance Matching

To obtain optimum parameters of a MLA, an optimum impedance matching must be achieved so the transmitted energy travels from a transmitter to a MLA without a standing wave on the feeder cable. The problem is not only the direct loss due to a mismatch (with other antennas acceptable even over SWR=2) but also due to that under a mismatch a MLA fails to act as a magnetic field radiator, and the feeder cable becomes a part of the loop. This is a critical point in using magnetic loop antennas, unfortunately often neglected.

In addition to the proven FCC as a loop coupling device, the author of the MLA-ER designed a new coupling method, „triangle match“. By varying the side length ratio (summit angle) one can well vary the impedance transformation between the coupling and main loop, and adjust a SWR optimum close to 1.0.

If during setting up a MLA-ER the triangle-match shape was not optimal in Step 1, we can change the side ratio by trial-error procedure, (with a minimum power input, please.)

Conclusion

Each expedited MLA-ER was individually and carefully tested on all bands, with the marked capacitive pads (10 MHz, 14 MHz, 18 MHz, 24 and 28 MHz), with 100 W RF input power.

Exceeding this maximum power can be critical and the main loop can be inadvertently damaged.

See Fig. Replacing the loop is not expensive but shipping cost may exceed its value. Use therefore a care mainly when starting the operation, and keep the power at a safe level.

When using the maximum power of 100 W (better from 50 W) please keep at least a 2 mm gap between the active loop ends and the distance ring, see Fig., otherwise

MLA-ER/30-10

The audio record is signal of station JH1HDT received in Czech Republic. CW signals have been received on MLA-ER/30-10 (L) situated 1 m above ground.- in centre of the historical town. Look also www.qrz.com (page OK2ER).

Photogallery

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