Peter, OK1DPX , gave me one MLA-S antenna for testing. This antenna was designed by OK2ER for a portable use on short-wave bands. The design is that of MLA-SMART series.
First, why a MLA? (Magnetic Loop Antenna). It is very suitable for short-wave bands as it is much smaller than full-size dipoles, verticals and directional antennas. One can locate it on a balcony or in a hotel room where it would not attract the attention of neighbors or property owners. Contrary to the opponents is not at all an „ersatz“. One can make interesting QSOs with it. It is quite compact, light-weight and easily transportable.
The MLA-S utilizes the innovative concept described in detail in . Using the coaxial cable sheath as well as the inner conductor, and a novel circuit, it can cover the short-wave bands from 80 to 10 m in three ranges. Range switching is easily done by a jumper on the box containing the tuning capacitor (Figs. 1, 2). The jumper is either open or shorted, or has a capacitor in it. The tuning capacitor is a commonly used „low-voltage“ type which allows to use up to 30W RF power into the antenna. I have not seen any other MLA with such a wide-band coverage.
Mechanical design is simple, robust and light-weight. It uses common components and a low-cost design to the best result. One can disassemble it into a backpack, and carry it easily with 0.83 kg mass. The main loop of MLA-S is made of a good quality RG214 coaxial cable, its diameter is 75 cm, and the ends are connected to the tuning capacitor box by PL connectors (Fig.2). To fold it for transport the cable loop may be made in a small ring. An accurate tuning is accomplished by a reduction gear 3 to 1 on the capacitor, and the scale has 50 divisions. The unfolded 75 cm dia.loop can be shaped to a circle by hanging it by two strings attached to the cable rings, and to ahother ring on the vertical column (Fig.3). The column and its end are composed of five plastic parts made on a 3D printer. These 5 plastic parts are fitted together, so the column is easily assembled and disassembled.
The main loop is fed by the Faraday loop (FCL) with a BNC connector attached to the screw-adjustable ring which is positioned in height on the column (Fig.3).
My comments to the mechnical design:
The MLA-S must be located at some minimum height above ground. How to make it:
- A camera tripod can be used (Fig.1). In the plastic column end block there is a 1/4“ threaded hole, up to 17 mm long. Camera tripods often have only short screws (e.g. 4 mm). The plastic end should also sit well on the tripod flat plate, otherwise the threaded hole of MLA-S may be damaged.
- The MLA-S may be hung head-down on a suitable tree branch like in Fig.4. This way it sits above ground while the tuning capacitor is easily reached by hand. Before hanging the MLA-S on a branch it is recommended to adjust a verified FCL position on the column (maybe you could be unable to reach it by hand). This location (2) has an advantage that a rain water would not get into the jumper sockets on tuning-capacitor box. Also cover the tuning-capacitor axle under the knob.
The position 2 may be tested also at home, e.g. on a chandelier (fig.5) if your family allows.
The MLA-S design is not, however, designed for antenna use when hanging. The column may disintegrate, so use thin pieces of paper in column joints. The antenna is very light so it could work.
- A minimum SWR adjustment is done by FCL height, tilt or turn against the main loop. The height and turn position can be secured by the set screw. I found it easier to adjust FCL tilt. I used a non-professional way to fix the position, see Fig.6.
Measuring Electrical Parameters
To measure SWR I used MFJ-259 B while MLA-S was located on a camera tripod inside a room in a panel apartment house, and as far as possible from all larger conductive objects like heaters, windows, refrigerator... I wanted to find out what are MLA-S frequency ranges, what a good SWR can be achieved, and what are band widths at individual ranges, for SWR limits of 1,5 and 2.0
- I can testify that on all band ranges I achieved SWR of 1: 1.0 with a suitable position of FCL. I did not turn it but tilted and adjusted its height. Over each one amateur SW band I did not need to change FCL position. FCL vertical position varied in height from -25 to +30 mm, the tilt from the main loop was 6 to 50 mm.
- Frequency coverage by tuning was as follows (3 full knob revolutions)
jumper min.frequency max.frequency short 3.364 Mhz 6.340 Mhz 100 pF capacitor 6.35 Mhz 11.44 Mhz open 9.965 Mhz 35.37 Mhz
- Maximum frequency shift from an adjusted frequency in each SW band
Center frequency [kHz] +/- kHz for SWR ≤ 1.5 +/- kHz for SWR ≤ 2.0 3530 3,2 6,3 7036 9 17 10112 9 18 14060 10 22 18100 13 24 21048 14 30 28060 18 41
- I have tested the reproducibility of tuning at 7 and 14 Mhz bands so that I marked the knob position for SWR 1:1.1, turned the knob by a full turn, moved the antenna a bit, then set the knob to the marked spot, then read the SWR. I was interested how the tuning I can reproduce and if small disturbance like changing loop shape would affect the tuning.
In the second test I moved the MLA-S from the room to house roof, and restored FCL position. I was nicely surprised to find that after the move I hit the earlier tuning to SWR 1.1.
My question was „If I také MLA-S and a transceiver on a hill, should I také MFJ-259B along“?
Testing in Operation
As the solar activity was low I could try to use MLA-S only at 40 and 20 meters (I had no comparison antenna for 80 m band). I compared signal strength from the MLA-S with that of another verified antenna. I used a mechanical switch to feed the Kenwood TS-570D or TS890S.
The level on S-meter was converted to dBm by earlier taken correction curves. For S more than 9 it not necessarily holds that 1S = 6 dB as required by IARU. I have mainly used strong stations with S 9 or more at the reference antenna. This also rejected interfering signals to an acceptable level.
I also tried to make QSOs. I set the power to 5W (exceptionally also to 20 or 30 W as later shown).
This way I tested that MLA-S survived the higher power without any damage.
- MLA located on a veranda covered with glass windows (Fig.7) The reference antenna was the Butternut HF9V vertical located on house roof. The veranda faces East. I communicated with stations located in the North-East to South-East sector. The MLA-S bottom on the tripod was 1.35 m above floor. On one side from the antenna was a metal frame of the glass window, on the other a plastic frame also lined with metal.
On 7 Mhz the antenna rotation did nor affect SWR. On 14 Mhz, the close frame and window allowed to get SWR 1.1 only by turning the loop some 30 degrees from an ideal direction.
On 7 Mhz I made 9 QSOs by telegraph mode, with 5W input, with Ukraine, European part of Russia, Rumania and Greece (1700 to 2300 km distant). In reception, the signal from MLA-S was 12 to 18 dB lower than from the reference antenna.
On 14 Mhz and with 5 W input I made 3 telegraph QSOs with European Russia, Bulgaria. The signal from MLA-S in reception was lower by 12...15 dB than from the reference antenna.
- MLA-S located on a flat house roof (Fig.8). The reference antenna was again the Butternut vertical HB9V on the same roof, over some 12 meters apart, MLA-S located on a tripod , 1.35 m above the roof, oriented East-West. On 14 meter band, 11 telegraph QSOs were made with 5 W input, with stations in Azores, Spain, Ceuta and Melilla, USA, Russia, Bulgaria (1800 to 6400 km distance). Signal received from MLA-S was 12 to 14 dB below that from the reference antenna. Later at night I made 2 telegraph QSOs with 30 W input, with Eastern USA stations. At that time the received signal from MLA-S was only 8 to 12 dB weaker than that from the reference antenna. MLA and vertical have different vertical radiation pattern, and certainly the propagation conditions changed (Distance some 6400 km).
- MLA-S in a garden (Fig.9). Here I used two reference antennas: one was my MLA design, with loop diameter of 1.3 m, for 7, 10 and 14 Mhz, with +/- 9 kHz band width for SWR 2. The other was an „inverted V“ dipole for 7 Mhz, with the top at 11 m height. MLA-S was again located on a tripod at 1.35 m height. Local conditions for 7 Mhz were at the test time not perfect for QRP operation, there was a lot of interference, so I made only one QSO to Italy, with 20 W SSB. I could, however, compare received 18 signals from the three antennas (stations in Italy, Slovenia, Netherland and Germany). At 7 Mhz, the signal from MLA-S was 12 to 24 dB weaker (average 15 dB) against the dipole. My larger MLA was 6 dB better than MLA-S at 7 Mhz, but it was 6 dB worse than MLA-S at 14 Mhz.
The MLA-S is a folding antenna, extremely light-weight and easily transportable. The cost for its small size and an increased resistance of the main loop due to the PL connectors used for an easy disassembly is a poorer performance than that of larger MLAs. On the other hand, after a proper tuning at some frequency it may be mis-tuned without any deterioration of SWR. This is an avantage in an outdoor operation with a manual tuning. The reproducibility of tuning is so good that after pre-tuning at home (tuning capacitor and jumper, FCL position) the user can go in the field without a precise SWR meter. Some fine tuning at the spot can be done only by a SWR indicator like in the FT817. MLA-S can be operated even hanging on a tree branch.
While MLA-S has been designed as a compromise to be used in QRP portable operations, it is also well suited for use on balconies. It has a small size almost clandestine, so nobody around is disturbed. The maximum input power of 30 W is quite high for such use.
Along with the authors of  I would like to point out that the antenna should be tuned under a low power, and stay away from it at 5W.
In a suitable location, MLA-S performed almost equally well on a open veranda as it did on a flat roof at 14 Mhz. Its performance cannot be equalled with full-size dipoles or directional antennas like Yagi. With its small size its versatility over the full SW range is fascinating as well as it is easy to switch to another frequency range.
Operation testing was done by telegraph (CW) mode on 7 and 14 Mhz bands, and the results were quite impressive (DX 6400 km). Those who favor digital modes with a high redundance (WSPR- now FT8) can use a low power and antenna small size to achieve communication over larger distances than I did by telegraph.
I wish all owners a lot of nice QSOs and good experience!
Jirka, OK1CJN, e-mail email@example.com
Translation: Jiří Polívka - OK1-5037
 O.Burger, M,Dvorský, Magnetic Loop Antennas, Slightly Different Each Time, V-th edition, Ostrava, April 2020, ISBN 978-80-270-6978-3