This is a short article about several MLA models designed by Olda OK2ER that I had a chance to try. I do not include any precise measurements, as I will leave this to the experts with proper equipment. Here I try to summarize my own (and sometimes subjective) experience with these interesting SW antennas.
When writing about MLA antennas I must start with a bit of my own history. In 2019, after a long break, I turned my attention back to the CB radio. However, meanwhile I moved to a new QTH, which is probably the worst case for SW work: large block of flats in Prague, with terrible QRM a no access to the roof. Standard antennas could only be somehow hung from the balcony and still they suffered from the QRM produced by hundreds of wifi routers and (worse) thousands of cheap chinese AC adapters and (probably the worst) the lift engines all around.
With MLA, I was hoping to solve the QRM and also the tight space requirements. I had some basic knowledge about magnetic loops and I also built some for listening only. However, transmitting antenna is a completely different story. I must admit that I even attempted to construct my own MLA according to some simple design found on the web, but I failed miserably. I lacked the necessary theoretical knowledge and measurement equipment, which these antennas definitely require.
So I decided to buy a factory-made MLA and soon I found that Olda OK2ER is a leading expert on MLAs for HAM (and also CB) use. I decided to order the MLA-CB model (now probably discontinued), which was just the model MLA-ER pretuned for the 11m band. The design is remarkably simple: the main loop is made from widely available copper pipe for plumbing purposes, the tuning capacitor is realized as a sliding stub plugged into both ends of the pipe; the tuning is therefore done by slowly shifting this stub in and out. The matching loop is realized just by a piece of solid copper wire. Remarkably simple design that can be easily duplicated, if you have the proper know-how. I'm sure a lot of time and experiments and measurements went into this design. For example, later I found that Olda purchased a thermal camera in order to track the energy losses in his antennas. The antenna, however, had a minor construction fault when arrived: the platform was not holding the loop strongly enough (fixed by some glue).
The antenna achieved what I was hoping for: to be able to work the CB band even in my local hellish QRM conditions. I received very good reports even in my conditions, that is 3rd floor out of 6, the MLA placed at the balcony facing the courtyard. The disadvantage of MLAs in general is that their tuning is extremely narrow. After tuning the antenna operates well in the range of say 20-30kHz. But it is possible to tune them extremely sharply almost to 1:1 SWR, which is extraordinary. The tuning of MLA-ER/MLA-CB is a delicate process: the tuning stub is extremely sensitive and moving it by 1mm causes change by 100kHz in the tuning. Therefore, precise tuning takes a lot of effort and trial-and-error. Moreover, the matching loop needs slight adjustment when retuning, in order to obtain correct impedance. I found that the matching loop shape that works best for me is a rather thin oval and the impedance is then adjusted by the diameter of this oval. Olda suggested to twist the loop into almost 90 degrees angle, but this did not work for me. This is maybe a weak point of the design: it is very hard to do it without an antenna analyzer. I use the NanoVNA which is perfect for this purpose.
When tuning for the first time, it took me about 15 minutes to obtain 1:1 SWR and 50Ohm, but soon I gained experience and was able to do the tuning in, say, 3 minutes. This mode of tuning is suitable for participating in local OM rounds but is unusable for contests and other events that require frequent retuning.
For local QSOs the vertical orientation of MLA is preferred. It it mildly directional, thus allowing me to partially filter out the worst sources of QRM. I also experimented with the horizontal orientation for some SSB DXing. During the band opening in summer I was able to work most of Europe with 12W SSB only. The horizontal position and thus horizontal polarization filters out a lot of local noise and still is OK with the regulations: directional antennas in horizontal polarization are forbidden in the CB band, but the small horizontal MLA is omnidirectional. I received very good reports and the stations did not believe I'm achieving this with only 12 watts. The only problem was that the platform of MLA-CB is not suitable for holding the antenna in horizontal positional and I had to find some way how to fix it.
Now is the time for my public "thank you" to Olda. Here in Czech Republic the telecommunication regulations are very benevolent, allowing unlicensed operators to use several so-called "shared frequencies" in almost every interesting VHF and up band, under some restrictions.
I thus felt little need to obtain a HAM license, of course being not able to work SW. But after seeing a way how to operate SW from my home QTH using MLAs, I decided to take the exams and I became OK3TV.
This means my operation range was greatly expanded. The MLA-CB (that is, just a MLA-ER model slightly pretuned to 11m by distance rings attached to the tuning stub) works also in the 10m band. I was able to compare the performance of this MLA with a 10m vertical (modified Sirio gainmaster) and there was only very little advantage for the vertical. The antenna is rated for 100W, I tested around 50W with no problem. Well, apart from various electronic devices failing due to the strong magnetic field.
Due to its lengthy tuning process, the MLA-ER is best suitable for narrow band digital modes like FT8 (which I don't do) or CW (I keep practicing, but I'm still very clumsy CW operator). I was looking for ways of a more fine tuning, which would eliminate the trial-and-error during the sensitive shifting of the stub capacitor. After writing to Olda OK2ER with some suggestions, Olda answered that he had already solved this by a remarkably clever trick: close to the main loop a revolving copper disc is attached on the screw-thread. By rotation the disc changes its distance to the loop which then changes the capacity by a small amount, thus allowing fine tuning.
I was promised that during one of my trips around Olda's home I can pay him a visit and for a bottle of wine, this disc is mine. And I really visited Olda, but my visit went completely different than I expected: we talked for several hours about many topics and I ended up not only with the tuning disc, but also a version of MLA-ER for horizontal mounting and also the crucial part of the MLA-S antenna: the high-voltage tuning capacitor.
The MLA-S is the antenna that I experiment with currently the most. The tuning capacitor is manufactured by (I believe) Slávek OK1TN and is designed to handle several kilovolts that appear on the main loop. I have a version that handles 10W from TRX, but I heard that Slávek designed a better model with much bigger power rating.
I made the main loop and the matching loop from RG213 cable, which is quite sturdy and when fixed on the top does not need a further support. The main loop is attached to the capacitor via standard PL connectors. With the right length of the loops, this MLA tunes from 80m to 10m. There is a jumper which can be plugged in to activate another circuit of the tuning capacitor. With no plug inserted the antenna tunes from 40m up, when shorted the antenna tunes at 80m and when plugged a correct capacitor, it tunes at 40m. I found that the angle of the main loop and the matching loop matters: on some bands the best tuning is achieved with zero angle, on some bands I had to attach the matching loop with 45 degree angle.
I paired the MLA-S with the FT-818 TRX. I must say that on the lower band the efficiency of MLA-S is not good. On has to be very lucky to make a QSO on 80m with 6W only. The antenna works best on 20m band and of course 10m, although I feel that MLA-ER performs slightly better on 10m.
Of course I compared the results of MLA-S with wire antennas like end-fed half-wave or windom, and of course these big antennas perform better. The exception is proabably 10m, where one benefits from the directionality of the MLA. But hey - compare the space and installation requirements of long wire antennas with this small MLA!
All in all, trying to make QRP contacts with MLA-S is an interesting sport and one really appreciates every QSO. I was able to work on SSB many, even very distant European countries on 20m (I was even heard in some expeditions pileups!) and some countries on 40m. I had only very few contacts on 80m. I'm also still waiting for a transcontinental QSO, but I hope this will improve when I find courage to work more CW.
The tuning works as follows. First I do a crude tuning to the desired band using NanoVNA and finding the right matching loop angle to optimize the impedance. Then I connect the TRX, select the frequency and keep turning the capacitor until the background noise increases. Then I shortly key and check SWR on the TRX display. When it is bad I slightly adjust the capacitor and repeat. The tuning could be finer, but it is usable, I'm able to retune in, say, 15 seconds.
The best thing on MLA-S is its transportability: when folded, the big loop from RG213 fits together with all other equipment in a very small backpack, which makes it a great companion for travelling. This antenna is also silently tolerated by my XYL - I really don't want to imagine what would happen if I attempted to install a longwire antenna during the vacation.
Currently, inspired by Olda, I'm experimenting with finding the right size of both the main loop and the matching loop to tune the MLA for the 6m band, where the efficiency should be very good.
My resume is that small MLAs are definitely worth trying for any HAM or even CBer. Although being compromise antennas with many drawbacks, I guarantee you will be surprised that such a small antenna can provide such results.
I tested the MLA-S (MMT) QRP antenna from my weekend QTH in Divice with power of 4W, using FT8. I had the antenna set up next to the kitchen window. Although I worked with little power, I managed to make some fairly decent connections in a short while, mostly on the first call. Connection distances (QRB) were about 300 to 1000 km. I made a connection with station 1200 km from my Prague apartment with 10W and an antenna set up on balcony on the second floor. I am pleasantly surprised by this, because according to some information from the internet, it is not possible to transmit with MLAs from apartment buildings, where steel reinforced concrete is used. I am glad that my MLA also works in Prague, where our house policy doesn't allow me to set up an antenna on the roof. Thanks to MLA-S (MMT), I can now be on air during the week.
I’m very happy to finally own an OK2ER MLA, and what’s more one of the new 2020 “SMART” series - SN 003, equipped with a FCL.
This antenna makes an extremely solid and well-built and professional impression. The 3D printed parts add much to this. The weight of around 1 kg is low compared to copper + large metal capacitor designs, but probably a bit heavier compared to coax-only “travelloops”.
Connected to my transceiver just on receive tuning the antenna felt very nice. No problems with finding bands or band changing. Sensitivity seemed excellent. Tuning is very smooth. The geared capacitor makes settings easy. Surprisingly, adjustments on the higher bands - where you would normally expect broader antenna bandwidths – are quite critical – one mm knob movement can make a difference. There is no hand effect whatsoever. This is UFB - many other manually tuned loops seem to suffer from this very annoying effect.
Band switching is fast and convenient with band settings of 80/60; 40/30, and 30-10m associated with the two plugins and open connection. A minor inconvenience is the location of the jumpers / C at the back of the unit. However, you quickly get used to it. The jumper holder on the central loop cable clip is a great idea.
The attached SWR charts were produced with my NanoVNA/VNASaver. It was amazing to find that SWR’s very close to 1:1 are easy to achieve with the coupling loop more or less in the same position. Tuning the antenna to resonance by ear lands you directly on or very near to the lowest SWR spot. Another half a second tx “tune” and perhaps tiny adjustment and you are set with essentially a 1:1 SWR. Bandwidth is very low on 80m (<10 kHz) - higher on the other bands of course; proof of the antenna’s high Q.
To me this is an outstandingly well designed, small and lightweight loop antenna considering it tunes from 80 to 10m. This evening, in absolutely miserable band conditions I was able to have a CW QSO with a Swiss station calling CQ with a 569 signal on 30m. The contact was difficult for both sides with several overs due to QSB. But he was running 100 W and copied my 5 W into the loop for a complete QSO! Amazing again. This is big fun of course.
While this antenna may perhaps not win any design prizes, it certainly works very well and is very stable.
My experience with MLA antennas.
I am using MLA-M with Elecraft K3S (80m, 10W, digital mode). This type is a very good for QRP on digital mode. Easy matching.
Also, I am using MLA-T with Kenwood TS-990S (15W, 160m digital mode, 80m digital mode and CW mode, 60m digital mode) more than one month without any problem. In my opinion, the matching with remote tuning control box CB4M is the best!
I took part in the MLA workshop organized by BTV Company and made my own MLA-ER antenna last month. Till now I used it only on 20m band with KX3 (5W). This antenna is very suitable for portable operation (SOTA, Flora-Fauna etc.).
MLA antennas are the best solution in case of limited space for building antennas. I plan to test this type of antenna on sailboat during my IOTA / IOCA activity from Croatian Islands next year.
The loop MLA-T PRO works fine. I've worked with loops before, so no big surprise. I know what loops can do! Made some good QSOs on 40 and 80 with the loop on the ground in the room. I still have to make QSOs on 160 and 60, but I'm sure that will be OK too. Remote tuning goes well. Changing bands is not difficult. After changing bands you'll be tune again. No big trouble, but a memory would be nice. So, I'm very pleased with this loop. Very good quality. Looking forward to using it in our new QTH. We are moving to a flat, so that's why I bought it. Also, I am very pleased with your service. Great!
To start evaluating this type of Magnetic Loop Antenna (MLA), we must first say that this is a „compromise antenna“. The fact is that we can get something by sacrificing something else.
The small size and an easy transportability, a nice mechanical design, simple assembly, easy mounting, this all would attract mainly mobile users. Making MLA-ER ready to operate is really easy and no skills are needed. Basic tuning (band switching) is quite easy by swapping capacitive segments. So far all sounds fine, but then...
To make any magnetic loop antenna operate at the peak of its theoretical performance (and not to have a nice technical artefact only), a MLA must (contrary to a piece of a wire that can operate always and anywhere) meet some additional physical conditions. Some are determined by a good technical design and technology used by the manufacturer, other conditions should be met by the user at the final location. The resonant-loop systém must be optimized (tuned), otherwise the promised and promoted features of the MLA-ER would never materialize. From my own experience I must say that using only a SWR meter to tune MLA-ER is bothersome, inaccurate and simply poor. A beginner would be certainly lost. I can confirm everyting what the author had written. The best for tuning is using an antenna analyzer, AA. Then the tuning can také less than a minute or a couple of minutes. A „professional“ may succeed even without an AA, but those are rare birds. They might know and require no instructions.
To conclude my evaluation of MLA-ER: This antenna is mainly suited for digital operations where it is not needed to tune over a larger frequency sector in a band. Certainly I would not recommend this antenna to beginners without an experience in antenna techniques. They can really be quite dismayed by a poor MLA tuning which is required to use all magnetic loop antennas.
Any tiny change in the vicinity like turning the loop by several degrees, or hanging wet laundry on a drying hanger close to the loop, can mistune the antenna. With an AA the problems are gone. The evaluated MLA-ER is a good and low-cost solution for many radio amateurs who cannot build their dream antenna. The MLA-ER can be also used in a situation which is not optimal, but this would be another story of thinking and using it.
The book is inspiring and I think is unique. Somewhere on the web I found a negative comment about it from someone that wanted over the counter recipes but the truth is that building a good MLA is not easy and it's not cheap either and involves understanding the concepts and experimentation.The section about the theory is very useful along with the references and the next section about the development of your commercial MLA's is pretty interesting specially for the use of the design with multiple turns. Please let me know if there is a new printing with additional information as I read on the site.
I have checked your products and I am impressed with the build quality. I'll keep an eye on the MLA-A as a candidate when I need one for portable use or for digital modes. I think that it is more elaborated compared to others, similarly priced, commercial MLA's. I will certainly recommend visiting your web site to others looking for these antennas and I like that you make it clear what to expect from an MLA.
Thanks again, best regards and best 73.
With regards to your great MLA book. Thanks,
Hi, I purchased a copy of your book at the ARRL headquarters the other day and found it very helpful and well -written.
I tested the MLA by skimmers on 30, 20, 17 and 15 meters. Evaluation: comparable reports to those with a LW, sometimes also better. As conditions were not good at 12 and 10 meters, I still have to try it there. Now I am installing 60m band in my ATS TRX, I would like to try the MLA also at my other QTH and on 60 m band.
I have to say that MLA-C works quite well for me on 40 and 60 meters but on 80 meters I do not have a large number of qso’s on CW, maybe because of my geographical qth (I only have land to east or north east and south, Europe and Africa) or maybe because of the low SFI. Nevertheless, I am very happy with the antenna and for the price and for the shape of the antenna (we do not need to install a mast with guides, for instance) it deserves what I paid for it. Thanks, Olda.
I examined it again and really I can only congratulate. With a completely invisible antenna in the room I can normally work on 80m. Tuning can be quickly drilled. Now I did on the first call and the EM7 G3 for 579 and 599 After my CQ also responded skimmers . All QSOs and test for skimmers was WKD on the my out 50W. I think, min. 3 db reserve to LP 100 W. So what can I say, good luck in JA and elsewhere. Super, my congratulation!!!
My QTH is on 9h floor in a 13-story building. My antenna sits on a chair near a window. I detected no change by locating it on my balcony. After a while of operating my station I became convinced that MLA is a genial invention of its kind. I normally believe only in things I can prove myself. Although I am an electronic engineer and a teacher at a high school of electronics, I do not trust too much antenna theories. First I want to try it, then I can evaluate it. My excitement of MLA-M was high and exceeded any expectation. The magnetic loop antenna is no chimaira but an excellent antenna, mostly suited for those having problems to locate suitable antennas on apartment blocks and similar situations.
Hello Oldrich, Very great job ! Thank 73
HI, I recently read the book Magnetic Loop Antenna slightly different each time and I am interested in your MLA-M with tuner loop antenna. Ihave built several versions of magnetic coupled loop antennas myself which have worked well but was very impressed with your design and the look of your workmanship.
I am using both MLA-M and MLA-T at a complex housing of a city. It is well known that most dwellers live in a restricted area here in overoccupied Tokyo. However, we do not want to give up operating with HF antennas at our veranda. So we must beg for small but efficient antennas, MLA-M and MLA-T. feel very happy to use MLA-T especially on top bands.
Magnetic loop represents totally different feeling of the air. Because of the nature of MLA, it uses primarily magnetic part of radio waves, performs with a very strictly defined directional effect and works as a frequency preselector. Thus, in compare with the common antennas, you can use those effects to almost eliminate the disturbing and jamming signals, focusing on the interesting weak ones. And you can use MLA for transmitting, too... The experience is great. I recommend it.
In addition to the generally known reasons WHY a MLA, I would offer more comments on MLA-M. The matching is perfect! On all bands it literally sucks out all TCVR power out, even without the need to use an antenna tuner. Very great during thunderstorms- I can use it indoors during a storm, when it also rejects QRN. A good reference antenna: given my limited land area, I cannot build more antennas next to each other, switch them and compare by skimmers. Thanks to MLA I can use it as a reference while I build other antennas and test them one after another. MLA is good due to its good match at all bands, I cannot afford another larger one as a reference.
A joke to finish: our grandsons were excited with Star Gate series. So they connect an old notebook to my MLA and start entering „addresses“. After I hear „seventh symbol insterted“, I can see toys and fluffy bears flying through MLA rings as they „fly“ to other planets... So the MLA entered as an indivisible part of our interior and my life style... D
I had an exciting QSO last night (23/07/2017, 0303 UTC) on 80 meters using the MLA-C. I worked RI1ANO in the South Shetland Islands of Antarctica on JT65. The distance and bearing from my location to that of RI1ANO is 12,288 km at 164 degrees. I have to admit that I’ve never thought such a thing to be possible for me on 80 meters and I’m still amazed by what happened.
What amazes me even further is that my MLA-C is on a north side balcony. It must be magic.
There are times that I wish for the MLA-C to work better than it does on 40 meters but I truly love the antenna on 60 and 80.
Dr Olda. I use aerial MLA cca 1.5 years. In first I use tcvr from MFJ who had 5 W output. Now I have tcvr KX3. I live in town Plzen where is a big QRM and QSOs with that antenna were nothing much. But on my weekend house - that was very success !! I had QSO with 10 watts -stations from Arizona and also from Florida. MLA is beautiful antenna for QRP QSOs! 73 and MLA forever!