MLA - S (MMT) QRP v.3 + nanoVNA
Advantages of MLA-SMART can be best described as „ small, light-weight, low-cost, wide-band.“ Max usable RF power input: 10W max on all bands. Operating SW Bands: 80m, 60m, 40m, 30m, 20m, 17m, 15m, 12m, 10m, CB. Tripod not included.
MLA-S (MMT), the indoor magnetic loop antenna, is a new product of the Czech BTV company which has continued in MLA development for years. This is the first commercial MLA version built using the concept of patented magnetic loop antennas known as MLA-SMART. The antennas in this category differ from the earlier versions by a more complex architecture of the basic L/C resonant circuit. Classical MLAs have one or more loop turns made of a metal pipe to which an external resonant capacitor is connected. In MLA-ER design (by OK2ER) the capacitor is made as a quasi-virtual capacitor created by inserting a metal pad into the loop pipe cavity. This antenna has not become too popular despite its simplicity, mostly due to its demanding tuning procedure. The concept of MLA-SMART design improves the MLA-ER idea, while its complicated tuning is avoided. As the loop inductance in MLA-SMART a multi-layer conductor is utilized in the L/C circuit, in which the second layer is used as the virtual capacitor of the L/C circuit. The third layer can optionally be used as a capacitive coupling section of the antenna loop.
Advantages of MLA-SMART can be best described as „small, light-weight, low-cost, wide-band.“
So far there were few magnetic loop antennas on the market which would be capable to operate over the full SW frequency range, or even on 160-m band. One of such antennas came also from the BTV design workshop, and was named MLA-M (multi-band). Over seven years, some 700 of these unique two-turn MLAs were sold. The new MLA-S (MMT), with an added value, can fully replace the MLA-M. Its advantage is that at the top SW band segment, by avoiding the shorted second loop turn, MLA-S (MMT) has its efficiency improved ten times (10-dB difference).
MLA-S (MMT) antenna is manually tuned, and manually turned on a tripod. The standard 1/4“ thread in its box base fits to any robust camera tripod. See Figs.5. . The patented design of MLA-SMART L/C circuit allows to switch the antenna to any of three short-wave segments by one jumper, Fig.2. Over any of these segments the antenna can be continually tuned by a classic variable capacitor, Fig.3.
The short-wave segments selectable by MLA-MMT jumper are:
- 80 and 60 meters,
- 40 meters,
- 30 through 10 meters, see Fig.2 a,b,c.
For a correct MLA operation, two basic conditions must be met : a sophisticated design (manufacturer's task), correct tuning (user's task). The correct tuning includes also a correct impedance matching to a feeder. Contrary to other antennas, the new MLA-S (MMT) is provided with the „triangle match“ that allows to match the antenna with SWR close to 1:1. See fig.4.
Without meeting the last condition the antenna cannot radiate effectively as the complete antenna systém does not behave as a magnetic loop antenna but rather as a „coaxial antenna“ terminated into a mismatched „expensive L/C circuit“.
Many MLA manufacturers do not think about the second important condition, and their antennas cannot be perfectly matched. All BTV MLAs are provided with a configurable coupling devices, to optimize the radiation of the RF power from a transmitter. (Most often, by a rotatable FCL – Faraday Coupling Loop, or the „triangle match“.)
Operating SW Bands: 80m, 60m, 40m, 30m, 20m, 17m, 15m, 12m, 10m, CB
Usable RF power input: maximum 10W.
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.