G8JNJ Reverse Engineers and Reviews the MLA-30 HF Loop Antenna

Last month we posted a collection of reviews about the MLA-30 which is a budget magnetic loop antenna designed for receiving HF signals. The overall consensus from the reviews was that it worked decently for the price, but of course could never live up to the high end loops that cost hundreds of dollars.

Recently Martin (G8JNJ) reverse engineered the active circuit used on the loop from photos taken by M0LMK and has made some observations on it's performance, noting that it's design isn't very good. First he notes that the amplifier chip is a Texas TL592B two stage video amplifier which isn't that great for this application. His measurements show an OIP3 of 20dBm, a P1 saturation of -3dBm and a noise figure of 12dB.

Of interest, he explains that the creator of this loop has designed it poorly as the impedance match of the loop to low pass filter is very wrong, resulting in a very poor amplitude/frequency response. He shows how the response can be improved with a few termination resistors, but is still not great.

MLA-30 Frequency Response. Ideally should be flat.
MLA-30 Frequency Response. Ideally should be flat.

If you're interested in a cheap magnetic loop antenna, Martin suggests DIYing the M0AYF design which he says works a lot better.

We note that the "YouLoop" design is also in the works as a product that will apparently sell at close to manufacturing cost. The YouLoop is a passive loop idea by the creator of the Airspy that consists only of a simple 1:1 transformer and coax cable as the loop. It works best with high sensitivity radios like the HF+ Discovery.

12 comments

    • Martin - G8JNJ

      I wouldn’t bother trying to modify it, it’s not worth the effort.

      You have to remove the potting compound to get to the circuit board and it is highly likely that other components will get damaged in the process.

      Just enjoy using it as it is.

      • none

        I see. There are some easy Mods like put a serious Antenna Cable onto and or change the Bias-T.
        So when no other mods are easy to process i stick with them.

        • Martin - G8JNJ

          I wouldn’t bother changing the coax as the cable losses are negligible and the loop amplifier defines the overall noise figure and Signal to Noise ratio.

          The existing bias tee is adequate, as the bias tee choke inside the amplifier is the main constraint with respect to the overall frequency response.

          There is one modification that is relatively easy and worth doing.

          That is to add a 1000uF capacitor across C2 in the circuit diagram.

          http://www.g8jnj.net/MLA30%20Biasing%20Tee%20Outline.png

          This dramatically reduces the level of 60 and 120KHz noise from the 5v to 12v DC-DC switching convertor that is present at a fairly high level on the RF output.

  1. Ren

    Sorry but the proof is in the usage. Throughout the night this guys A/B tested both loops and the MLA-30 at least at his location was quieter. Both loops were mounted in the same location at the same height.
    https://www.youtube.com/watch?v=oX4ZzDTdbnQ

    This article reads more like an ad for AirSpy, which I’ve now grown accustomed to seeing here disguised as content. Technical details of the MLA-30 are fine but most people want to know how it performs. Better quality components did not make the W6LVP or Wellbrook perform 10 times better as the price would suggest. It is this type of elitism which hurts the hobby and provides a psychological barrier for some people who think they need a loop costing hundreds of dollars when they don’t. Sure, “make your own” you say. Not everyone is technically able to or interested in making their own. There should be cheaper options for them and the MLA-30 is a cheaper option which performs as good and sometimes better than the Wellbrook and W6LVP loops costing 10x as much.

    RTL-SDR started out as being a place for people to experiment with cheap, readily accessible tuners. The MLA-30 fits that spirit more than boutique antennas costing $500.

    • admin

      It can’t hurt for someone to analyze the circuit and point out the flaws for us to improve upon at home. Some reviewers did already point out the non-flat freq response. Pointing out the flaws is also how the RTL-SDR went from a DVB-T tuner with lots of issues that needed to be fixed for many projects, to a relatively stable radio product that can just be used without mods.

      It’s not possible to generalize performance based on one guys A/B test. Maybe the wellbrook had too much gain for his radio, maybe the loop size wasn’t optimal for the freqs he was looking at, maybe the rad pattern of the loops were slightly different with the MLA-30 just happening to be better in his location, maybe the lower sensitivity of the MLA-30 prevented SDR overload. I’d still say the MLA-30 is a good loop for the money, but ignoring the design flaws doesn’t make sense. But yeah, it’s still going to work well for people as it’s a loop, and loops generally work better than other antennas in noisy suburban environments.

      This blog is not affiliated with Airspy or other companies… On this blog we just talk about Airspy, SDRplay, HackRF etc products too since they’re also good low cost SDRs (albeit at the next price level).

      That YouLoop is i’m guessing going to be dirt cheap since it’s just a single transformer. However as it’s passive I don’t know how it would compare against the MLA-30 on non-low NF radios. Maybe a YouLoop + cheap HF amp is going to be better and cheaper than the MLA-30. Just pointing it out so people can have all the options.

    • Martin - G8JNJ

      Hi All,

      I have built and tested a lot of active antennas and I can assure you that the MLA-30 is not as good as more expensive designs such as the Wellbrook.

      However for many folks, especially those using RTL dongles in direct sampling mode, it may be good enough.

      In the Youtube video comparisons were made between two antennas and the comment made was that the signal strengths were the same. However if you look at the waterfall display on the SDR you can clearly see that the noise floor is about 10dB or more higher when using the MLA-30.

      This is the crux of the matter, when receiving signals it’s not about the signal strength butt he signal quality and that is usually referred to as the Signal to Noise Ratio (S/N).

      S/N is a measure of the wanted signal relative to the receiver (or antenna) noise floor expressed in dB.

      An active antenna has a finite S/N ratio. This is defined by the amount of signal that can be intercepted by the actual antenna element, any mismatch losses between the antenna and amplifier, the noise figure and gain of the amplifier and the level of signals that can be amplified before distortion occurs, and the point at which the spurious components that are produced start to exceed the noise floor.

      Most small loops (approx 1m diameter or less) do not have sufficient sensitivity to be able to detect the natural noise floor, which is the overall limiting factor for all types of receive antenna. With a well engineered loop it may be possible to get close to this value, but the loop has to be made from a large diameter conductor or flat strip in order to minimise the overall loop inductance. A thin wire has too much inductance which degrades the S/N performance, especially on frequencies above about 10MHz.

      The MLA-30 has a sweet spot centered around 6MHz due to the input low pass filters being mismatched with the amplifier input impedance. On other frequencies the performance tails off quite noticeably and it is not as effective as other designs.

      If you have a very high noise floor at your location and you are using a receiver with limited dynamic range, the MLA-30 will probably perform OK, as the amplifier noise output will probably be masked by the level of received noise and the overall difference between the noise floor and maximum strength of received signals (dynamic range) will probably be well within the capabilities of the overall setup.

      The You Loop relies upon the receiver having a sufficiently low noise figure and good dynamic range, which is why it works so well with the Airspy HF receivers but it may not works quite so well with lower end receivers, or if a long cable run is used.

    • Ladislav OK1UNL

      Hi Ronald,
      interesting, but with loop average cca 600mm (0,6m) has a good receive performance from 6-15MHz.
      There is a terrible Bias tee. Martin G8JNJ published schematic diagram. There is one solution called- linear power supply.
      So, is a problematic improve RF performance suggested by G8JNJ. There is a both side PCB and components are impossible solder.
      Me built original Hula loop antena from ebay kit(PCB + Through hole devices, not soldered) This one allow tune loop by varicap. Me ordered MLA-30 is on the way now. Says generally LZ1AQ loop is not too much expensive but significantly better antenna.

  2. Den

    Weird to read. So much positive reactions
    about the mla-30 somehow it does work for the gros this looks more like a advert for the youloop..

  3. Ladislav OK1UNL

    So well, me supposed and published few day ago. Me built “Hula Loop” antenna, from kit on ebay.com Similar to this one:
    http://www.techlib.com/electronics/antennas.html
    Must agree IC is TL592 or NE592. Depending on factory production.
    Few days ago me wrote this piece of info to M0LMK discuss boar. But was not approved. So sorry.
    I suggest these mods.
    Install BNC connector to ABS box.
    Using low loss coax. cable better than RG-174 is a necessary.
    For “Heavy Duty” installation is a better replace loop wire by bicycle rim (average 600mm)
    Using linear power supply instead USB port.

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