A Remotely Tuned Magnetic Loop Antenna

If you are a radio amateur, you may be familiar with the magnetic loop antenna. It’s different from most conventional wire antennas, taking the form of a tuned circuit with a very large single-turn coil and a tuning capacitor. Magnetic loops have the advantage of extreme selectivity and good directionality, but the danger of a high voltage induced across that tuning capacitor and the annoyance of needing to retune every time there is a frequency change.

[Oleg Borisov, RL5D] has a magnetic loop, and soon tired of the constant retuning. His solution is an elegant one, he’s made a remote retuning setup using a stepper motor, an Arduino, and a Bluetooth module (translated here). The stepper is connected to the capacitor via a short flexible coupling, and tuning is performed with the help of a custom Android app. We’d be interested to know what the effect of a high RF field is on these components, but he doesn’t report any problems so it must be working.

He’s posted a video of the unit in operation which we’ve posted below the break, if you’ve ever had to constantly retune a magnetic loop you will appreciate the convenience.

We’ve only brought you one magnetic loop antenna in the past, whose special feature was that it is portable.

31 thoughts on “A Remotely Tuned Magnetic Loop Antenna

      1. Interference is unlikely to be an issue – mag loop radiates perpendicularly to the surface of the loop, there is almost no energy at all being radiated in the loop’s plane. That’s why these antennas are also used for direction finding, locating transmitters and interference sources, etc.

        My guess for the plastic shaft is mostly insulation – the capacitor can have few hundreds of volts on its plates, depending on the RF power being used – and mechanical flexibility to allow for imperfect alignment (metal would be likely too rigid).

      2. No. If you watch the video where it zooms in that is a braided steel cable. Yes, it does have a plastic coating but RF is not like DC. It can probably jump right through that stuff.

        What I was curious about though is why use a flexible cable? I was expecting a metal shaft or if you are worried about interference then maybe a solid plastic one.

        1. If you have a motor and a pot both rigidly mounted to a pole, with a rigid shaft between them, and the motor isn’t *perfectly* lined up, then the motor will either bind or stress the weakest part of the setup, which is almost certainly the internals of the pot or it’s solder joints. A hand drilled box strapped to the pole isn’t exactly precision manufactured, so there needs to be a flexible linkage to absorb the misalignment.

  1. Boy, that looks like a lot of extra technology “just because you can”. And you have to remember to turn on the antenna before you can use it, and turn it off again when done (9V batteries are kind of expensive). If it were my setup, this would deter me from using the radio in bad weather. Since presumably there’s already a coaxial cable going from the shack to the antenna, I would opt to just put the stepper motor out there, with a piece of 4-conductor telephone cord (either the flat stuff or the round CAT-3) run alongside the coax. This would put the electronics indoors and close to where you can feed it power from a wall wart. You can still use Bluetooth if you really like operating everything from your phone, but the battery issue would disappear.

    1. Yea I’d definitely power it somehow. I don’t know that I’d like to use another long cable and have to deal with a potential strike against it. I’d probably try to take one or two of the solar garden lights and have them charge some batteries for it. Or something with a better panel.

    2. I am not quite sure what you mean by “a lot of extra technology just because you can”?

      A long unshielded 4 conductor cable is a wonderful thing to get RF induced in and also the energetic switching pulses from driving the stepper would radiate from it like from an antenna, causing wideband EMI whenever tuning. That’s really bad when you have to actually listen to the signal you are receiving while trying to tune this antenna (mag loops are commonly tuned “by ear”, to maximum noise/signal level of a weak signal – have one like that myself).

      The cheap BT module is actually a clever workaround for that. All the switching and possible EMI mess is on short wires and in the null of the antenna, so will neither pick up RF nor cause you to “hear” it. Driving it using a phone/tablet saves him having to build a control box for it.

      Also these antennas are mostly meant as portable/temporary setups. so weather-proofing and battery life is rarely a concern. Also the stepper likely runs only few steps every once in a while and 99% of the time all the electronics is in low power mode, saving the battery. If it was a permanent installation he would probably set it up differently – but bringing a two wire power line to power the tuner is still easier than long 4 wire setup driving the stepper.

      1. @Jan Ciger
        “weather-proofing and battery life is rarely a concern”
        I will tend to agree upto a point… depending on where you live weather proofing may be a concern, north west with the rains, and the weather or even snow problems etc. Or even here in AZ where the dust and dirt then freak rain storms like right now turns it all into mud, weather resistance at least to block some of all that to give it longer life so things dont short out… that would be helpful….usualy just a gasket(s) is all you need.
        But if one lives in a good climate then even a gasket isnt needed.

        As for battery life, this i agree totally, and if one is so concerned with battery life from then from the looks of it, get a rechargeable 9v, put a/2 solar cells ontop of the box with a small charging circuit, cause the box has room for it that is for sure. Alot of room actually simple NiMH 9v(same size as alkaline) and a charging circuit, heck 2 ‘solar light’ cells work great at trickle charging a 9v, cause I use them to do my setups, since the 9v isnt used all the time, and very little ma are pulled to keep the circuit running and the trickle charging it more than the pull…. well lets just say I havent changed my setup(s) in about 7 years. So it is possible.

    3. What weather?

      It’s a mag loop. Aren’t those usually used either portable or in an attic? If there are leaks in your attic then your first priority should be patching your roof, not anything involving an antenna. As for portable use… who does that outside in the rain?

      As for the batteries.. not a problem for portable operation. Definitely an inconvenience in an attic.

        1. No. I can’t tell for sure if that is a rooftop or not. What I do see is there are bushes that appear to be on the same level and more telling.. there is a nice tall railing. If this is a rooftop it’s a rooftop that is meant to be a regularly trafficked area. Permanently mounted transmitting antennas are not usually mounted in such places both to protect the antenna and to protect passers by. Portable setups though… an easy access rooftop is perfect.

          Also, if you watch the video you will see a tripod in the background. I think that is what it is mounted on in the picture although I can’t be certain because the bottom where the legs would be is cut off. Again, perfect for portable use but not so great as a permanent setup. In rain and snow it will rust. In high winds it will probably blow over.

          How about the Alex Loop box.. does that look like it is meant to be permanently mounted outside.

          I don’t see it written in the article either way but this looks very much like a portable setup to me.

    4. So instead of a tiny wireless box that is a really bad receiver for RF energy at the frequencies this antenna operates at, you’re advocating putting a nice long wire all the way back into your shed that will be an EXCEPTIONALLY GOOD antenna for those same frequencies?

      Should I explain why that is a very bad idea?

      1. Pelrun: No. Should I explain how easy it is to filter this out? Wireless is ALWAYS a bad idea unless you have a good reason wires won’t work. And wires almost always work. And like I said, he already has a wire running to the shack anyway. I tell people this ALL the time in video productions. They want the freedom of wireless mics even when they’re just sitting in a chair the whole time, and then they complain when there’s interference or any of the other dozen things that go wrong with wireless mics. And I got this ALL the time when I was in tech support, with people who HAD to have wireless keyboards, mouse, and printer to connect to the printer sitting on the same desk.

        No, I’m not living in the past; for every device that I know of that depends on a wireless connection, the wireless part is always where the problems are.

        1. Are you actually a ham, or is your only exposure to RF issues wireless mics? This isn’t a 100mW mic transmitting audio 200m to a sound desk, where audio quality is a problem. This is an antenna potentially transmitting *hundreds of watts* (hams in some regions can do 1.5KW!) that will induce hundreds of volts in any wires that are the right (wrong) length near the transmitter. Often a receiving antenna for these signals is just a long wire strung up between two trees!

          RF injuries near high-power transmitters are no joke, and a large part of qualifying for a ham license is understanding the dangers of these power levels and how to safely mitigate them. Running a bare wire from the antenna back to your shack is out of the question. But a 2.4GHz wireless link isn’t going to care about high-power HF signals.

          1. Stupid WordPress put my reply in the wrong place.

            Pelrun: I have experience, years ago, with transmitters producing as much as 3 kW, and yes, I had to deal with interference. Easier than the problems I have to deal with today with wireless links, including Bluetooth.

            My ongoing rant against using wireless everywhere is just a geezer thing – I sometimes find it hard to fathom that a low power radio transmitter and matching receiver and associated batteries and chargers cost less than the wire and fittings to connect two items three feet apart. But it’s each person’s choice, which demons they want to fight.

          2. Your intuition is precisely backwards here.

            The problem is you’re focusing on the wrong thing. Using a wired link for this is a SERIOUS ELECTROCUTION HAZARD. The interference you’re worried about doesn’t affect the wireless link at all (because it’s a tiny device operating at a very high frequency that is physically incapable of being sensitive to HF band transmissions regardless of power) but the long wire you suggested CAN pick up that transmitted signal and CAN damage your hardware or yourself.

            Also, the link is not “three feet apart” – the video is just demonstrating the device, not showing how it is used in practice. Antennas for high power transmission are placed with as much clearance around them as possible, ideally tens of metres. and the operator is obligated to assess and enforce a safe zone around it. The only conductor should be the feedline going from the transmitter to the antenna, and everything is grounded extremely well to prevent dangerous voltages from being generated anywhere near people.

  2. I know nothing about RF.
    But I’m storing away the flexible coupling for later use.
    I hadn’t considered using something like that to control an existing rotary switch or pot before.

    1. It’s actually a good technique for transferring torque when you have to deal with small misalignments. You can hand-drill a hole in a box and not faff about with spacers or filing a mis-drilled hole larger, or finding your motor doesn’t entirely fit where the shaft dictates it has to go. The flexible shaft lets you be misaligned a bit and tilted a bit and it won’t bind or rip your screws out or tear the pot off the pcb.

  3. I’ve built a similar rig using an ESP8266. I can use my phone or laptop to access the web interface and tune a magnetic loop antenna.

    The only issue I have is you can hear the stepper running. It does get louder as you get closes to in tune so it kinda helps but is otherwise annoying.

    One of these days I need to get around to building a box for the electronics and documenting the project to share with the world. Camping season is starting up, so maybe I’ll finish it as it is the ideal antenna for the RV.

  4. FWIW… “Comparing the performance of an inverted vee dipole with a small transmitting loop on 40m Posted by Richard Newstead on May 08, 2017”

    http://www.sotabeams.co.uk/blog/comparing-the-performance-of-an-inverted-vee-dipole-with-a-small-transmitting-loop-on-40m/

    TL;DR: “The antenna configurations tested are typical of a SOTA deployment, and the experimental results suggest that the Inverted V antenna is about 15 dB better performance than the P-Loop antenna as tested.”

    Hmm… At first glance it doesn’t look very good for the Loop thing. What do you think?

    SOTA is “Summits On The Air” or some-such – where Hams climb hills and mountains to make contents and add them to a log of “Achievements”. Essentially this means “Portable” operations where you go to the field with your radio and a portable antenna of some-sort.

  5. how about upgrade it to a auto-tune antenna, which can detect the signal level, and auto tune to the best level?
    what is a simple way to detect the signal level and build a PID loop?

  6. I have experience, years ago, with transmitters producing as much as 3 kW, and yes, I had to deal with interference. Easier than the problems I have to deal with today with wireless links, including Bluetooth.

    My ongoing rant against using wireless everywhere is just a geezer thing – I sometimes find it hard to fathom that a low power radio transmitter and matching receiver and associated batteries and chargers cost less than the wire and fittings to connect two items three feet apart. But it’s each person’s choice, which demons they want to fight.

  7. I’ve wondered if it would be possible to use a large number of “big transistors” to act as varactors. With large enough depletion regions, could you actually create a varactor that would be capable of replacing an air- or vacuum-gapped cap?

    1. It is possible. I’ve done this with power transistors (using just one of the junctions – BC I think) to get relatively large capacitance – around 50-100 pF, as I recall. It’s been a while, but I think I was getting about a 3:1 capacitance ratio.

  8. Thanks for translating my article about remote running of magnetic loop an antenna, I hope soon to finish new version with small SWR meter and auto-tune antenna, which can detect the signal level, and auto tune to the best level. Also it will less power supply and control via wi-fi

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