[Jeri] Builds A Magnetic Loop Antenna

Most new hams quickly learn that the high-frequency bands are where the action is, and getting on the air somewhere between 40- and 160-meters is the way to make those coveted globe-hopping contacts. Trouble is, the easiest antennas to build — horizontal center-fed dipoles — start to claim a lot of real estate at these wavelengths.

So hacker of note and dedicated amateur radio operator [Jeri Ellsworth (AI6TK)] has started a video series devoted to building a magnetic loop antenna for the 160- and 80-meter bands. The first video, included after the break, is an overview of the rationale behind a magnetic loop. It’s not just the length of the dipole that makes them difficult to deploy for these bands; as [Jeri] explains, propagation has a lot to do with dipole height too. [Jeri] covers most of the mechanical aspects of the antenna in the first installment; consuming a 50-foot coil of 3/4″ copper tubing means it won’t be a cheap build, but we’re really looking forward to seeing how it turns out.

We were sorry to hear that castAR, the augmented reality company that [Jeri] co-founded, shut its doors back in June. But if that means we get more great projects like this and guided tours of cool museums to boot, maybe [Jeri]’s loss is our gain?

[via r/Amateur_Radio]

43 thoughts on “[Jeri] Builds A Magnetic Loop Antenna

  1. I’d love to build a magnetic loop antenna. The bulk of the construction is easy to do.

    The main killer is the capacitors. Vacuum varicaps are hugely expensive and you have to be very careful that they haven’t lost vacuum. Even butterfly caps of sufficient power can cost a quite a bit.

    1. Vacuum varicaps are not actualy the lowest loss capacitors in this frequency range, Q is usualy in the range of a few hundered. Modern high performance ceramic chip capacitors from companies like American Technical Ceramics have far greater Q. This has the disadvantage of not been variable however.

      If you want to make your own variable capacitors it is also quite easy! you don’t have to use air as a dielectric, what about PEEK? unless you are transmitting at high power you should not need the vacuum type.

      1. I was surprised to find links on youtube to make your own tune-able/variable capacitors. I have links on my youtube channel you can check out.

        You can make variable capacitors from a can inside a can, tubes inside of tubes, sheets overlapping and even tune-able plate style. I was thinking adding a threaded rod to one can, where the rod is free to rotate and stationary on one end and a nut on the other end for the can or tube style and the other tube or can is fixed in position to really be able to tune finer and more accurately.

        I was just thinking now similar to my idea of placing the Medocino Motor in a vacuum tube and making a magnetic clutch… I wonder what the magnetic field of an easier to glass tube seal and blow (really melt) the ends closed make vacuum capacitor using a magnetic chuck to rotate the shaft of a plate variable capacitor. I am not sure if the magnetic chuck design is feasible. The glassblowing would be easier so you wouldn’t have to worry about a sealed bushing/bearing shaft that will hold the vacuum when rotating.

        Just like batteries, resistors, capacitors, inductor coils, electromagnets, relays, variable inductors and other electronics components, we can all make our own variable capacitors. Then try to do that MacGyver style or maybe you can say Junk Yard Wars style. :-|)

        I haven’t even read into yet manufacturing specs for the vacuum components. However… yes… we can make our own variable capacitors rather easily using commercial off the shelf items.

        Good call Dodo also, I was thinking a switch and a finer tune cap.

          1. Comercial vacuum varicaps use a set of berillium copper bellows and circular interlocking plates as per this image:
            the vacuum increases the maximum voltage between the plates before arching compared to air dielectric. So if your not operating at high power the vaccum is just and additonal complication.

            Vacuum varicaps tend to have a lot of sliding contacts the make the bellows arrangment and berillium copper is not as conductive as conventional electrical copper so the Q isn’t as good as an equivlently dimensioned air dielectric capacitor (that would have a lower voltage rating). The highest Q variable capacitor I am aware of in the capacitance range suitible for HAM magnetic loop antennas is the tuning capacitor in a HP Q-Meter 4342A:
            This must (for correct operation of the instrument) have Q>>1000 upto 70 MHz ! and was a custom component for the meter.

          2. Excellent, thank you sir. I still haven’t even touched on the details other than know the higher voltage rating and like you noted lowing the point of coronal discharge.(arcing) until now. I wonder with a strategic designed magnetic clutch to factor in Q even acting as an antenna and better materials for the plates/bellows a better lower Q variable capacitor can be made, Do you happen to know why they switch materials to beryllium copper in the vacuum tubes? Softer for better seal, sublimation or am I totally missing the point?

          3. That’s nonsense. There is no clutch inside a vacuum cap. They have a continuous path of smooth copper in the form of a wide area bellows. You are absolutely wrong and giving out completely false information.

            The Q of a typical vacuum cap is in the many thousands, many of them are in the Q= 10,000 range.

            This is just a stone cold fact. The very high Q is one of the major reasons why vacuum caps are used in Q-critical circuits, and circuits with very high RF currents.

          4. @Tom When I made the comment I was brainstorming, writing my “Thoughts” here in a post and still earlier on in my electronics and RF engineering studies and skills. I wasn’t giving out false information only writing down my “Thoughts”. I have typos and you might of misunderstood what I was stating.

            I was “wondering” what a magnetic clutch would do to a variable capacitor. I basically was inventing a component that doesn’t exist (or at least that I am aware of existing). Basically, you’d have to seal a contact on each end of the sealed tube that wouldn’t require a bushing & shaft to go through is what I was mentally writing. Then have a finger on the one side of the contact that would rotate with the magnetic clutch dial. I am still wondering what the effect the magnetic field would have. That is all.

            Did you read that and comprehend that, you might be misunderstanding what I was inventing… not misleading… I was questioning

            “I wonder what the magnetic field of an easier to glass tube seal and blow (really melt) the ends closed make vacuum capacitor using a magnetic chuck to rotate the shaft of a plate variable capacitor. I am not sure if the magnetic chuck design is feasible. The longer term seal would mean owners wouldn’t have to worry about a sealed bushing/bearing shaft that will hold the vacuum when rotating. That is… if the magnetic chuck doesn’t adversely effect the electric and magnetic field properties.

            Make sense? Now what are your thoughts?

    1. @Chris, you are correct. The HaD post entry text should be changed:

      “Most new hams quickly learn that the high-frequency bands are where the action is, and getting on the air somewhere between 40- and 160-meters is the way to make those coveted globe-hopping contacts.”


      “Most new hams quickly learn that the high-frequency bands are where the action is, and getting on the air somewhere between 10- and 160-meters is the way to make those coveted globe-hopping contacts.”

      Diff: Change 40 to 10 above…

      Actually, “globe-hopping contacts” are fairly rare in the 40 through 160 meter bands, but not unheard of (small-signal DSP digital modes not considered). As you go to longer wavelengths and employ modern DSP digital modes, very low frequency (long wavelength) frequencies do become interesting, but difficult to implement.

      1. Yup. I should have been more clear that I was referring to the HaD article, not the video linked to. I’ve made some pretty long contacts on 40m, but it definitely is not reliable for global communication!

    1. Kind of an open ended general question the more I look at it. Effective design is going to be key and if you’re effective at being able to resonate with the desired frequency you’re listening to or transmitting you can’t have a higher loss unless that is what you wanted. Say to clarify what I meant, is if your design is like an active array as echodelta noted in his thought written if I interpret correctly… and your specifications are correct then you will have more gain and less loss with each additional loop if oriented in the not null direction. If you think like a yagi is designed from a more passive radiator design to be effective… then still if you’re tuned in specification… more loops will have more gain and less loss with each additional loop if oriented in the not null direction. That can get you thinking and ask more questions if you need more clarification. I like the book TRANSMITTER HUNTING: RADIO DIRECTION FINDING SIMPLIFIED as that was my first book on the subject other than the crystal radio kits that I hardly can remember the details regarding from Radio Shack if I recall correctly.

  2. Cool! I have been thinking about a magnetic loop as my next project after the current batch are wrapped up. Maybe watching Jeri do it will help.

    One problem for me… I want to put mine in my attic. I have a rotator to mount it on and plan to motorize the variable capacitor so adjustment will not be a problem. My only problem is that I am going to have to do most of the work actually IN the attic as the finished loop will not fit through the access hole. That means squatting, balanced on rafters ankle deep in blown in insulation. Oh well.. just an extra challenge!

  3. Hi Jerry!

    Over the last years, I have seen a lot of your videos, so I would rather classify as a follower and not as a criticee to your ideas, because i like them.
    I am a fully licensed ham (cept 1) since 2015 and have studied STLs for quite a long time, i finally also built a fully functional STL for 80-20m in 2016. So I do know some things about STls, especially their advantages, usability and problems.. One of the most serious problems is that of effeciency in the 160m band, You will only get about 0.63% of effeiciency when using such a small circumference. I don’t see the capacitor to be the main problem here, because i have built a lot of similar capacitors rating about 6 kV and more before…

    VY 73 de OE7WPA

  4. For the receiving antenna, what would be the approx. size magloop that would give comparable performance to vertical monopole antenna? Calculations I find on the net show effective heights of only few cm versus few m for vertical, but:

    I read many stories claiming magloop superiority due to magnetic field reception vs. electric field reception for vertical monopole. Advantageous in buildings maybe?

    1. For small size RX-only you’re probably better off using an active whip rather than a tuned loop. Tuned loops have the big problem that they’re very narrow band. For the size Jeri is doing she’ll have a bandwidth only barely big enough to do SSB on 160m. If you’re not trying to transmit you can just put a bit capacitive plate on the gate of a jfet and get decent performance.

  5. For a receiving antenne, its size does’nt really matter. It’s only a question of your receiving equipment, About half a meter in diameter is more than adequate, i think. What makes STLs very interesting is their small size and the more “horizontal” propagation of their EM waves, so that you can reach much further stations during DX.

    Vy 73 de Werner, OE7WPA

  6. Replying to my own post before,a magnetic loop (STL Small Transmitting Loop) primarily receives the magnetic field of an electromagnetic wave and therefore doesn’t pick up as much QRM (Man Made Noise of your house or neighbourhood) as other antennas do. This is very important for receiving weak signals especially in large towns. I am living in a very classical european flat with a lot of neighbours and a lot of concrete surrounding me and no option to erect a full size antenna within the garden or on the roof of the building. My STL has to find its place on my Balcony (2nd store) for all my shortwave radio amateur connections. Its made of 22mm copper tubing, diameter 1.5m meters (balcony limits) and wors well for me on 80 to 20m (7-15 MHz). Relyable contacts are within 3-6.000 kKm range…

    VY 73 de Werner, OE7WPA

  7. “Most new hams quickly learn that the high-frequency bands are where the action is … Trouble is, the easiest antennas to build … start to claim a lot of real estate at these wavelengths.”

    This seems confusing to me; I was under the impression that higher frequencies required smaller antennas, not the other way around.

    1. Anteanna size does go down with frequency. The HF bands he mentioned, the ones that are routinely capable of supporting communications over the horizon via bouncing off of atmosperic layers, 160 to 10 meters, however, have antennas that are in the rage of 20 to 260 feet. Even higher frequencies (VHF and UHF) have much smaller antennas, but are not able to routinely reach beyond line of sight.

    2. You are correct. The author transitioned intent I think with the “and getting on the air somewhere between 40- and 160-meters is the way to make those coveted globe-hopping contacts.” That goes from a higher to lower frequency discussion also in the sentence. Seems to work, though can be written better… though, great article topic.

    3. ‘High frequencies’ (3-30Mhz) for hams is very different to the ‘high frequencies’ of today’s electronics (Ghz).
      Hence the confusion for imagining antenna size.
      No point changing the terminology with the times I suppose….

  8. Back in the 70’s I did a lot of experimenting with 11 Meter band antennas. I spaced 1/4 wave antennas 1/4 wave for bi-directional sensitivity. But 11 Meters is easy because 1/4 wave spacing is only 9 ft! I can understand the reason behind a loop antenna because any sort of straight element directional antenna would be huge!

  9. 50 feet = 15.24 meters
    3/4 inch = 1.905 cm

    And for the Dan: 160-meter = 524.93 feet and 80-meter = 262.46 feet :)
    It’s actually amazing that people from the US also use ‘meter’ for band description, I wonder how that was managed since so long ago. Some long forgotten master of persuasion must have worked wonders.

    1. It’s because dividing 300 by the frequency gives the wavelength in meters, easier to just leave it as such than to have to then convert it to feet :-) Also, even in the US, most scientific applications use the Metric system, which is probably why as well, although I am sure there is a formula that would give one the wavelength in feet.

  10. Some of the comments here are just wrong. Vacuum capacitors typically have very high Q, some have Q>10,000. They have no sliding contacts internally.

    Two turn loops, by the IRE definition of radiation resistances, have the same radiation resistance as a single turn loop the same area. They have the same NET current in the same spatial area causing radiation. This is why almost all commercial loops that have gone through engineering and testing are just single turns.

    A small loop is just as much affected by earth as a large dipole. It is not more immune to earth effects. The reason people thing it is more immune is that the very high losses dilute or swamp out the more easily noticeable effects, like impedance changes. The pattern is lower angle off the loop’s radial dimension because the polarization is vertical. We really should not compare it to a horizontal dipole and think the change is peak radiation angle is caused by “immunity to earth effects”. That idea just is not true.

    When receiving on HF, the pattern is virtually everything. But when transmitting the important thing is the absolute field strength at the desired angle and polarization. Peak radiation angle does not matter. The absolute LEVEL of radiation for a given input power **at the desired angle and direction** is what matters. It is not uncommon to have an antenna with peak radiation at 90 degrees elevation (90 degree TOA) that has more absolute field strength at 15 degrees (or some low angle) than an antenna optimized for 15 degree TOA that has poor efficiency.

    The worse possible place for the tuning capacitor in the loop is at the bottom near the mounting structure and feed system. This is because a very strong electric field from the loop surrounds envelopes the capacitor area. There will be many kilovolts there, creating a strong electric field, at radiated field levels in the order of a few watts per meter^3.. The capacitor really belongs up in the clear away from feedlines, mounting, and ground.

    This all said, almost anything “works” and makes contacts.

  11. It seems to me that using a vacuum variable capacitor as a tuning capacitor for regularly going back and forth over multiple bands must have a severe handicap over something like a butterfly capacitor. The handicap is the way a vacuum variable is constructed using a bellows assembly. On one side is a vacuum, on the other is atmosphere. Working back and forth the bellows will eventually work harden and crack. Pop, there goes your financial investment. A good butterfly can be run end to end of its range on a daily basis without financial catastrophic failure. If a vacuum variable was just used to tweet tuning over a narrow range it would last a long time or if a ham were tuning on only one band then maybe this would be viable. I’m considering building an 80 meter mag loop and so may use a vacuum variable for that. If I used a door-knob capacitor in parallel with a vacuum variable I may only need a small variation in capacitance to tune the whole band. Just a thought to mull over.

    1. The loop is a very high impedance point. For receive the type of capacitor won’t matter. But when transmitting, the capacitor will see relatively high voltage.

      So the capacitor has to handle it. Vacuum variables are one solution, but as you say, they aren’t really made for regular retuning. An air variable may not have the same voltage rating (hence may arc over) , especially nowadays when variable capacitors are less common, so finding them with sipufficient voltage ratingbmay be xpensive and hard to find. You may need a high capacitance variable capacitor, depending on the frequency and loop size.

      The Q of the capacitor is a factor too.

      Like I said, for receiving only the capacitor is less critical. Though as someone commented last year, an active antenna may be simpler than a loop for receiving, at least forbmany uses. Especially if yiu can get tge active antenna outside and away from buildings.


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