The Trombone Magnetic Loop for 50MHz

A while back I found another discarded piece of the orange polyethylene-aluminum-polyethylene tubing from which I already built several loop antennas. This piece was small, about 1.5m, and thicker than the tubing I’ve been using so far: the diameter of the aluminum tube is 16mm rather than 14mm. I decided to turn it into a 50MHz (6m) magnetic loop.

The main challenge when building magnetic loops is the tuning capacitor. After considering several designs, I decided to build a trombone capacitor using the loop itself as one plate, using 16mm polyethylene-aluminum-polyethylene tubing for the loop and the outer plate of the trombone, and a small piece of the 14mm as the inner plate. This is essentially a refined version of my unusual loop; in the unusual loop I squashed one end of the loop and stuffed it into the other end to form a capacitor. This time I would stuff a thin tubing into both ends of the loop, creating two capacitor that are connected in series by the thin inner tubing.

The thin tubing was too thick to slide into the thick tubing, so I had to file some of the outer polyethylene layer from the thin tubing. In the picture at the top, you can see scratches from this process and you can also see that the aluminum got exposed in some places. This is nothing to worry about, because the inner coating of the thick tube is about a millimeter thick, giving plenty of insulation. After filing away some of the polyethylene, the thin loop slid into the thick one.

Sizing the loop, sizing the capacitor section (the small piece of thin tubing that slides into the open ends of the thick-tubing loop), and tuning the loop were all difficult, because I don’t have an antenna analyzer. I could sometimes detect the frequency to which the loop was tuned by the peak in the noise, but not always, especially if the resonant frequency was way off. Also, the KI6GD loop calculator that I usually use to size loops and tuning capacitors refuses to go above 30Mhz. I initially started with a loop that was too large, so it resonated way above 50MHz even with no capacitor at all. I then decided to scale down the loop from a 25MHz design; this suggested a 38cm loop, which turned out to be a good size. I made some rough calculations as to how much capacitance each centimeter of the trombone would produce; I did not attempt to measure the capacitance, which perhaps I should have. The capacitor section too started much too long, but after I cut it to about 9cm I was able to tune the loop near 50.1Mhz, which was my goal. Tuning took a long time. Once the loop was tuned, I secured the trombone in place using a lot of electrical tape. It’s been several weeks since, during which I moved the loop many times, and it remained tuned. Even being in the sun does not seem to detune it significantly. The coax is coupled to the loop 1/5-diameter loop made from stiff copper wire. The coax connector and the coupling loop are secured to an ABS base using lots of epoxy glue; the base is secured to the loop using two cable ties.

A relatively large magnetic loop that requires very little capacitance to tune has a relatively wide bandwidth. That, together with the fact that the 50MHz allocation here is only 200kHz wide (of which the lower 100kHz is used mostly for beacons), means that a fixed-tune loop is good enough for me.

I don’t know how good the capacitor is (how high its Q) and I didn’t measure this, but in theory the Q should be high, because polyethylene is a good dielectric; Peter Rhodes used polyethylene bags (e.g. freezer bags) in his PIC-A-TUNE antenna tuner to build high-Q high voltage capacitors. In practice the coating of my tubes might have some additives that lower the Q, so I can’t say anything definite.

I have not found too many other designs for 50MHz loops. Ed Bosshard describes a couple on his web site (both larger than mine). Juan Antonio Siles Sanchez published two videos documenting (in Spanish) the construction of a square copper loop with a trombone capacitor; the loop itself forms the outer tube of the trombone, as in my antenna, and a threaded rod forms the inner part.

Higher in the VHF and certainly at UHF, magnetic loops do not make much sense, because the size of half-wavelength antennas and even full-wavelength loops is manageable, even if you do not have a lot of space. But the 3m that a 50MHz dipole would require may still be large enough to make a magnetic loop attractive.

So far I’ve made only local contacts with the loop, but there is no reason that it would not work for long-distance communication when conditions are good enough on the band. Relative to the wavelength, this loop is larger than my coax loop is at 14MHz and 21MHz, bands at which I’ve made many long distant contacts with the coax loop.

An update from early June 2011: Yes, the loop works fine for contacts with distant stations. So far I’ve made two SSB contacts with European stations and heard many more in several modes. The loop also seems to stay tuned, which is something I worried about a bit.


15 Responses to The Trombone Magnetic Loop for 50MHz

  1. Sid Boyce says:

    Excellent use of spare materials.
    The first magnetic loop I used was 1m diameter of 3/8″ copper tubing, it was hand tuned with a spare single variable capacitor which arced over beyond 7W, but with 7W on 40m I worked all over Europe with the antenna on the upstairs landing.
    I’ve used magnetic loops for 160/80 and my current one is 8 feet diameter tuned with a vacuum capacitor at the bottom to lessen wind loading, it’s capable of 100W on 80/40 (22mm copper and 45 degree elbows).
    The larger 13 feet diameter loop for 160/80 (28mm copper and 45 degree elbows) blew down in high winds and the vacuum capacitor smashed. I have another vacuum capacitor and some time I hope to rebuild the 160/60m loop with careful attention as to how I can get it to stay up in winds as high as 80 mph. All my 80m contacts Stateside were made with the 13 foot diameter loop and 100W.
    I’ve used gamma matching as I’ve had trouble getting the coupling loops to tune properly across 2 bands.

    • Sivan Toledo says:

      Thanks Sid.

      With my coax loop, I can get an good match with a coupling loop from 30m (sometimes down to 40m) to 12m. On 10m, I always get an acceptable match (the transceiver does not complain), but not nearly as good. Perhaps it’s a similar problem to yours. But with a coax loop, I can’t try a gamma match.

  2. Sid Boyce says:

    My previous posting should have read “160/80” instead of “160/60”.

  3. Victor Koren says:

    Did you compare horizontal Vs. vertical polarization performance?
    Victor – 4Z4ME

    • Sivan Toledo says:

      Hi Victor,

      Yes, but not very methodically. I made some comparisons, by ear, and after a while decided that the vertical orientation (the loop is not parallel to the ground) is better. This was while listening to local stations, so it might not be optimal for distant ones. What do you suggest?

  4. Sid Boyce says:

    I did not do any proper tests either. I had a G5RV up that was quite a bit dog-legged to fit my property. Sometimes on 80m I could RX a signal better on one than the other, but that tests probably only relate to my setup and location.
    One excellent feature of a magnetic loop is the ability to null out interference. Listening on 40m with the G5RV there was a high and constant noise level. When I first hooked up the magnetic loop my impression was that 40m was “quiet tonight”, then I remembered the theory, I rotated the loop 90 degrees and up came the familiar noise which comes from the electric pylons about 80 yards away.
    Not only noise could be nulled out but splatter coming from a station in a different direction to the one being listened to.

  5. Sid Boyce says:

    With the loops I have tried 1/5 diameter RG-213 coax coupling loops starting with it a bit larger and pruning as I tested. I could only get a decent SWR on one band, OK on 80m and a good match unattainable on 40m, so I’ve since settled for the gamma match.
    The first small loop was built for 40m and the 1/5th loop worked well. I didn’t check on higher bands as I have a 2-ele quad for 20m – 10m.
    I don’t know if the proximity to other antennas affected it.
    One day I had the G5RV connected to the ATU trying to tune the G5RV with the ATU’s noise bridge, it was then I noticed that the loop was detuning the G5RV. I also observed that the G5RV detuned the loop.
    I had to make sure that they weren’t both tuned around the same frequency.

  6. Victor Koren says:

    When the loop is horizontal the radiation polarization is horizontal too. I guess the other stations use also horizontal polarity on 56MHz, so it is interesting that the vertical polarization works better. May be on DX it will be different.
    Victor – 4Z4ME

  7. lz2ilr says:

    shalom, it looks like we have both been working in similar directions. i have several loops made from this tube over the last 6 months, from 80m 2 turn loop to 10m cross polarised. they all use the same tuning as yourself!. i am now working on multiple loop configurations inductively coupled. i also work from the junk pile but i aquired an old grid dip meter to test resonance.
    I would be very glad if you would want to colaborate on some tests to test repeatability of designs using our different pex-al-pex pipes.
    here in bulgaria the brand is normally ‘pexal’ and is white.
    you can mail me at [email address removed]

    what is your callsign?

    ed kemp
    LZ2ILR (ex. M0XEK)

  8. Mark Rothstein says:

    Hi: I have also been experimenting with trombone capacitors. As the dielecric I use polybutalene heat shrink tubing in multiple layers until there is just enough room to slide up and down in the larger tubing. Polybutalene has a good dielectric constant and very low dissapation factor. I am a semi reitired Chemical Engineer so I am familiar with electrical properties of polymers.
    Mark Rothstein, P.E. KC3JV

  9. Chris says:

    I had not seen the term “trombone capacitor” before, but I can see that this method of construction is a *really* good idea for making loop antennas with a robust transmitting capacity, simply. Now I really want to get my ham license to try this out. Thank you!

  10. Sid Boyce says:

    The trombone capacitor has been around for quite a while and there was an HF magnetic loop described in a QST article that used one which was motor driven.

    Other schemes such as using coax cable to supply the bulk of the capacitance and having a smaller variable for fine tuning are used and also capacitor switching with vacuum switches is another method.

    A capacitor is simply 2 or more pieces of metal separated by a dielectric material, how you achieve it is up to your imagination and what materials you can get hold of.

    There are many schemes and BTW you don’t have to wait for a license in order to try the technique as these antennas are very effective both on receive and transmit.

    They are easy to build and use and are fun antennas that don’t take up much space.

  11. Currently the guys are using Pexal tube with a cap of copper tube slid in the ends on HF for Wspr.
    One has made an adjustable cap (copper tube with a threaded nut).

    I have several Magloops made from 12.7mm copper pipe, my 2 element indoor loop can be spotted 18600km with just 5 Watts :o)

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