The Coax Loop

It turns out that the first transmitting loop I built, which I thought was not efficient enough, is actually working fine. It is a modular and portable coax loop. It’s very similar to Julian Moss’ Wonder Loop. The loop itself is made out of a piece of a surplus coax assembly consisting of 2.5m of LMR-400 coax with N connectors on both ends. I found it at a surplus store and also bought there two matching bulkhead connectors and an air variable capacitor. I mounted the bulkhead connectors on the sides of a small box first:

I then took them out, soldered a coax braid to each one of them (both inner and outer conductors), and screwed the small box to a larger plastic box that houses the variable capacitor. The capacitor itself is a 4-section unit, of which I used only two in a split stator configuration. (This configuration essentially uses a series connection of two capacitors, each formed by one stator section and one rotor section; the rotors are connected by the metallic shaft.) The variable capacitor is tuned using a reduction drive.

The coupling loop is a piece of house wiring. Here too I used a choke balun between the coupling loop and the BNC connector.

To test the loop, I secured the capacitor box and the coupling loop to a wooden pole using straps of webbing. Here is a picture of the antenna supported by a tripod inside the balcony. To make contacts, I hang it outside the balcony.

Hung outside the balcony, the loop enabled me to make contacts with Europe with 16W on 20m PSK, so it is radiating reasonably well. It is much smaller than the water-pipe loop (80cm diameter versus 125cm), so it is less efficient for a given value of Ohmic losses. It also has more Ohmic losses, because of the resistance in the N connectors and in the variable capacitor. So the fact that is it radiating is encouraging.

The main attractive aspects of this antenna (and of course of Julian’s Wonder Loop and earlier loops) is that it is modular, portable, and easy to tune:

  • It is modular because I can use longer coax assemblies for better efficiency and for lower frequencies. The modularity also allows me to exchange the coax I have for semi-rigid coax, which will probably have lower losses. The surplus store where I bought the coax has a range of coax assemblies with attached N connectors, so trying out better/longer coax is a real possibility, not a theoretical one.
  • It is portable because the coax detaches from the capacitor assembly and from the coupling loop, so the whole thing is easy to pack and transport.
  • The variable capacitor, especially with the reduction drive, is easy to tune. I use a resistive SWR bridge with an LED to indicate match. I just turn the knob quickly until the LED blinks (good match), then turn the knob back and forth slowly until I find the null again, and that’s basically it. My body affects the tuning  a bit (as do nearby structures), so sometimes it’s necessary to leave the capacitor just before or after the null to obtain a match when I’m no longer near the antenna. But it’s pretty easy.

The main problem with this antenna is the weight of the capacitor assembly. It is quite heavy. When the loop is mounted on a vertical pole, as in the picture above, this is not much of an issue, since the pole transfers the weight to whatever it stands on. But when I stuck the antenna out the balcony on a horizontal pole, the weight of the capacitor box at the far end of the pole makes it difficult to secure the antenna. I guess that I can use Julian’s strategy of putting the capacitor at the low or near end of the pole, to reduce the problem. The water-pipe loop is much lighter so it’s easier to support.

For testing and light use, manually tuning the capacitor worked well. It will obviously be easier to tune the antenna with a stepper or geared motor, but the mechanical construction required is not trivial. (Even connecting the reduction drive to the capacitor was not trivial; Ezra Shaked helped me with this by creating a coupling from a potentiometer shaft.)

6 Responses to The Coax Loop

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  4. Sid Boyce says:

    I’ve used magnetic loops for quite some years. The first was 1m dia. of 3/8″ inch soft copper tubing, gamma matched and indoors. I worked all over Europe with 7W limited by the capacitor plate spacing.
    The next which crashed in high winds consisted of 3.69m dia. of 30mm copper tube tuned with a vacuum capacitor for 160/80m and used mainly on 80m with 100W accounts for all my Stateside 80m contacts.
    The current one for 80/40m also made with 45 degree elbows, 2.44m dia. of 22mm copper tube and tuned by a vacuum capacitor, this time at the bottom of the loop to lessen the wind resistance and gamma matched at the top.

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