I had been wanting to build a loop that would work on 40, 60, and 80 meters. I built this one a couple years ago and never really had time to really test it out or even complete it until this past weekend. Our NVIS field day was the perfect chance to get it out and put it to a great test.
I still need to install a drive motor to be able to tune the capacitor from the operating position. I'm debating leaving the wood frame as-is, or building one out of PVP pipe. For best performance, I need to get it up in the air at least another 3' to 4'.
Construction of the main radiating loop is 31' of 1/2" Heliax hard line coax configured into 2 turn that are spaced 6" apart. The diameter is just slightly over 5'. The vacuum variable capacitor is connected to the outer "braid" of the coax. The coupling loop is made from a piece of solid 12ga wire stripped from a piece of 12/3 house wire. Coax is fed to the coupling loop by way of a BNC to binding post adapter.
I purchased this capacitor from RF Parts. At $190, it is an EXCELLENT buy for a small transmitting loop antenna. Rated at 7.5 KV with a range of 10-205 pF, and plenty of threaded holes on each end to make mounting it very easy. I believe the body of this very sturdy VVC is made from solid brass, then chrome plated. When cleaning off the ends at the threads to ensure a good connection, I was met with a brass surface immediately.
This antenna will easily tune on 40m, 60m, and 80m. I have not tried to see if it will tune on 160m, but I suspect there is not enough range in the capacitor to do so. I will at some point try another capacitor with a larger range. I would love to see how it works on 160m for local contacts and NVIS.
The antenna performed very well on 80m and 60m, and was exceptional on 40m. This field day has definitely breathed new life into getting this antenna completed! And once the weather warms up, that's exactly what I plan to do.