SW RX/QRP loop antenna from a scrapyard

by Karel Julis, OK1UHU, ok1uhu@qsl.net

As a side-effect of my professional work appeared this very cheap and easy design of a higher-SW loop antenna, which can be made from materials, found at your local scrapyard. In a fact, I needed some well performing antenna for 13.5MHz RFID cards experiments. I decided the single-turn loop.

Almost everything you can disclose from the pictures. You need a piece of an old TV antenna's aluminium boom (about one meter), about 4 meters of 2mm dia copper wire (from the mains electricity installation wires), 33pF trimmer capacitor (robbed from a wreck of an old tube AM radio), some pieces of PCB material, some screws&nuts, one BNC female and about half a hour of time. The detail photos show the top connection of the wires, the tuning circuit and a front view of the antenna.

The diameters of the loops are 70cm and 23cm (funny numbers, aren't?). With the trimmer alone, the antenna tunes very well (with SWR almost 1:1) at 18 and 21 MHz bands. With 15pF ceramic tube capacitor in paralell to the trimmer, it tunes at 14MHz (and 13.56MHz ISM, too). The antenna needs a very exact tuning for the particular operating frequency, even when moved from CW to SSB band part! Tune the antenna with smallest power usable for your SWR meter, or you get the fingers burned, hi! That's not a joke - about 5 watt produce such a voltage!

Thus, PAY ATTENTION! This antenna with the described construction is suitable for RX or true QRP only! With just a few watts (5 and above) there is too high voltage at main loop ends (it means, at the capacitor, too) for the ordinary ceramical tube capacitor (200V) and the trimmer (about 500V) to withstand! If tried higher, you burn the capacitor, make sparkfires in a trimmer and carbonize the PCB (HF thermal effect because of lossy glassfiber)! For such a disaster, I guess 50 watts PEP(!) and more as pretty sufficient.

But QRP world is beautiful enough, don't you think?