A Wideband Receiving Loop
November 19, 2010 4 Comments
The antenna I’ve been using the most recently is a wideband receiving loop. The wideband nature of the antenna allows me to quickly find out what’s going on on different bands without having to re-tune the antenna.
The loop element is the one I built for the active tuned loop. The amplifier was designed by John Hawes; I built about two years ago in an enclosure designed for wire loops. The wire loops did not work well. But I found that the amplifier worked well with the aluminum tubing loop element, so I built a new enclosure that can be weather-proofed. This setup works very well.
The original enclosure, shown below on the left, was great for experimenting with different loop elements, but the connections where exposed to the elements when the amplifier was left outdoors. I used it not only with loops, but also with a short dipole (old telescopic TV rabbit ears); it worked, but not terribly well. The loop element works much better.
The antenna works well from at least 7MHz to 14Mhz, which is the range of my Softrock radio. In the map below you can see WSPR reception reports from one night, mostly at 7MHz. It also works at 18MHz (the Softrock received some WSPR signals there even though it’s input filter attenuates them significantly). I suppose that the antenna works well below 7MHz, but I can’t test this right now. An article by Chris Trask shows that the resistance of a 1 meter loop element is low up to about 15 MHz, where it starts raising very quickly (more on his article later). So I suspect that the amplifier, which has a balanced 50Ω input, is reasonably well matched to the loop only up to about 15MHz. I’ll test the loop at higher frequencies at some point.
The main problem with wideband preamplifiers, like the one I used, is that they are prone to overloading by large signals at frequencies far from the one you are interested in. I did not experience any obvious overloading problems. But it’s of course possible that if somebody starts transmitting close to me, the amplifier will overload. Chris Trask’s article, which I mentioned above, describes a wideband loop preamplifier that’s a lot more sophisticated than the one I built. It’s dynamic range is probably much higher. But since I did not experience any problems yet, I was not eager to build the more complex circuit. (His article, wideband loop antenna amplifier, is available on a Yahoo group; you need to register to access it.)
My build of John Hawes’ amplifier is almost an exact reproduction, except that I used a ready-made 1:1 output transformer from Coilcraft, a WB1010-PCL with a 780μH inductance, rather than a ferrite torodial transformer. It works well.