Once I’ve made some refinements, I’ll do a detailed post, but over the weekend I built and tested a homemade 3-foot tunable AM antenna. This is a passive antenna which uses a variable capacitor to tune the antenna to the appropriate resonant frequency to match the frequency you want to tune with your radio. Here is a picture of the “finished” rig:
Not the greatest picture, but you can see the 11 windings of the main coil (made from a 50-foot length of 20-ish gauge speaker wire by separating the two leads and soldering them end to end, thus the two colors), the inside pickup coil (made from 16-gauge speaker wire because there wasn’t quite enough of the other wire left over from the 100-foot length), the coax running from the pickup coil, and the tuning capacitor on the back side right. I used information from Bruce Carter’s site, including the calculator, and some of the links at the bottom of that page. I bought the tuning capacitor (and some other odds and ends) from The Xtal Set Society, which sells various radio-related goodies for do-it-yourselfers. I recommend getting the capacitor with the 8-to-1 gear reduction built in, which makes fine-tuning much easier. I placed the order online on Monday night and the package arrived on Saturday.
As far as results, I’m a little limited at the moment because I don’t have a support for the loop, which makes directional adjustments a pain. One of the key behaviors of this antenna is that not only do you pick up a lot of magnetic signal in a small size due to the multiple loops, but the antenna is highly directional along the plane of the loop. Even in a fairly radio-quiet corner of my home, there is enough interference that the best orientation of the loop is more about minimizing interference than direction-finding on the station. Thus, the performance of the loop isn’t radically better than my much smaller commercial Terk Advantage loop. (I should mention that the Terk does work pretty well for the price and small size.)
However, I did an outdoor test this evening with the usual DVB-T dongle and Ham It Up upconverter hooked up to my laptop. I like to use 850 KOA out of Denver for testing at night. It’s a 50,000 Watt clear-channel station, so it gets out a good skywave signal at night, but it’s still more than 500 miles away. With the Terk, I was able to get a decent listenable signal, but there wasn’t much variation when I rotated the antenna to different orientations. With the 3-foot loop, when aligned northeast-southwest to line up with the direction to Denver, I got a very good signal about 15 dB stronger than the Terk and the noise level was only increased about 5 dB for a 10 dB improvement in signal-to-noise ratio. However, when rotated about 90 degrees, KOA was nulled out and I was picking up a good signal on a Spanish-language station that was inaudible in the original orientation. (I’m actually not quite sure what station that is, maybe XEM out of Chihuahua operating on daytime power after dark.)
I’ll figure out what that other station was some other time, but the point is that the loop seems to be working as it should. I work a day job, so I generally DX at night, and the outdoor thing may not be ideal, but the loop still does work indoors, too. I need to come up with a good mounting system, and also a better wire connection system. I’m contemplating building a bigger loop since it has to be mounted anyway. But, that will probably be better with thinner wood supports and larger end caps to wrap the wire (like many other designs do).