50 years of Antennas for Shortwave Listening

Home | Digital Photography | SW Audio Clips | SW QSL Cards | RX320 Control

After 50 years of fiddling around with various antennas for shortwave listening, I finally reached the following conclusions:

Forget antenna tuners.
Forget dipoles.
Forget folded dipoles.
Forget inverted V's.
In fact, forget everything except a long, long wire!

And, the longer the wire, the better the reception.

One year, I bought a 500-foot reel of 300-ohm twin-lead, and built a set of folded dipoles "tuned" for each shortwave band. No difference from band to band, and nothing better than a vanilla long wire.

I've tried them all, with receivers ranging from a 10-tube Hallicrafters SX-96, to an Icom R-71a, and nothing worked better than a long wire.

And the best long wire I ever used was 500 feet long! I had loaded a fishing reel with #26 magnet wire, walked along the side of a wooded road, and held the fishing pole over my head while draping the wire from tree-to-tree, catching the lowest hanging branches (maybe 8-10 feet off the ground max). No terminating resistors, no ground rods, no "1/2 wavelength above ground," no nothing. Best antenna I ever used!

Of course, part of the advantage came from being out in the hinterlands, with no digitally-induced RFI for 50 miles!

And I would have let it rest at that, except I retired this year (2009), and had never tried a "magnetic loop antenna." Idle hands, you know. And the rumors about "being less sensitive to RFI" really piqued my interest. My house's feng shui is baaaaad -- it sits at the universe's karmic center of digital RFI.

So, my first attempt was a "standard implementation" of something I found on the web:

I also have a 50-foot long-wire over the roof of my house for comparison. Much to my surprise, this loop worked almost as good as the long-wire when attached to my RX320 receiver -- in general, signals were 2 S-units lower with the loop. But, indeed, noise pickup was about 4 S-units lower, and the antenna could easily be rotated to almost null out some of the noise sources.

The real surprise, however, came when I tried the loop antenna with my itty-bitty portable receiver -- an Eton G6 -- you know, one of those imitations of a copy of a simulated fake shortwave receiver. It looked like an atomic bomb had gone off: trees flattened, fish and people blown out of the water! The loop antenna and the G6 outperformed my RX320, and my old R-71 (both using the long-wire)!

I was still limited by the amount of digital RFI, but I got to thinking: this antenna, while it will travel well in the back of my CR-V, is not really suited for real travel, for example, the next time I go to China. And it sure beats wasting an hour stringing the magnet wire among the tulips and treetops.

So, with no time constraints, I tackled a redesign that would fit in my suitcase. Made of 1/4" fibreglas tubes intended for building kites, here is the result, broken down for packing (that's a 1-foot ruler for comparison):

And here it is, fully assembled in about 23 seconds (that's the G6 radio next to it on the table):

Instead of using the smaller diameter pickup loop from the original design, I instead used a small ferrite toroid transformer to couple the loop to the coax feed line. I tried three toroids from Universal Radio, T80-2 iron powder, and FT82-61 and FT82-43 ferrites. The #43 ferrite proved to work the best -- 3 turns for the loop side, and 6 turns for the feed side. The presence of the toroid had two unexpected effects on the performance of the antenna: (1) it slightly lowered the high-end of the frequency range (from 20 mHz down to 18 mHz), and (2) it completely broad-banded the tuning below about 5.5 mHz. With the orignal loop, and a 14-390 pF variable capacitor, I got a very sharp tuning range from about 4.0 to 20.0 mHz, with a bandwidth of around 100-200 kHz at any point within that range. With the ferrite-coupled design, I got a wider bandwidth, about 200-300 kHz, from 5.5 mHz to 18.0 mHz, and broadband effectiveness from 2.2 to 5.5 mHz.

With the RX320, the signal strength was comparable to the original loop design, but with the G6, the signal was even stronger!

For reference, the square is 26 inches on each side; the wire is #20 stranded. I had made the orginal round loop from #14 solid wire (hey, ya never know when I might want to pump 1000 watts through it!). Consistent with 50 years of prior experience with receiving antennas, none of the "conventional wisdom" applied: it really didn't make any difference where the smaller pickup loop was located, as long as it was inside the larger loop. Similarly, the location of the toroid made no difference -- locating it at the bottom of the loop, adjacent to the tuning capacitor, made for a simpler and more compact design. The round loop weighs 2 pounds; the square loop weighs 100 grams.

I modified an ordinary C-clamp, and a Bogen "car window tripod", to allow mounting the loop antenna on the edge of a table (for indoor use) and on the edge of my car's window (for outdoor use):

Now, all I need is sunspots!