Got Noise?
The lower frequency bands are often so full of man-made noise that you simply cannot hear many stations because of the noise. It’s about S7 here in central North Carolina, on 40 meters, when using an inverted Vee, even though I’m on a one-acre wooded lot.
The noise comes from all sorts of electronic equipment, computers, power supplies, lighting systems and all manner of industrial equipment. Not to mention summer static on the low bands.
I recently acquired and rebuilt an early model Hammarlund HQ-129-X receiver, which I really like. When I first listened to it, I found that 40M was so full of noise that most stations could not be copied. The same noise level had always been true as well, on my Yaesu transceiver.
My main (and noisy) antenna is a 137 foot doublet about 55 feet high in an inverted Vee configuration. It is fed by a 450 ohm ladder line (this is really about 370 ohms) which in turn feeds a 1:1 current (a.k.a. Guanella) balun built into the front of a home-made tuner. This can be tuned to a 1:1 SWR on 80 – 10 meters.
I also had a 450 foot low elevation (8 – 10 foot high) unterminated random wire antenna that snaked in a sort of “lightbulb” shape through the forested rear of my one acre lot. I tried using this on both the Hammarlund and my Yaesu xcvr, but it was as noisy as the doublet. What to do?
I laid this antenna on the forest floor in a rough “loop” shape and brought the ends together. The ends were then fed with #14 450 ohm twinlead (which is actually about 370 ohms). The Hammarlund was built for a 400 ohm antenna input and its antenna peaking control works on all bands, indicating that it likes the antenna.
Voila! Much of the noise disappeared and 40M came alive with signals. It makes an excellent receiving antenna; there is probably a slight loss of signal but the signal/noise ratio is so much better than before. The roughly 450 feet of wire provides a rather large “capture area.”
Some sources state that low horizontal loops are quiet because:
1) If insulated wire is used, there might be less static noise from wind (mine is buried under the leaves so this doesn’t apply).
2) The loop’s higher takeoff angle tends to null low angle noise signals.
3) It might have less common mode noise since it is balanced.
If feeding a modern 50 ohm input receiver, add a 1:1 current balun in front of the tuner. For receiving only, you can also use an old TV antenna transformer, which transformed a 300 ohm TV folded dipole to the 75 ohm TV coax. If you have no tuner and no old TV transformer, try a 9:1 balun.
Remember that you are not going to transmit on this antenna! I have no idea about the results if you do.
You do not need a huge loop like I have. If you can manage even 75 – 100 feet of wire run along the bottom of a wood fence or hidden in the shrubbery in a sort of loop-ish manner, it will work fine. A square with 15 foot sides will do the job.
There are a number of efficient designs; most much smaller than what I’m using. Some will provide directivity. I notice that quite a few are fed with coax but I prefer 450 ohm twinlead because of its extremely low signal loss when used as the lead-in. A number of references regarding on-ground antennas are below (there are many more; Mr. Google is your friend):
Matt Roberts, KK5JY, analyzes the on-ground antenna
KG3V describes his experience with his loop on ground, particularly on 160 and 80 meters.
VA7ST describes a Matching Transformer for LOG’s and Beverages. His work is very useful. As I mention above, I prefer 450 ohm twinlead because of its extremely low loss. Use a balun at the tuner to make it a bit easier to match, but if you use a 9:1 balun, or an old TV balun you can probably feed a 50 ohm transceiver directly with no tuner being needed. Of course, the antenna/feedline impedance will change with the frequency being used.