What follows are my random thoughts and comments about these loop antennas in general. Take all of them with a grain of salt - I'm a satisfied builder; not an expert!
Small wire loops have the same or better performance! After spending all this energy and time describing coax loops, you'd be amazed to hear that I am about to replace them all with a single 28-foot circumference wire loop and a balun for convenience. All the coax loops do (wound in the way described on this site) for you is provide a way to make a balanced antenna out of it, and also have a built-in balun to eventually transition itself into an unbalanced coax feedline.
I recently purchased a Unadilla W2AU 1:1 dipole voltage balun (for another project) and made a wire loop with it in the meantime. When tuned, the small wire loop (with wire as thick as the diameter of coax braid) performs the same as the coax loops. In fact, I can tune from 160 to 10 meters with it, whereas with the coax loops I'm somewhat limited to a 2:1 frequency ratio. (On the higher frequency bands, the wire loop becomes larger than .10 wavelength and starts to lose it's directional pattern) The body-hand-object detuning issue isn't as bad as I thought. I've made a very quick page out of the project which you may want to keep an eye on as I improve it.
Preamps: Lately I've found that I can do just fine without a preamp. I didn't realize I was so addicted to overall signal-strength and was not concentrating on the signal-to-noise ratio. This is always disconcerting when trying to do an A/B comparison with another wire antenna which usually is much stronger than the loop. In the end however, I can listen to the loop for days at a stretch whereas with the wire antenna, all I can take is about one-hour's worth of monitoring and fiddling with filters before I throw in the towel.
Recent ICOM radios and preamp notes: Most users of recent-vintage Icom radios will probably want to run with the single 10 dB preamp, or preamp #1 ON. The reason for this is that without the preamp, the sensitivity on ssb is around 0.5 microvolts. This a a sane design feature to help prevent overload, and is not a flaw. Other manufacturer's radios of recent vintage that do not have preamps are usually rated at around 0.25 ssb microvolt sensitivity or less. The Icom radios usually measure at around 0.16 microvolts with preamp#1 enabled. This is an important point to remember if you are comparing results with recent-vintage radios that don't have preamps to Icom radios that do have them.
Don't kid yourself: You won't open or close the bands with this antenna. It is designed to null local noise and raise the SNR ratio, which helps make up for it's lower gain, and is what differentiates it from just some other tuned 10 foot piece of wire. Is it a good replacement for all wire antennas? NO - I'd have a decent long wire up in a heartbeat if I lived in a quiet environment.
Do you really operate in a quiet environment just because your S-meter doesn't move off the peg? Build one of these and find out! In fact, what started this whole business was being taken on a short ocean-going fishing trip. I brought a radio with me and almost jumped overboard when I heard what a REAL noise floor was like! It was as if an amateur astronomer was allowed to use the Hubble telescope. So far in my search, the small rotatable loop is the only thing that even comes close without being a total dummy load. While I loved the noise-nulling feature, it took me a day or so to get adjusted to the overall slightly lower signal levels. Then it dawned on me: I was hearing more than ever before, and it was actually enjoyable just like on the boat! Win-Win in the SNR department!
At one time I thought I had a pretty quiet noise floor on 40m because it didn't move the meter and wasn't harsh sounding like most noise. I thought it was the natural noise floor. Surprise! The loop revealed that I could null out this mellow-sounding fake floor.
DSP Noise Reduction: I sold my outboard DSP gear because I no longer needed it. While it was great for lowpass, highpass and notch filtering, the hollow, distorted sound it created when doing random noise reduction on SSB just sounded too artificial to me. I almost never need to use the noise-blanker either. I'm sure some high-priced rigs really sound much nicer, but I wanted to do something about the noise before it got to the receiver. After the loop, any DSP noise-reduction is just icing on the cake.
Acceptable Performance: Now that I'm not so preamp-crazy, it seems to me that if I can obtain an occasional S9 signal strength (and an S1 noise floor) with the loop on a skywave signal when the band is open, I'm doing ok. (On 80m I have reached +20 on some skywaves) S-meters are notoriously inaccurate, non-standard and non-linear, so I use them as a very generic reference and basically eye-candy. This may be an oversimplification: anything over S9 is a waste. Receiver performance does not improve, and in some cases starts to degrade above S9. Which brings me to:
FREE 30 dB Amplifier! Place a piece of tape over your S-meter. Obviously said tongue-in-cheek, it is amazing at how addicted I am to watching it while monitoring. Once I learned to ignore the S-meter when using my loops, it was amazing to peek up at it and see the strength of what I thought was a good signal. Try it for a day or two. I love my S-meter so I took the tape off. (grin) Once a signal is above the band's natural noise-floor, it's all about the SNR ratio, and not about brute signal strength. Any ratio from 10 to 30 dB is fine with me. Translation: any signal reaching S5 rocks!
80 Meters: Reduce storm-related static crashes! Last year I didn't get much time to evaluate the loop during storms. In 2004, I've been very successful in tracking and nulling distant storms. Rotating the loop to reduce 20 over 9 static crashes down to about S3 is very gratifying. I've noticed that local storms are hard to null, but distant storms seem much easier to null out. (I pull the plug during local storms anyway.) Obviously nulling out storms doesn't always work due to nature and the fact that the loop null is pretty sharp. For the most part however, the crashes that used to be so bad that forced me to turn off the receiver are now tame enough that I can listen to the band all night if I want to.
160 Meters: For most of my amateur career, 160 meters was a joke simply because of the noisy locations I lived in and the length required to put up a decent antenna. The 25 foot circumference loop brought a whole new band to my consciousness. You can really listen to the band and discern the distinct sound it makes all on it's own. It isn't just a lower-version of 80 meters. Fascinating... 2005: the new 50-foot circumference loop rocks on 160!
Electromagnetic Theory: What about TEM waves, E-plane, H-plane, Near-Fields, and Far-Fields? It's a fascinating subject and I'm not qualified enough to even begin to explain it. I can offer one bit of advice should you want to study it: keep in mind the difference between feedpoint impedance and field impedance -- they aren't the same thing.
It is my hope that these pages may help you overcome some noise problems of your own, and wish you the best of DX!
73 de WN6F
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