Audio Spectrum Analyzer App for the iPad
A few weeks ago, I put up a new antenna, a delta loop for 43 meters. Since it was dedicated for a single band, the performance should be very good. My plan was to write a article here about how well it works, compared to my existing antenna, the 635 foot Sky Loop. That article never materialized, because… the delta loop doesn’t work any better than sky loop. What went wrong?
Shortwave listeners, it seems, are addicted to two types of new things: new radios, and new antennas.
We’re sure that the latest and greatest radio will substantially improve reception, reject QRM, and let us hear lots of stations we could never hear before. And Software Defined Radios promise to do all this and more (just ask Al Fansome). While a new radio often does offer conveniences and advantages over the old one, usually they turn out to be mostly minor improvements (unless you’re switching from say a portable to a desktop communications receiver, or finally giving up that old analog tube radio for a newfangled solid state rig with digital readout).
The same holds true, it seems, for antennas. Sure, if you’ve previously had an indoor antenna, and finally are able to put up your first outside antenna, the improvement will indeed be dramatic. You most likely will hear new stations that you never could pick up before, and the reception of existing stations will be substantially improved. You’ll also end up not hearing some things you previously did, like your plasma TV.
And switching from say a 50 foot random wire to a dipole or T2FD will also produce a noticeable improvement in reception. Not as much of an improvement as going to an outside antenna, but still significant.
But after that, it certainly does seem to be a case of diminishing returns.
When I switched from the T2FD to the Sky Loop, I did notice an improvement in reception, but it was not what one would call amazing. It was better, certainly, and worth the effort. But I went from a 132 ft T2FD to a 635 ft sky loop. Most of the improvement was on the lower frequencies and MW, as one would expect. Reception on the higher frequencies, say above 20 MHz was either the same or worse. Also probably as one might expect.
But, like the gambler looking for that last final big score, we SWLs have to try for the ultimate antenna. The one that will let us hear otherwise impossible DX. Like a pirate on 6925 kHz during the daytime transmitting from Montana. Possibly also being heard in New Zealand. To hell with the laws of physics!
So I ran numerous NEC models on various configurations of the delta loop, optimizing the dimensions and height for the best possible reception. Ignoring the fact that minor changes in things like ground conductivity cause huge changes in antenna performance. And that I have no idea what the ground conductivity is here, anyway. Plus, it probably changes when it rains. Also, the takeoff angle from the antenna varies quite a bit if you change the height of the antenna by a foot or two. Did I mention that my yard is heavily sloped?
But, I did the calculations, cut the wire, shot the fishing line up over the trees to pull up the rope, and installed the new delta loop. Then ran coax to the shack, connected it to the radio, and ran some tests that evening, to see how much better the performance was. It wasn’t. Signal levels were lower than with the sky loop, and more importantly, the signal to noise ratio was the same or worse. Plus, I had an antenna that basically worked for one band, whereas the sky loop is good from MW up.
So, I think I’m going to stick with the sky loop. No need to switch antennas, or use an antenna tuner. It just works. Although, if I take the delta loop and reconfigure it as a horizontal resonant one wavelength antenna… hmm… time to run some NEC simulations!
Y’know, one trick you can do with a delta loop is to run it through pulleys. Then, feed it on one side. You can change the takeoff angle (and quality of reception) by moving the feedpoint up and down.
This isn’t just theory–I used to run Field Day with a ham that did this, and it was remarkable the difference (on 20 meters) moving the feedpoint would make on reception.
The feedline was 72-ohm balanced twinlead, but I’m sure 300 ohm twin lead would work just fine.
Do you use an antenna tuner? They can make a big difference.
You are comparing one folklore or myth antenna with another folklore or myth antenna, and expecting to prove bad science or baseless opinions or rumors are real. When a common baseless opinion about antennas deviates from science, and someone does a test and looks at results factually, results are often not what the baseless performance rumors predict.
Those antennas are comparing exactly as theory, engineering, or good science predicts:
1.) Receiving on HF is based on directivity and the level ratio of unwanted signals to desired signals, not gain or physical size. Since those antennas do not have directive patterns that point at a desired signal while excluding other directions, none will be outstanding unless through some dumb luck a null falls in the direction of unwanted stuff and a peak on a desired signal.
2.) For transmitting, the average gain over a dipole or some standard reference antenna in the directions of interest is all that matters. Since none of those antennas have significantly different average gain than a dipole, and almost certainly have less average gain than a good high dipole, they should not be outstanding.
3.) Size doesn’t matter. Effective aperture or capture area is strictly a function of gain and frequency. Capture area is NOT determined by size, unless it is a standard antenna like a well-designed dish with nearly perfect feed system. For all antennas, capture area is strictly related to gain and frequency. Your big antennas probably have similar or less gain than a dipole, and so have similar or less capture area than a dipole. A properly built 4 element Yagi, for example, has a little more capture area than a 2 WL per leg Rhombic. This is because the Yagi normally would have a little more gain than the Rhombic.
Bigger looks good, and makes people feel good, and everyone loves a magical niche antenna, but you don’t see successful competitive stations using them. This is because they quickly learn they can’t compete against others and win with them.