Saturday, August 17, 2013

20 Meters Delta Loop - First Look

I finally freed up a few uninterrupted hours during calm, clear weather to install the 20 meters delta loop (described and shown on the ground here) on top of the Site-B mast. As you can see in the picture below it is not too high -- the bottom is about 7 meters above ground, with the apex a little over 13 meters up. The dipole (TH1vn) height on the Site-C tower is centred on this range (10.5 meters). Rope stays stabilize the fibreglass mast. You should be able to spot the RG-62/U ¼-wave transformer connected λ/4 down from the apex, complete with coax common-mode choke.

I had intended to raise it one further 4' section of fibreglass mast, but I had to abandon that plan. The reason was one of personal safety due to lifting an unwieldy structure while standing at the edge of the roof. I was not at risk of falling! I rigged up a safety harness that allowed me to move fairly freely along the roof line without being able to go over the edge. (Safety first!) I was more in danger of dropping the antenna and damaging both the shingles and the antenna.

As installed, the northwest corner is close to the roof. This is not ideal. There is RF noise emanating from household and neighbour's computers, and the bottom is closer to the eaves trough than is ideal. Coupling to the eaves trough, soffit and house wiring will detune the antenna. However the pattern is unlikely to change much in comparison to a slightly-greater height. For an experiment it'll do.

I had great hopes for high-band delta loops. This experiment was designed to minimize cost and labour should the antenna not meet expectations. If the experiment has a positive result I will proceed to add more bands to it and weather-harden the structure.

Before I talk about how it performs, let me first recap my hopes for the delta loop:
  • Omnidirectional, low-angle pattern
  • Low-angle (DX) radiation at modest height
  • Broadband, low-SWR match
  • Inexpensive and physically robust
There is ample discussion (and endless argument) about the relative merits of full-wave loops versus dipoles, including directional arrays of both (quads and yagis, respectively). However few actually do real-world comparisons. I do not expect large differences, but I hope to be able to make a clear choice. Models are nice, though never definitive. In the past I've tried both styles of antennas, both single and multi-element, just never at the same time that would allow A/B direct comparison. Now I can. Consider these points:
  • Height has a strong influence of antenna performance, regardless of polarization. While low-angle radiation angle does not appreciably improve for higher vertically-polarized antennas, all antenna gain an advantage by getting above the local environment. There is a lot of metal in houses and utilities that can degrade the antenna pattern, especially at low angles.
  • Performance of vertically-polarized antennas is very sensitive to ground quality. The poorer the ground the higher the near-field losses and absorption of low-angle radiation in the far field. EZNEC and other modelling software can estimate the effect but despite your best effort what you will encounter in the field can be quite different.
So...with 24 hours of use how do the antennas compare? Is there a clear winner? What does it even mean to say that one antenna wins?

First, the antenna works. I've worked a number of stations with it. The resonant frequency is much lower than designed, almost certainly due to nearby metal. The SWR dips to 1.0 around 13.5 MHz, well outside the band. Even so it only rises as high as 2.0 at 14.350 MHz. Despite being 92Ω, the ¼-wave transformer is working.

Note: All the following comparisons were done on receive. I did not bother any contacted stations to help me with transmit A/B tests. Each comparison took the time to allow for the vagaries of multi-path and Faraday rotation due to the different position and polarization of the antennas. All quantities were "eye-balled" on the S meter, not measured with any instrument.

There are a few areas where the delta loop is a clear winner over the TH1vn comparison dipole:
  • Closer stations are attenuated. Stations in the eastern half of North America are often 1 or 2 S-units weaker, and sometimes as much as 4 S-units weaker. This makes DX'ing more pleasurable, and would be a benefit during DX contests.
  • With a tuner in line it performs very well on 17 and 30 meters, much better than the TH1vn.
  • Very broadband, as noted above.
  • Omnidirectional, though not as pronounced as expected. It is not always the better antenna for DX off the ends of the dipole (South America and Asia).
Now let's talk about the important comparison. Which is the better DX performer?
  • The dipole wins on almost all near and medium-distance DX. I did not expect this. This includes DX off the ends of the dipole. Recall that dipoles do not really have nulls off the ends, just at very-low angles. At higher angles there is substantial vertically-polarized radiation.
  • The dipole is superior to the delta loop into Europe, the Middle East, west Asia, west Africa, Central America and northern South America. Into western Europe (EA, F) the dipole is up to 3 S-units better.
  • The antennas are roughly equivalent into Siberia (UA9), Brazil and (surprisingly) VK.
  • The delta loop is better into southern South America (CX and LU), Japan and UA0.
During these tests the K-index was 3 or 4, so there was some geomagnetic activity. I mention this since it could cause higher absorption at low angles. If true this would favour the dipole. Testing will continue to determine if this is the case when propagation improves.

If first impressions tell me anything it is that this antenna is not going to pass the test. It is not only the DX performance but also the susceptibility to EMI from home electronics. Even when the delta loop is better, if the DX is weak it is sometimes hard to copy due to the noise. The dipole, being farther away, is much quieter.

Regardless of the outcome I am pleased with this experiment. I'm learning new things. I am already thinking about alternatives that would meet my needs. Summer is wearing away and with it the remainder of antenna-raising weather. I have some work ahead of me.

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