Thursday, August 22, 2013

Putting the 20 Delta Loop on 17 and 30 Meters

Antennas that are smaller (shorter) than λ/2 tend toward poor performance. Not only does the SWR increase -- increasing transmission line and matching losses -- the radiation resistance can get so low that ohmic resistance in the antenna can become dominant. You won't notice this (much) for frequencies only a little below that point. But by the time you reach λ/4 it is a critical factor.

On 17 meters a 20 meters delta loop is longer than 1λ, while on 30 meters it is over 0.7λ. I therefore expected reasonable performance in regard to antenna losses and pattern.

As I promised in my final look at the recently-retired 20 meters delta loop, I am following up with my observations of the antenna's performance on both these other bands. The modelled patterns of the antenna on 17 and 30 meters are below.

Notice how on 30 meters the radiation pattern is even more omnidirectional than on 20 meters. It is generally true that the smaller an antenna the more isotropic it becomes. The opposite is true on 17 meters. Just don't become too enamoured of this result since short antennas can have serious performance issues, as mentioned above. However on 30 meters, for this antenna, the impact is small.

The modelled SWR on both 17 and 30 meters is high, and it measures high as well. There are surely substantial losses in the transmission line, which in this instance is approximately 15 meters of RG-213/U plus a λ/4-tranformer made from RG-62/U. Even so the antenna performed reasonably well on both of these non-resonant bands, with the help of a high-power antenna turner on the transmitter end of the coax.

The performance is unlike what I experienced with the TH1vn on 17 and 30 meters. While both tuned well with the same tuner, the losses were noticably higher. It performed so poorly that it was pretty much useless. Exactly why this should be so is not clear to me. It can't simply be the transmission line quality (which works just fine up through 10 meters, on bands where the antenna is resonant). Perhaps the traps are showing a low parallel impedance on those bands. and therefore acting somewhat as resistors. The question while interesting is not important to me at the present. It's enough that I know what the antenna cannot be expected to do.

In operation the delta loop did quite well on 17 meters. I had no trouble working most any DX I heard that was not especially weak. Europe, Siberia (UA9) and South America stations were logged. I even tried to break the E44 pile-up. I didn't succeed, which is no surprise at all. I heard but did not work (or call) stations further afield, including KH6, VK and JA.

On 30 meters I had less success. The lower bands can be deceiving since the noise level is higher. Even an inefficient antenna can hear with similar S/N ratios as better antennas. The real test is whether the DX hears you. I worked a couple of Europe and Caribbean stations, but it wasn't easy. I have to score this antenna as poor on 30 meters. Or use more than 10 watts to compensate for the losses.

With that I will put to bed the discussion of this antenna. However I am not quite done with delta loops, which I still expect to put one or more up for the lower bands where low horizontally-polarized antennas suffer. Who knows, maybe a dipole will do better there as well. I intend to test this at some point.

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