[TowerTalk] ionosphere Re: More Interference Pattern

[Brian Beezley]

Jim Lux said:

"There are a fair number of people who have done polarization diversity
receiving. A big X is popular (because it's easier to build than a big 
cross, and the two sloped elements have similar ground effects)"

I've been interested in polarization diversity for HF reception for 
years, but I've never done anything about it. I too thought sloping 
dipoles would be the way to go and for the same reason. I'd slope them 
from each side of a single pole.

However, when fooling around on my computer one day with a tilted linear 
array over ground, I selected circular polarization by mistake. Before I 
corrected it, I noticed that the axial ratio was nonzero. That's 
impossible with linear elements, I thought. I reduced the model to a 
simple dipole tilted 45 degrees and still got a nonzero axial ratio 
broadside to the antenna. Within a few minutes I had hung a folded 
dipole for the FM broadcast band from my ceiling fan. It was easy to 
verify than when sloped, the antenna received CP. What generates it is 
the image antenna below ground, which is perpendicular to a wire sloped 
45 degrees, together with the phase delay for the ground reflection. 
Imperfect ground reflection yields elliptical polarization, which 
depends on the ground constants. I wrote it up here:

https://k6sti.neocities.org/tilted

A lot of people use slopers (quarterwave wires) or sloping dipoles on 
HF. Directivity is off the end but CP occurs broadside so it might not 
be obvious. Still, I've never seen a fading benefit reported. You'd 
think someone would have noticed.

So you don't need two wires to get CP. But with a single wire, you're 
stuck with RCP or LCP and no adjustment of axial ratio. What I always 
had in mind for HF CP was adaptive S/N maximization using two antennas. 
An algorithm adjusts the amplitude and phase of one signal path to 
maximize S/N. I'm not sure the best way to measure S/N, but if you 
adjust for minimum signal internally then flip the phase 180 degrees for 
the output signal, you at least maximize the amplitude, if not S/N. This 
system shouldn't be bothered by each antenna itself being CP.

I'm not sure how much benefit this system would provide in practice. On 
160m where fades can be excruciatingly long, it could help a lot if the 
fade was due to crosspolarization. At the very least it would be fun to 
play with.

Brian