On Sun, 14 Feb 2010 20:51:21 -0800 (PST), Art Unwin
wrote:

Has there ever been a study that shows the relative consistency of
received signal polarity to see if it would be advantageous for multi
polarity receive antennas?

Yes. I did one for a company doing exactly that at various
frequencies between 120 to 450Mhz. Sorry, but I don't have a copy of
the report. For convenience, we use ham frequencies for most of the
testing. What we found is that once a signal is reflected, the
reflected signals polarization is fairly random. There are few flat
plate reflectors in both man made and natural objects. The measured
result was a mess of varying polarization angles.

You can expect similar results for HF signals reflected off the
ionosphere with the added complexity of Faraday rotation.

However, it is beneficial to build polarization insensitive antennas.
In a common dipole, there's very little loss for polarization mismatch
until you get very close to perpendicular. There, the signal drops
off quickly. Filling in this hole is considered to be a good thing.

You can get a crude idea of how it works using an Adcock DF antenna
array, or just two cross polarized dipoles. Since you're not building
a direction finder, the crossed dipoles are easier to explain. Just
setup two perpendicular dipoles with the center feeds fairly close
together. Connect two well matched receivers to the two antennas.
Connect the IF or audio outputs to the vertical and horizontal of an
oscilloscope. The resulting Lissajous pattern will give you a rough
idea of the polarization (assuming the signal arrives from above).
Pick a strong steady signal like WWV. You'll probably see the
polarization change erratically when the skip is in. (Last time I did
this was 20 years ago). You'll also see that vertical and horizontal
parts of the Lissajous display to wander around in amplitude fairly
independently. This is the main advantage of a polarization
independent antenna. The antenna will automagically select the
strongest polarization to feed the receiver.

There are circularly polarized HF antennas, but I'm too lazy to Google
for them tonite.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558