On Mon, 10 Aug 2009 06:29:03 -0500, Bob Bob
wrote:

Keep in mind too that cross polarized (circular) point to point links
(ie with a CP ant at each end) suffer from odd reflection attenuation
(ie the polarization sense gets reversed by reflection)

That's one of the big advantages of CP for point to point links. The
polarization reversal on odd numbered reflections means that multipath
is greatly reduced.

In 70cm UHF experiments I did back in the 80's I found out that a
horiz-horiz system (base to mobile) outperformed a circular-circular by
at least 12dB when moving. This wasnt actually the base reason for the
experiments so I didnt make accurate measurements.

The book "Microwave Mobile Communications" by Willaim C. Jakes Jr
(1974) has a few words on the subject. As I recall, Ma Bell concluded
that neither linear or cirucular polarization is good enough and that
some form of diversity is required.

Incidentally, "The Practical Handbook of Amateur Radio FM and
Repeaters" (1981) (Tab 1212) by Pasternak and Morris, has chapter 31
on CP tests on the WA6VQP repeater on Loop Mtn. They draw a polar
plot of the measured repeater antenna pattern and note that they get
the typical "flower" pattern, with attendent deep nulls with linear
polarization. With CP, the nulls are far less pronounced.

My tinkering in the 1970's was specifically to reduce severe multipath
fades along a section of freeway in Smog Angeles. It worked, but with
some loss in signal stength from mismatched linear and CP
(theoretically -3dB).

OT Art, but I hope interesting.

How random is the propogated linear antenna HF wave polarization and
does it vary much with single hop and/or high angle? That might be a
starting point for determining how useful CP on HF might be.

I did some tinkering with measuring the polarization of skywave
signals using a rotating loop antenna. Including Faraday rotation, my
guess(tm) is that it's quite random and changes rapidly.



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