On Mon, 28 Sep 2009 14:13:31 -0700 (PDT), wrote:

I have been looking for a decent CP design to try out on my
repeaters .
For many years I have been looking at the reasoning for FM
broadcasters having used CP pol with great success.

I also have a small indication that linear antennas (eg:linear
collinears)and non-linear antennas(eg:folded loop dipoles) by design
may have slightly different characteristics in the far field that
tend towards a greater degree of cross/CP from the mechanical design .

Methinks you'll be better off with eliptical polarization, where
perhaps 70% of the power is vertically polarized, and the rest is
horizontal. The idea is that most of the energy will be properly
recieved by the traditional vertically polarized mobile antenna. The
remainder will be to fill in the gaps, where the polarization changes
to something not so vertical.

Search Google patents:
http://www.google.com/patents
for "omnidirectional circular polarized antenna". Also, look at FM
broadcast xmit antennas. Some really strange stuff there.

Anyone had good success with installing a CP repeater antenna(2m or
70cm) to assist with the deap fade nulls in mobile uplink to the
repeater.

Yes. I can't find my previous rant on the subject, so I'll try again.
Around 1968 thru 1970, I was at Cal Poly Pomona doing some experiments
with CP UHF antennas. The problem was that the local repeater was
experiencing deep Raleigh fading as mobiles moved along the San
Bernardino Freeway. The ideas was that CP would reduce these fades so
that listening to the repeater didn't sound like it was accompanied by
a machine gun. It worked but with a price. The fades were
dramatically reduced, but so was the coverage area. Antenna gain was
down at least 3dB as was maximum range for the repeater. I thought it
was a good tradeoff, but not everyone agreed. The stations that were
marginal (usually because they were too lazy to install a proper
antenna) were gone. However, the reliability of the local mobiles was
greatly improved. Range won over fade reduction and the antenna was
replaced after a few months of testing.

There was an article on the subject in "The Practical Handbook of
Amateur Radio FM and Repeaters" by Bill Pasternack and Mike Morris.
1981 by Tab Books.
http://openlibrary.org/b/OL4421123M/practical_handbook_of_amateur_radio_FM___repeaters
http://www.alibris.com/booksearch?qsort=p&isbn=0830612122
Sheesh. I paid about $5 at a hamfest.

Since it's not online, I scanned the applicable chapter, chopped out
some irrelevant junk, and posted them to:
http://802.11junk.com/jeffl/circular-polarization/
I'll convert them to a single PDF after I figure out why Irfanview is
generating garbage. Bug me if you have trouble reading the JPG's.

I want to try rhp for TX and lhp for RX - anyone tried this
combination before?

Yep. I tried that to improve TX-RX isolation. I gave up. Mounted on
a single pole, with separate phasing harnesses, I never could get the
antennas anywhere near 50 ohms. There was also just too much coupling
between the RH and LH sections for that to work. Another explanation
might be that at the time, I didn't have a clue what I was doing, but
we won't go there. The rather high VSWR certainly didn't help with
the isolation as it detuned the tx and rx cavities (formerly a
duplexer). Some rather crude testing showed that we were better off
with a single antenna, the stock duplexer, and a single CP
polarization. There was also a problem with distant stations. They
wanted to use CP on the base station to reduce the effects of
multipath (usually off the infamous L.A. smog inversion layer). That
would have required two antennas for each base station. I suggest
that you stick with one circular polarization for now.

Incidentally, note the scanned photo of the Loop Mountain site
http://802.11junk.com/jeffl/circular-polarization/cp-02.jpg
and compare it with the previous intermod factory and post-storm
removal exercise at:
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn03.html
One of the reasons I'm current intact and still sane is that I don't
own any repeaters and find excuses not to do any tower work.


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