Brad,
FM broadcasters do the dual polarization antenna thing. Actually, they
use circular polarization where the vertical and horizontal signals are in
quadrature, that is, 90 degrees out of phase with each other. If the signals
are in phase the result is a wave that has a 45 degree wave front. This
isn't the results that you want. With 90 degrees difference a single
polarity antenna (that's most of the common ones) will always receive an
equal signal regardless of it's orientation. This is good. Any other phase
difference will have peaks and valleys as the receiver is rotated.
The down side is that this signal will be 3 db lower than if both source
and receiving antennas are the same polarity. Broadcasters can make up for
this 3 db loss by doubling their power out. Their licensed ERP is the amount
in the horizontal pattern and they can run up to the same amount in the
vertical plane. A 50 thousand watt ERP station can run another 50 thousand
watts ERP in the vertical plane.
Getting a truly circularly polarized antenna can be a real challenge but
most of the modern ones are within a db or so. Years ago (1968 maybe?) we
experimented with interspersed horizontal and vertical bays, separate
verticals under the main horizontal array, and some other configurations.
Didn't work worth a hoot. Finally put up a roto-tiller type in 1983 and it
works OK.
Several people use what starts out as a horizontal circular dipole but
about one half of one end of the dipole is turned up and the other half is
turned down. I have no experience with them but they are popular among FM
broadcasters. They look like they might be an applicable design for either
two meters or UHF repeater antennas. I don't know that anyone has published
the design specs for them but they look very interesting. It may well be a
patented design, though, or an industrial secret. Maybe someone here has
some insight into these antennas. Or maybe someone has come up with a way to
build roto-tiller antennas.
Good luck with your antenna project. Keep in mind the desirability of 90
degree difference between vertical and horizontal patterns. Let us know how
you make out.
73,
Al, K9SI
Date: Wed, 14 Apr 2004 11:04:00 -0700 (PDT)
From: bradley glen <[EMAIL PROTECTED]>
Subject: dual polarisation uhf repeater antenna
In th the process of making my tests on a dual
polaarisation antenna for my uhf repeater (one of
many).
I am persueing something that I should have done a
long time ago.I have had reasonable success with
opting for a site that needed 270deg coverage which
made what I have done within reason.
I have taken a standard commercial 4-stack-removed the
top dipole-replaced it with a 6dB collinear for the
vertical polarisation.
The remaining three folded dipoles I have place in the
horizontal pane at 0-90-180.(still experimenting with
the spacing)
The idea and theory is to reduce the deep nulls in
mobile and portable comms.
I would like to hear from anyone that has any comments
and experience in doing this-suely I can not be the
first to do so.
I have a keen interest in dual pol antenna arrays so
if you have any articles I would appreciate the
sitesand your comments.
Regards
Brad Node : 8300
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