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From: hoffmann%marie@mc.lcs.mit.edu (Ron M. Hoffmann)
Newsgroups: net.ham-radio
Subject: Broadcast polarization
Message-ID: <1325@brl-tgr.ARPA>
Date: Tue, 7-Jan-86 16:31:20 EST
Article-I.D.: brl-tgr.1325
Posted: Tue Jan  7 16:31:20 1986
Date-Received: Fri, 10-Jan-86 06:43:58 EST
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The use of circular polarization is television broadcasting
has been, as far as I know, restricted to certain subscription
stations operating in the UHF band.  By transmitting with, for
example, right-hand circular-polarization and equipping the
receivers with a similar antenna, they can effect considerable
attenuation of odd-number reflection ghosts, since these would
have opposite polarization sense.  And actually, it is really
just the first reflection that is a problem, second, third...
bounces are usually too far attenuated to create a visible
problem.  Actually, the visual aspect of the problem is only
secondary to the technical problem of decoding suppressed-
synch type scrambled transmissions.  Multipath makes decoding
a stable image more difficult.

Now, in the case of broadcast FM stations, some proportion of
the carrier is transmitted in both the horizontal and vertical
planes.  This is mostly as a concession to mobile receivers
that commonly posses vertical whip antennas.