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Path: utzoo!watmath!clyde!bonnie!akgua!mcnc!unccvax!dsi
From: dsi@unccvax.UUCP (Dataspan Inc)
Newsgroups: net.physics
Subject: Re: Cancelling EM waves
Message-ID: <296@unccvax.UUCP>
Date: Tue, 17-Sep-85 09:03:03 EDT
Article-I.D.: unccvax.296
Posted: Tue Sep 17 09:03:03 1985
Date-Received: Thu, 19-Sep-85 04:43:17 EDT
References: <543@sri-arpa.ARPA>, <542@petrus.UUCP>
Organization: UNC-Charlotte
Lines: 35


     True, directionalisation of the antenna system is common in AM
stations.  However, the use of directional antennas in FM is not 
very common.  The problem is obtaining sufficient directionalisation
in the XY-plane (say the Z-axis is normal to the point on the earth
where the tower is) without introducing severe "multipath like" distortion.
It would, for example, be very very easy for an FM station to just
stick up an ordinary Yagi-Uda directional antenna, but as you were driving
on a road in the "minor lobes" or nulls, you would get severe interference 
due to constantly changing phase and amplitude from the various elements.

     However, directionalisation in the family of Z planes is common,
to obtain antenna gain. Here, you are simply "squashing" the doughnut
of a single dipole as you add more bays (not exactly true) so that
you don't radiate all that RF into free space.  The rate of change of
signals from the various elements would be very small (you'd have to climb
Pike's Peak in 2 minutes or so to get the "picket fencing" in thsi
plane!)

     In addition, most (if not all) modern FM stations use circular
polarisation, which is very tough to directionalise in the XY plane.
(I have yet to see "Cavity Backed Radiators" (tm) used in FM, which
have approximately a 120 degree coverage range. They also have no
gain over a dipole) FM stations are solely assigned on distance / power
criteria, and short spacing is rarely a problem (except in North Carolina,
and South Carolina).

     By the way: just what is it that is circularly polarized in a CP
antenna? Is it just that the vertical and horizontal E-fields are lagging
in phase quadrature (90 degrees) or something? A good (and rational)
explaination of this would be very helpful.

David Anthony
DataSpan, Inc.
.