Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site unccvax.UUCP 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. .