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Posting-Version: version B 2.10.2 9/18/84; site brl-tgr.ARPA
Path: utzoo!linus!philabs!cmcl2!seismo!brl-tgr!floyd
From: floyd@brl-tgr.ARPA (Floyd C. Wofford )
Newsgroups: net.physics
Subject: Re: Cancelling EM waves
Message-ID: <1582@brl-tgr.ARPA>
Date: Wed, 18-Sep-85 15:36:28 EDT
Article-I.D.: brl-tgr.1582
Posted: Wed Sep 18 15:36:28 1985
Date-Received: Sat, 21-Sep-85 11:10:48 EDT
References: <543@sri-arpa.ARPA> <542@petrus.UUCP>
Reply-To: floyd@brl-tgr.ARPA (Floyd C. Wofford (MMW) <floyd>)
Organization: Ballistic Research Lab
Lines: 17

In article <542@petrus.UUCP> mwg@petrus.UUCP (Mark Garrett) writes:
>++
>a point where you are an even multiple of wavelengths from both transmitters,
>and will observe twice the amplitude.  Just as much energy is being radiated,
>but the pattern will be more complicated.  This is used quite often for AM
>(FM?) broadcasts.  There will be an array of six or eight transmitter towers
>placed such that the resulting waves cancel in one direction and add in the
>other (more or less)........
>-Mark Garrett

Pick up an antenna theory book.  This will describe the large scale effects
of such a system.  A line of radiating dipoles is a linear array.  The same
thing is done in two dimensions, hence a planar array.  Vary the phase of
the currents in the dipoles and space the dipoles some integer multiple
of half a wavelength (arbitrary).  You will be able to direct the main
beam (or beams, depending on the array size) to any direction.  This type
of antenna forms the front-end of some of the nicest radars built.


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