Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site petrus.UUCP Path: utzoo!watmath!clyde!burl!ulysses!ucbvax!decvax!bellcore!petrus!mwg From: mwg@petrus.UUCP (Mark Garrett) Newsgroups: net.physics Subject: Re: Cancelling EM waves Message-ID: <542@petrus.UUCP> Date: Mon, 16-Sep-85 11:45:12 EDT Article-I.D.: petrus.542 Posted: Mon Sep 16 11:45:12 1985 Date-Received: Wed, 18-Sep-85 03:18:48 EDT References: <543@sri-arpa.ARPA> Organization: Bell Communications Research, Inc Lines: 25 ++ > Here's the question... If I place [two identicle electromagnetic sources > operating at some frequency] half a wavelength apart so that > they are 180 degrees out of phase, the waves will cancel. Now I appear > to be getting no energy out of this system, at least not in the form of > EM waves. I am still putting as much energy into the system. All I did > was move one of the devices. What is happening to the energy? Is there > an output in another form of energy? Is it building up in one of the > devices somewhere? > Dan The key is to think of the problem in three (or even two) dimensions. True, along the line passing through the two points, you will have exactly canceled the transmission; and also at certain points in the plane or space. But generally, there will be regions of destructive interferance and of constructive interferance. That is, you can find 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). This way, a station in New York can transmit over an area of which New York is not the center, to avoid interference with a station in Philly while covering Connecticut. -Mark Garrett Brought to you by Super Global Mega Corp .com