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From: pierson@cimnet.dec.com
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: naive question about RF propagation
Message-ID: <797@ryn.esg.dec.com>
Date: 15 Jan 90 22:59:08 GMT
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Organization: Digital Equipment Corporation
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In article <28774@amdcad.AMD.COM>, phil@pepsi.amd.com writes...
>We all know that AM broadcast radio fades when you drive under a bridge
>but FM doesn't tend to nearly as much. Why is this?

	Think of it as a shielded enclosure.  Shielding effectiveness varies
	with the size of the opening (road, in this case) with "size"
	_measured_ in _wavelengths_ at the _frequency_ of interest.  In the
	case of the bridge, this makes it a good shield at 0.55-1.6MHz and a
	poor one at 88-108MHz.  As a useful rule of thumb, any opening with a
	single dimension greater 10% of a wavelength will start to leak at 
	that frequency, and above...

>Does RF at the FM broadcast frequencies propagate better?
	I would not have used the word "better".  Each does propagates 
	differently.  IF the 88-108MHZ signal were "off axis" to the road
	opening and IF there were no random metal around to reflect it, it to
	would likely fade...

>I thought the higher the frequency, the more it exhibited "line of sight"
>behavior.
	I once lost the base transmissions from a nearby city police 
	department, while still getting the mobiles.  A little mapwork showed
	that a new, high level, Interstate bridge was blocking the line of
	sight to the (high) base antenna.  The mobiles were radiating through
	the space UNDER the bridge...

>Or is it because AM shows the signal reduction more directly?
	AM does show changes in signal strength more directly.  Thats why there
	is...

>Shouldn't AGC take care of that?
	AGC, however, if the signal strength goes down too far, the AGC can't
	bring it back up.


Thanks
dave pierson			|the facts, as accurately as i can manage,
Digital Equipment Corporation	|the opinions, my own
600 Nickerson Rd
Marlboro, Mass
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