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From: keithe@tekgvs.LABS.TEK.COM (Keith Ericson)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: naive question about RF propagation
Message-ID: <6699@tekgvs.LABS.TEK.COM>
Date: 19 Jan 90 01:17:32 GMT
References: <28774@amdcad.AMD.COM>
Reply-To: keithe@tekgvs.LABS.TEK.COM (Keith Ericson)
Followup-To: sci.electronics
Distribution: na
Organization: Tektronix, Inc., Beaverton,  OR.
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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? Does RF at the FM
>broadcast frequencies propagate better? I thought the higher the frequency,
>the more it exhibited "line of sight" behavior. Or is it because AM
>shows the signal reduction more directly? Shouldn't AGC take care of
>that?

It's because the AM signals are low frequency = long wavelength and
those long waves can't fit into the opening of the tunnel like the
hi-frequency FM signals can.

Similar reason FM/high-frequency signals don't propogate as far as
AM//low-frequency:

   Low-frequency Radio signal-----+
                                  |
                                  V
                                  **                         
                               *       *
   Hi-frequency signal-+     *     ^     *
        _   _   _   _  V_   _    /   \    *
XMTR   | |_| |_| |_| |_| |_| |_/       \   *                 RCVR
                         *   /           \   *                *
         *              *  /  MOUNTAIN     \  *              *
           *          *  /                   \  *           *
            *      *   /                       \  *       *
               **    /                           \  **
                   /                               \
                 /                                   \ 
 
kEITHe
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