Xref: utzoo sci.electronics:9507 rec.ham-radio:16761
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From: karn@jupiter..bellcore.com (Phil R. Karn)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: naive question about RF propagation
Message-ID: <18988@bellcore.bellcore.com>
Date: 16 Jan 90 02:22:45 GMT
References: <28774@amdcad.AMD.COM>
Sender: news@bellcore.bellcore.com
Reply-To: karn@jupiter.bellcore.com (Phil R. Karn)
Organization: Bell Communications Research, Inc
Lines: 29

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?

I might as well take a stab at this one.

The reason has to do with the relative wavelengths of the two signals;
the modulation method isn't nearly as important. Standard AM broadcast
signals have much longer wavelengths than FM. AM signals are "large"
relative to the dimensions of a bridge, while FM signals are "small".

Radio waves have trouble propagating through small (relative to their
wavelength) windows in conducting material. That's why you can easily
see into your operating microwave oven without frying your eyeballs. The
holes in the metal screen in the door are very large compared to the
optical wavelengths you see with, but they're very small compared to the
wavelength used for cooking.

I've noticed that you can see a difference when driving even between
signals at the opposite ends of the AM broadcast band. Stations at the
top end aren't nearly as affected by driving under a bridge as stations
at the low end. (Note that the AM broadcast band covers almost a 3:1
range in frequency and wavelength.)

Phil