Xref: utzoo sci.electronics:9506 rec.ham-radio:16757
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!asuvax!hrc!valley!pfluegerm
From: pfluegerm@valley.UUCP (Mike Pflueger)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: naive question about RF propagation
Message-ID: <480ec7fd.15840@valley.UUCP>
Date: 15 Jan 90 23:29:16 GMT
References: <28774@amdcad.AMD.COM>
Organization: AG Communication Systems
Lines: 38

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?
> 
> --
> Phil Ngai, phil@diablo.amd.com	{uunet,decwrl,ucbvax}!amdcad!phil
> Peace through strength.


Mostly it's due to the frequency.  The tunnel (or bridge) can be thought
of as a too-small waveguide - the signal is cut off.  FM wavelengths are
much shorter, and the tunnel passes them.

Practically, some signal will get into the tunnel, but you receiver is
not sensitive enough to receive it, so the AGC will still not work running
wide open.

I had a friend who was a DJ for an AM station, and when he rode in my car,
you actually couldn't hear him talk when we drove under a bridge.  (This
is an old joke, but I felt it applied here)

Actually, I had this as an EE problem to solve in college - how the tunnel
could be modified to pass signals of a frequency above cutoff.  One way
was to modify the u (mu) of the material in the tunnel - such as a special
gas, or styrofoam, obviously impractical.  The real way was to run a wire
through it which extended outside the tunnel.  The wire would conduct the
RF into the tunnel and allow reception.  I forget the exact details on
length of the wire, but this was in a course taught by John Kraus (Mr.
Antenna) himself, so I'm confident of the solution.
-- 
Mike Pflueger @ AG Communication Systems (formerly GTE Comm. Sys.), Phoenix, AZ
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