🚀 go-pugleaf

In article <797@ryn.esg.dec.com>, pierson@cimnet.dec.com writes:
> 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...

Wrong.  The tunnel/bridge is a waveguide, below cutoff at 550KHz-1.6MHz,
above cutoff at 88-108MHz.

> >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...

Actually, both AM & FM (and all electromagnetic waves) are governed by
Maxwell's equations, and propagate in the same way.  However low frequencies
tend to follow the curvature of the earth, while higher frequencies tend
to be more "line-of-sight".  But this doesn't come into play here...

Also, you can usually receive with some shielding between the transmitter
and receiver due to refraction and reflections.

> >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...

Often, such occurrences are actually due to destructive interference - a 
reflection and the original signal are 180 degrees out of phase and cancel,
giving a very sharp null.  This is also referred to as multipath.

John Kraus told me a story about the World's Fair in the New York/New Jersey
area where TV was first demonstrated to the public (ca. 1938).  An impressive
but unneccessarily large receiving antenna was erected and connected to the
receiver.  The transmitter was fairly high power and located only about 10
miles away.  When all was powered up, only a very faint signal was received.

The engineers were confounded.  They hooked up a couple of pieces of wire to
the receiver (ala today's "rabbit ears") and got a great picture.  Turns out
they used this for the demo, but left the large antenna up anyhow - it was
more dramatic.

Turns out the engineers later discovered a large, highly reflective, salt
bed was just under the ground halfway between the receiver and transmitter.
The direct and reflected signals were cancelling at the position of the
large receiving antenna.

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

Has nothing to do with why you get fading under a bridge.

> >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.

AGC does NOT bring the signal up - ever.  It only reduces strong signals so
they don't swamp the receiver.

> 
> Thanks
> dave pierson			|the facts, as accurately as i can manage,
> Digital Equipment Corporation	|the opinions, my own
> 600 Nickerson Rd
> Marlboro, Mass
> 01752				pierson@cimnet.enet.dec.com


-- 
Mike Pflueger @ AG Communication Systems (formerly GTE Comm. Sys.), Phoenix, AZ
  UUCP: {...!ames!ncar!noao!asuvax | uunet!hrc | att}!gtephx!pfluegerm
  Work: 602-582-7049        FAX: 602-581-4850      Home: 602-439-1978
Packet: WD8KPZ @ W1FJI     Internet: PLEASE USE UUCP PATH (NOT INTERNET)!