Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!asuvax!mcdphx!hrc!valley!pfluegerm From: pfluegerm@valley.UUCP (Mike Pflueger) Newsgroups: sci.electronics Subject: Re: naive question about RF propagation Message-ID: <4817a912.15840@valley.UUCP> Date: 17 Jan 90 17:51:38 GMT References: <797@ryn.esg.dec.com> Organization: AG Communication Systems Lines: 85 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)!