phil@pepsi.amd.com (01/13/90)
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.jans@tekgvs.LABS.TEK.COM (Jan Steinman) (01/16/90)
<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?> I'll leave the real answer to someone else, but thought readers might enjoy an anecdote on the subject. The following paraphrased letter to the editor appeared in Radio Electronics magazine in the mid-sixties: "When I drive through a tunnel, FM is okay, but all AM stations suddenly vanish. What should I do?" The editor at the time (whom I believe was the same guy that edits Popular Communications today) replied: "Stay away from tunnels! You're a menace to the entire AM broadcast industry!" (Hope this was helpful, Phil! :-) Jan Steinman - N7JDB Tektronix Electronic Systems Laboratory Box 500, MS 50-370, Beaverton, OR 97077 (w)503/627-5881 (h)503/657-7703
pierson@cimnet.dec.com (01/16/90)
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... >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... >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... >Or is it because AM shows the signal reduction more directly? AM does show changes in signal strength more directly. Thats why there is... >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. 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
pfluegerm@valley.UUCP (Mike Pflueger) (01/16/90)
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 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)!
karn@jupiter..bellcore.com (Phil R. Karn) (01/16/90)
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
wrf@mab.ecse.rpi.edu (Wm Randolph Franklin) (01/17/90)
In article <480ec7fd.15840@valley.UUCP> pfluegerm@valley.UUCP (Mike Pflueger) writes: > >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. Don't (some) subway systems use this so the crew can use radios? -- Wm. Randolph Franklin Internet: wrf@ecse.rpi.edu (or @cs.rpi.edu) Bitnet: Wrfrankl@Rpitsmts Telephone: (518) 276-6077; Telex: 6716050 RPI TROU; Fax: (518) 276-6261 Paper: ECSE Dept., 6026 JEC, Rensselaer Polytechnic Inst, Troy NY, 12180
pierson@cimnet.dec.com (01/17/90)
writes, in part: .. > >Don't (some) subway systems use this so the crew can use radios? >-- (this should probably be dragged over to rec.radio.shortwave, or rec.railroad 8)>>) The two systems i know of involve: Using wilfully leaky coax. An open weave braid allows noticeable amounts of signal out, and in. Some amplifiers and mixers and away you go... A system in NY City, allows the police to take their standard HT's into subway stations, while staying on the same channel. They found there was enough loss between topside and underground that they could equip each station with an above ground antenna, directional couplers, amplifiers, antenna(s) below ground. The loss "down the stairs" was enough to keep the underground transmitter from getting into the above ground receiver on the same frequency, and vice-versa... 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
carroll@bcsaic.UUCP (Jeff Carroll) (01/18/90)
<Mike Pflueger tells about Professor Kraus' solution to carrying broadcast signals in tunnels using a wire extending beyond the end of the tunnel...> Yup. Never thought about it myself, but that's a great idea. It'd be topologically equivalent to a coaxial cable, which has no cutoff frequency for the TEM mode. Now *here's* an interesting question, particularly for anyone like me who has spent half an hour in a tunnel during a traffic jam: Why do we allow tunnels to be built without such gizmos installed in them? Jeff Carroll carroll@atc.boeing.com
keithe@tekgvs.LABS.TEK.COM (Keith Ericson) (01/19/90)
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 Standard Smileys Apply :-)
tjw@unix.cis.pitt.edu (TJ Wood WA3VQJ) (01/30/90)
In article <19106@bcsaic.UUCP> carroll@bcsaic.UUCP (Jeff Carroll) writes: > Now *here's* an interesting question, particularly for anyone >like me who has spent half an hour in a tunnel during a traffic jam: >Why do we allow tunnels to be built without such gizmos installed in >them? In Pittsburgh, a city of tunnels, there is a wire running down the Liberty Tubes, and the antenna works well! But, there's none running through the Fort Pitt Tunnels or the Squirell Hill Tunnels and all you get is static. Terry WA3VQJ -- INTERNET: tjw@unix.cis.pitt.edu BITNET: TJW@PITTVMS CC-NET: 33802::tjw UUCP: {decwrl!decvax!idis, allegra, bellcore}!pitt!unix.cis.pitt.edu!tjw And if dreams could come true, I'd still be there with you, On the banks of cold waters at the close of the day. - Craig Johnson
elliott@optilink.UUCP (Paul Elliott x225) (01/31/90)
An engineer I once worked with told me that the tunnels near the Presidio
(military base in San Francisco) had these wires installed during WW2 so
the base commander (or whatever the military title is) could stay in contact
via two-way radio while driving around. I believe that the tunnel wires
originally had a tuned external antenna feeding the wire. You can see what
looks suspiciously like a diplole cut for the middle of the FM broadcast band
on the outside face of some tunnels. It probably works as an untuned antenna
for the AM band.
--
Paul M. Elliott Optilink Corporation (707) 795-9444
{pyramid,pixar,tekbspa}!optilink!elliott
"The dog ate my disclaimer."