[sci.electronics] naive question about RF propagation

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.

myers@hpfcdj.HP.COM (Bob Myers) (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? 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

The "higher frequency = more line of sight" is basically correct, but it's
also true that the higher the frequency, the shorter the wavelength - and
shorter-wavelength EM waves can "get around" edges and "into" semi-enclosed
spaces better than their longer cousins.  (Think of waveguides, etc..)


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers%hpfcla@hplabs.hp.com               |  sentient life-form on this planet.

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

reynhout@wpi.wpi.edu (Hagbard Celine) (01/17/90)

In article <6671@tekgvs.LABS.TEK.COM> jans@tekgvs.LABS.TEK.COM (Jan Steinman) writes:
>
>"When I drive through a tunnel, FM is okay, but all AM stations suddenly 
>vanish.  What should I do?"
...(He wasn't asking the question, but relating someone else's question.)

   ...which reminds me of something else...  Driving to downtown Boston from
Rhode Island (geographical data given simply because I don't know what tunnel
it is I drove under, what highway I was on, or anything) I drove through a long
underwater tunnel.  I was listening to an AM station's Boston Celtics pregame
show.  When I entered the tunnel, the AM station's signal was immediately
replaced by (I think) a traffic report, obviously very local to the tunnel.
This same broadcast was on EVERY AM frequency.  At the time, I was confused.

   From what I understand now, the AM signal dropped considerably, allowing a
lower-power broadcast at the AM demodulation circuit's RESONANT frequency to
be picked up.  This is done quite intentionally.  For some reason, 660Hz comes
to mind, but this doesn't sound right, and I don't know enough about rf to
say one way or another....but I'm sure someone (almost everyone?) out there
does...

   Any further explanation (assuming that what I have written is even close
to correct) would be appreciated...

   Andrew

-- 
   Andrew Reynhout   (Internet: reynhout@wpi.wpi.edu)
                     (BITNET: reynhout@wpi.bitnet)
   All hail Eris!

pfluegerm@valley.UUCP (Mike Pflueger) (01/18/90)

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)!

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

night@pawl.rpi.edu (Trip Martin) (01/18/90)

reynhout@wpi.wpi.edu (Hagbard Celine) writes:

>   From what I understand now, the AM signal dropped considerably, allowing a
>lower-power broadcast at the AM demodulation circuit's RESONANT frequency to
>be picked up.  This is done quite intentionally.  For some reason, 660Hz comes
>to mind, but this doesn't sound right, and I don't know enough about rf to
>say one way or another....but I'm sure someone (almost everyone?) out there
>does...

It has nothing to do with the AM demodulation circuit's resonant frequency 
(I assume you mean the IF).  Basically, the traffic report signal was strong
enough to make it through all the filters in the AM receiver (these filters
tend to be poor anyhow, compared to most other kinds of radios), and still be
strong enough to reach the detector with decent strength.  Since you're in 
a long tunnel, it doesn't have to fight off other stations.

-- 
Trip Martin  KA2LIV                           Trip_Martin@mts.rpi.edu 
night@pawl.rpi.edu                          night@uruguay.acm.rpi.edu
** Murphy's Law of Thermodynamics: Things get worse under pressure **

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."