AM radio reception inside passenger planes?
 

I have no trouble receiving FM radio broadcasts on a small am/fm radio
I sometimes listen to while onboard commercial jet airliners (flying
at cruise altitude), but I never seem to be able to pick up AM radio
stations. It's just static across the AM band.

Any explanation for this?

Any explanation for this?

FM radio generally operates at longer range than AM radio, however it's
limited to line-of-sight. However, when you're 40,000 feet up you can "see"
a lot of transmitters hence the FM signals.

Cheers,
Geoff Glave
Vancouver, Canada

In message HxHud.9448$eb3.8331@clgrps13, Geoff Glave
writes
Any explanation for this?

FM radio generally operates at longer range than AM radio, however it's
limited to line-of-sight. However, when you're 40,000 feet up you can "see"
a lot of transmitters hence the FM signals.

Cheers,
Geoff Glave
Vancouver, Canada



The window holes are much too small to let the much longer wavelengths
of the 'AM' signals through. The body of the plane is a very effective
screen. The 'FM' signals can squeeze in, but it helps if you have a
window seat. I've also listened to SW in the middle of the Atlantic.

Flying from the UK to Florida, on the other side of the Atlantic the
first FM stations you hear are usually speaking French (from Quebec)
It's quite alarming!
Ian.
--

"Geoff Glave" wrote in message
news:HxHud.9448$eb3.8331@clgrps13...
Any explanation for this?

FM radio generally operates at longer range than AM radio, however it's
limited to line-of-sight. However, when you're 40,000 feet up you can "see"
a lot of transmitters hence the FM signals.

Cheers,
Geoff Glave
Vancouver, Canada



I reckon you just answered the wrong question !!

The reason A.M. radio can not be received in a plane is that it is a Faraday
cage to the (lower frequency) A.M
frequencies, whereas the VHF frequencies can just about get through the
windows.

There may be a bit of frequency / range issue as well but top band and 80 mtrs
gets across the pond so
I don't think this is the issue here...

Nick

No one has mentioned that in many cases you need the pilot's permission to
operate a radio or other electronic device for that matter on a commercial
airliner. That includes AM/FM radios.

Radio emissions may screw up the plane's avionics.

See URL:
http://www.fordyce.org/scanning/scan.../scan_fly.html

It sez:
"The FAA does not allow inflight use of walkie-talkies, radio controlled
toys, AM/FM radios, portable telephones, or portable television sets, all of
which may affect aircraft radio and navigation equipment"

Also cruise ships may deny use of two way (FRS) or ham radios -- always
check with the communications officer.

For Hams always check with the person in charge on any commercial
transportation, busses, taxi's, ships planes etc.

Yeah yeah I know you did it without getting permission, but read the URL as
to what airlines have published.

And I know from personal experience that some cruise lines do not allow FRS
or ham radios transmissions.

--
RF Gotta Go SomeWhere



"nick smith" wrote in message
...

"Geoff Glave" wrote in message
news:HxHud.9448$eb3.8331@clgrps13...
Any explanation for this?

FM radio generally operates at longer range than AM radio, however it's
limited to line-of-sight. However, when you're 40,000 feet up you can
"see"
a lot of transmitters hence the FM signals.

Cheers,
Geoff Glave
Vancouver, Canada



I reckon you just answered the wrong question !!

The reason A.M. radio can not be received in a plane is that it is a
Faraday
cage to the (lower frequency) A.M
frequencies, whereas the VHF frequencies can just about get through the
windows.

There may be a bit of frequency / range issue as well but top band and 80
mtrs
gets across the pond so
I don't think this is the issue here...

Nick

Some Guy wrote:
I have no trouble receiving FM radio broadcasts on a small am/fm radio
I sometimes listen to while onboard commercial jet airliners (flying
at cruise altitude), but I never seem to be able to pick up AM radio
stations. It's just static across the AM band.

Any explanation for this?

Yes, there is. The AM cops have figured you out, since what you are
doing is illegal on commercial airliners. The FM cops are a little
slower, but they will pull the plug on you also, eventually.

Seriously, though, you are inside a metal cigar tube you call an
airplane, and you are being shielded by the body of the aircraft.
Although windows (portholes, not Gates), and the metal itself, don't
block out signals completely, you will see an effect from this (look up
"Faraday Cage" on google). AM broadcast is a very long wavelength
(hundreds of meters long) whereas FM broadcast is a smaller wavelength
(around 3 meters). If you were trying to throw a bunch of marbles
through an upstairs window, you would probably be able to do it. But if
you were trying to throw a bunch of beachballs through an upstairs
window, it wouldn't be as easy, right?

The aperture is the important issue. Although the airplane is not a
completely shielded RF-proof "screen room", it acts somewhat like one.
That is why avionics antennas are on the outside of the plane, not
inside. That is also why there is a teeny mesh grid in the door of your
microwave oven - they have to be that small to block the microwaves.

Using my example befo if you are throwing beachballs (AM broadcast),
or marbles (FM broadcast) or a handful of sand (microwaves), how small
would you want the window to be in order to block it?

OK, getting back to my first paragraph, if you are ever on a plane with
me, please let me know, so I can take the next flight. The local
oscillator of FM receivers is often on the same frequency as the VOR
stations that airplanes use to naviagate with, and can cause
interference. There are failsafe solutions that the pilot has, to deal
with loss of VOR coverage, but I don't want to depend on them because
you are listening to gangster rap at 32,000 feet. Get an iPod or something.

All the best,
Dave

On Sat, 11 Dec 2004 14:11:44 -0800, "Ed Price"
wrote:

You
are asking him to allow a potentially dangerous device to be operated just
for your convenience and entertainment. Switch roles for just a minute.

Hi Ed,

This would make sense (to switch roles) if the administration hadn't
trumped that call. Reports recently indicate that the FAA may soon
allow anyone, anytime, to make cell phone calls while in flight.

Anything goes for a price. The FDA has proven that it is no longer
the watchdog of medicine, and the FCC is the gateway for spectrum
bargains and marketplace sweeps.

With these acronyms, one may well wonder what the "F" stands for.

73's
Richard Clark, KB7QHC

What a load of horse ****.

You guys are acting as if the engines and flight control surfaces of
an aircraft are intimately tied to the plane's radio receiver, and the
slightest odd or out-of-place signal that it receives is enough to
send any plane into a tail spin.

All this while the air travel industry is considering allowing
passengers to use their own cell phones WHILE THE PLANES ARE IN FLIGHT
by adding cell-phone relay stations to the planes and allowing any
such calls to be completed via satellite. So I guess the feeble
radiation by my FM radio (powered by 2 AAA batteries) is enough to
cause a plane to dive into the ocean, but the guy next to me putting
out 3 watts of near-microwave energy is totally safe.

What about my hand-held GPS unit? Any chance me using it (during all
phases of a flight, which I do routinely) will result in a one-way
ticket to kingdom come?

Getting back to the original question (poor to non-existant AM
reception), I understand the idea of aperature and long wavelenths of
AM radio and the size of airplane windows - but what about the effect
of ALL the windows on a plane? Don't they create a much larger
effective apperature when you consider all of them? And since the
plane isin't grounded, isin't the exterior shell of a plane
essentially transparent to all RF (ie it's just a re-radiator) because
it's not at ground potential?

On Sat, 11 Dec 2004 19:10:18 -0500, Some Guy wrote:

Getting back to the original question (poor to non-existant AM
reception), I understand the idea of aperature and long wavelenths of
AM radio and the size of airplane windows - but what about the effect
of ALL the windows on a plane? Don't they create a much larger
effective apperature when you consider all of them?

This becomes a matter of the distance between them and the phase
separation at any wavelength. What you describe is a common technique
for coupling power between waveguides (in what are called directional
couplers). However, this is not the same thing as accumulating and
enlarging an opening because such couplers will add energy in one
direction, and subtract it in the other (which makes for their
directionality).

And since the
plane isin't grounded, isin't the exterior shell of a plane
essentially transparent to all RF (ie it's just a re-radiator) because
it's not at ground potential?

Ground does not always mean "at one with the dirt and rocks." At one
time it did, when cowboys put up talking wires, and indians pulled
them down. Ground has since come to mean "common" (which when you
think of it, brings us back to dirt, metaphorically). Common means
that everything is at the same potential. If there is no potential
difference, then there is no way to measure a voltage based signal.
In other words, it's a massive short circuit, and the only way to
sense a signal is to inductively couple to the short circuit current.

This takes us to the second killer courtesy of physics. High
frequency current travels on the surface of smallest, positive radius.
AM frequency qualifies here in spades, even though it is
conventionally called not HF but MF (even VLF qualifies as High
Frequency in this context). The aircraft frame thus presents both
curvature and radius such that the current confines itself to the
outside of the shell with an inclination for the narrow wings and tail
section, rather than the elongated body.

You might be tempted to inductively tap into this frame current, but
then you are on the negative, inside radius of the current carrier
(makes the tube interior self-shielding). Whatever current is
flowing, is on the outside of the skin, not the inside - that is,
until we consider skin depth and penetration. But then it appears
that experience described here suggests that not much of that frame
current penetrates inside.

73's
Richard Clark, KB7QHC

Some Guy wrote:
What a load of horse ****.

You guys are acting as if the engines and flight control surfaces of
an aircraft are intimately tied to the plane's radio receiver, and the
slightest odd or out-of-place signal that it receives is enough to
send any plane into a tail spin.

All this while the air travel industry is considering allowing
passengers to use their own cell phones WHILE THE PLANES ARE IN FLIGHT
by adding cell-phone relay stations to the planes and allowing any
such calls to be completed via satellite. So I guess the feeble
radiation by my FM radio (powered by 2 AAA batteries) is enough to
cause a plane to dive into the ocean, but the guy next to me putting
out 3 watts of near-microwave energy is totally safe.

What about my hand-held GPS unit? Any chance me using it (during all
phases of a flight, which I do routinely) will result in a one-way
ticket to kingdom come?

Too bad it's not that simple. But if you're really into this kind of
argument, do a groups.google.com search of the sci.geo.satellite-nav
newsgroup. There you'll find endless argument, speculation, and
rationalization ranging from well informed to completely clueless.
There's surely more than ample ruminating there to satisfy anyone,
regardless of your orientation or clue level; it's surely not necessary
to do it all over again here.

Getting back to the original question (poor to non-existant AM
reception), I understand the idea of aperature and long wavelenths of
AM radio and the size of airplane windows - but what about the effect
of ALL the windows on a plane? Don't they create a much larger
effective apperature when you consider all of them?

A bit larger, yes. But the attenuation inside is still very high, since
the windows are extremely small and spaced very close, in terms of
wavelength. Sort of like the screen of a screen room.

And since the
plane isin't grounded, isin't the exterior shell of a plane
essentially transparent to all RF (ie it's just a re-radiator) because
it's not at ground potential?

No, being at "ground potential" plays no part in shielding. Currents and
fields on the outside aren't magically allowed to violate basic laws of
physics and migrate through a good conductor just because a shield isn't
at "ground potential". For that matter, a box that is at "ground
potential" at the bottom is nowhere near that potential a quarter
wavelength up the side. No shield over a small fraction of a wavelength
on a side could work if "ground potential" were a requirement. Yet
room-sized shielded enclosures are routinely used into the microwave
region. Try your own experiment. Turn your portable radio on, turn up
the volume, put it into a sealed can, set it on a stool, and see how
much you hear.

Roy Lewallen, W7EL