Reception of radiosignals inside a Faradaycage is limited to frequencies
that are smaller then the holes in the cage.
"Geoff Glave" schreef in bericht
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

"Some Guy" wrote in message ...

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.

Not at all; however, there IS obviously a connection between
various flight control functions (such as, say, the autopilot) and
the information given by the avionics (esp. "nav" radios using
ground-based sources such as VORs, etc.). It's not going to
"send any plane into a tail spin", but it can certainly cause some
problems.

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.

You DO realize that these are on very different frequencies, and
that the emissions of an FM superheterodyne radio are very
likely to fall right in the aviation band, don't you? Hint: if you have
to go look up "superheterodyne" to understand this question, I
have serious doubts regarding your qualifications to comment on it.

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?

No. It's not the TOTAL area of the "apperatures" [sic] that
is important, it's the size of the individual openings. If this were
not so, then a conductive mesh could never be effective as a
shield.

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. "Ground potential" has absolutely nothing to do
with it. Hint: what do you think is the RF environment
within a perfectly conducting sealed enclosure, with
respect to outside sources, even if that enclosure is
completely isolated from any other surface or conductor?

Bob M. (KC0EW)

"Charles Newman" . on.sight
wrote in message ...
What is a Pitot tube anyway? I have seen a switch for most aircraft
in Flight Simulator marked "Pitot Heat", what is that?

A pitot tube is a tube which protrudes from the aircraft
body into the path of the air through which the aircraft is
flying. They are used for such things as determining airspeed
(which is the speed of the aircraft through the air, not over
the ground), and in some meteorological conditions are
prone to becoming clogged with ice. Hence, "pitot heat"
is just that - the switch in question controls a heater (most
often, electric) built into the pitot tube, which keeps in clear
of ice. Losing pitot pressure due to having the damn thing
plugged up is generally considered a Bad Thing, and
unfortunate events have been known to follow such an
occurence.

Bob M.

cabin. But has anybody ever heard a cabin announcement during flight
to turn off any devices?

Fred F.


There have been numerous postings in various scanner, shortwave and ham
groups by people who have been ordered to turn off their radio and other
PEDs.
More than one person has been ordered off, or met by the authorities on
landing and at least one passenger who refused to turn off a cellphone ended
up with some jail time after landing in the U.K. - it was pretty widely
reported a year or so ago.

Dave

My world is as an instrument rated pilot and one who services aircraft
avionics. And you must have missed my other post where I said PEDs
should be off at all times.

Fred F.



The I presume you specified AM because the LO operates outside aviation
frequencies (now that LORAN A is gone), unlike the LO in an FM broadcast
receiver which covers the VHF localizer and VOR frequencies very nicely.

Dave

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.


I can't provide technical details of the operation because I don't know
them; but I am familiar with a number of totally RF screened environments
where use of electronic devices are tightly controlled. However, internal
relays are used to permit operation of cell phones - which I always
understood were specific models which had been certified for such use.

Dave

Dave Holford wrote:

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

I am familiar with a number of totally RF screened environments
where use of electronic devices are tightly controlled.
However, internal relays are used to permit operation of cell
phones

The point was not how the planes are being equipped to handle
in-flight cell-phone use.

The point was that consideration is being made to allow cell phones to
be used while the planes are in flight. That intentional radiating
PED's are even being considered for in-flight use when so much hype
and concern is being given to the weak radiation potential of some
non-intentional radiators like am/fm radios.

BTW, what is the potential of the local oscillators of small hand-held
LCD-screen TV's to overlap with aviation frequencies?

On the way back, I spoke with a commercial pilot who was deadheading, on
this issue.
He said that it's not all that unusual to hear radio interference once they
have allowed the devices on, but when they are in cruise, they aren't
normally doing any urgent communications, so it isn't much of an issue. If
something comes up, then they will pass the word to shut down the PEDs.

On takeoff and landing though, the comms are much more rapid, and the
consequences of missing one transmission are much higher. They need to hear
all the comms, not just between themselves and the tower, but what the other
pilots are saying as well. Add to this, the fact that aircraft comms are
AM, which is inherently muddy, and it's easy to see why they take the extra
precautions.

"Some Guy" wrote in message ...
The point was that consideration is being made to allow cell phones to
be used while the planes are in flight. That intentional radiating
PED's are even being considered for in-flight use when so much hype
and concern is being given to the weak radiation potential of some
non-intentional radiators like am/fm radios.

Because, as has already been pointed out, of the
differences in emission characteristics (and specifically
the frequency ranges likely to be affected) of the two
classes of devices.

BTW, what is the potential of the local oscillators of small hand-held
LCD-screen TV's to overlap with aviation frequencies?

I believe they should be somewhat less than is the case
with an FM receiver, but they're still a bad idea for
the same reason. Note that the analysis of the likely
frequencies provided so far has dealt solely with the
first-order effects of the receiver's local oscillator; we
have NOT discussed harmonics or other unwanted
emissions.

The problem is most obvious with FM receivers because
the standard 1st LO frequency is 10.7 MHz, and the
top of the FM broadcast band is adjacent to the bottom
of the aviation band (108 MHz) - which means that
simply adding the LO frequency to standard FM
broadcast frequencies can take you instantly into overlap
with the bottom 10.7 MHz of the aviation band (and
unfortunately, that's where a lot of the radionavigation
systems within that band tend to be). But this does
not mean that receivers for other services would not
cause similar problems. VHF television covers
frequencies below and above both FM and
aviation (two bands, 54-88 MHz for channels 2
through 6, and 174 to 216 MHz for channels 7
through 13). It is certainly very possible that receivers
intended for these bands would emit in the aviation
band. Other adjacent services that may be of concern
include public-service and commerical communication
bands (i.e., police scanners) and the 2-meter amateur
band.

Bob M.

"Dave VanHorn" wrote:
They need to hear
all the comms, not just between themselves and the tower, but what
the other pilots are saying as well. Add to this, the fact that
aircraft
comms are AM, which is inherently muddy, and it's easy to see why
they take the extra precautions.


The design of newer comms doesn't help either. If they have automatic
squelch, set to break at say 1uV RF in, then obviously it doesn't take
much interference to break squelch. Then, they also may have "audio
leveling" -- a great feature when commonly using headphones -- but the
effect there will be to take a few uV of noise and amplify the audio
component to the level you hear when ATC hits you with as much as 50W,
and it's heard constantly between transmissions, to be hopefully
silenced when ATC talks. But not necessarily the case in monitoring
comms of other aircraft, where especially general aviation,
less-than-properly-functional 7W units can be relatively weak.

Fred F.