Radios in the cabin (was Re: ATC on passengers' headsets)

From: (Ed Hahn)
Date:         30 Aug 95 14:12:49 
References:   1 2
Followups:    1
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In s.a.a, (Mark Kinsler ) wrote:
>Henry Law <> wrote:
>>I'd appreciate watching a discussion among the more informed and
>>professional members of this group along the lines of:
>> <snip>
>I'd be interested in this, too.  Is it generally possible to listen to
>ATC radio from inside the aircraft on your own battery-powered receiver?

As I'm sure others will mention, radio receivers in the passenger
cabin are not permitted.  And with radio receivers, there is very good
technical reasons for this:

Most modern (digitally tuned) receivers use "heterodyne" technology to
synthesize the correct frequency for tuning.  What this means is,
there is a local oscillator inside the receiver, which when mixed with
the antenna signal, essentially filters out everything but the audio
signal of interest (aviation VHF is amplitude modulated).

In theory, this local oscillator is shielded, but in practice there is
always some leakage into the surrounding environment.

The VHF navigation signals which are commonly used are very low power
(10s to 100s of watts, unlike broadcast FM VHF (like 20,000 watts)).
This tiny oscillation from the local oscillator is, of course, much
smaller, but it is much closer to the aircraft antenna.

There have been documented cases where radio receivers (not
transceivers) have caused significant interference with navigation
equipment, so all radio receivers should not be operated in the cabin.
This includes cellular phones.

Other portable electronic devices (CD players, laptops) have
anecdotally been blamed for navigation equipment interference, but
unlike radio receivers, no systematic problems with these devices have
been found.

So you might ask, why can the aircraft radios be used without this
problem, especially onboard cabin telephones?  Well, in practice, the
equipment on board is much better shielded (ever see a MCU/ATR box
opened?), and that interference tests are commonly performed before
putting the equipment on board.

Sorry to ramble,

--------   Ed Hahn | | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.