"309" wrote in
oups.com:
There are also interference issues with AIRCRAFT SYSTEMS.

Most definitely. The only question is "how serious".

At an FAA DER Seminar a couple of years ago, they pointed out that
some "phones" would transmit spurrious signals off their frequency
that could couple with antenna cables (shielded or not), and interfere
with aircraft navigation systems (mainly VOR). It's not as simple as
many think (and I'm an aero guy, not an electron guy, so I won't
pretend to really understand).

The root cause is that most of the objects in the world don't
behave like a nice simple ohmic resistor where V=IR. In practice
I=f(V), where function f can typically be exponential, but can
be virtually any function.

Examples of non-linear objects: semiconductors, joints between
some metals, rusty bolts, mercury amalgam fillings, magnetic
components (e.g. transformers) and there are many others!

So what? Well, it turns out that this has two main consequences:
1 if there are two RF frequencies f1 and f2 impinging on the
non-linear object then the object will multiply the two
frequencies and radiate the results at harmonics of the
frequencies and of the sum and difference frequencies
(and harmonics of the sum and difference frequencies).
2 RF power can be rectified to produce a DC offset

The re-radiated frequencies due to (1) are unpredictable
in practice and splattered across band allocated to other
RF services. Example: warships can't use all their
RF equipment separately due to the rusty bolt problem.
HMS Sheffield was sunk by an Exocet because satellite
comms couldn't happen when the primary radar was on, and
that was the time at which the Exocet was launched.

(2) may or may mot be a problem. Typical electronic equipment
is able to cope with constant DC offsets associated with
constant RF power. But there are problems with varying RF power.

CDMA cellphones largely keep their tx power constant,
varying it slightly and occasionally when the transmission
path changes.

TDMA systems (e.g. GSM) have pulsed RF tx power at 217Hz
1/8 duty cycle and 8 times peak power. Put one of these
near to a phone or hifi or hearing aid and the results
are clearly audible. Such interference can completely
disrupt the operation of some equipment.