"starman" wrote in message
...

Anyone built a passive receiver for VHF or UHF? Could you listen to an
aircraft's communications as a passenger on the same plane? What does
the law say about using any aircraft receiver on a plane, whether it's
passive or active?


There was a circuit, very simple, for a germanium diode receiver for the FM
broadcast band. Basically, it consisted of a large loop and variable
capacitor making up the tuned circuit, a germanium diode, a resistor, a 100
pF disc cap, and a crystal earphone. Tuning was by slope detection,
although I can't see why such a device could not be made into a ratio
detector by center tapping the coil (or making two identical coils, and
tapping between them). Also no reason that you could not listen to an
airplane's broadcasts on such a device with the loop cut to those
frequencies.

As for the law, I don't think there actually IS one, only a convention
disallowing use of radio receivers/transmitters onboard commercial flights.
The reason for this is because the local oscillator of an FM radio falls
directly in the aircraft comms band anywhere above 97.4 MHz. A crystal
radio would not interfere, and would be impossible to detect. One for such
close proximity to the transmitter could be just a small coil, instead of
the loop, and could be built into something like a pocket radio case.