starman ) writes:
Michael Black wrote:

WShoots1 ) writes:
To go higher in frequency would require a diode capable of higher frequency.

My first crystal set, a Philmore, used a galena detector. My second one used an
out of spec microwave diode (1N23). G

Of course, a real problem with a "crystal radio" is that it's only got
one tuned circuit at the front end. Long before the diode becomes an issue,
the fact that that single tuned circuit will be pretty wide as you go up
in frequency will be a significant factor. Even at AM broadcast frequencies,
the selectivity of most "crystal radios" is marginal.

On the other hand, selectivity may not be a real issue as you move up in
frequency, since sensitivity is also an issue with "crystal radios". They
work with local stations because those stations are relatively strong.
Move higher than 1600KHz, the top of the AM broadcast band, and most people
will not come across local stations, so anything that is heard will be weak,
and have to be a pretty strong signal to begin with. In that context,
selectivity may not be a real issue, since there will only be a handful
of signals that can be received anyway.

Michael

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?

Some "crystal radios" for the aircraft band were described in the hobby
magazines thirty and so years ago. They specifically used the "it doesn't
radiate, so it's safe on an aircraft". I never saw a third party opinion
on it; clearly it is safer than borderline things that are allowed on,
but I have no idea if the rules actually specify specs, or a need to
meet specs.

The articles did say they worked well on the airplane, though what
you heard was mostly transmissions from the aircraft rather than ground.
The lack of selectivity in this case was not a consideration, since you
wanted to hear it all.

Michael