Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

How do crystal radios really work



More "Resonant Antenna" stuff...


Somebody on another list brought up the topic of crystal radios. You'd
think that I'd would know how they really wor. You'd think that I'd be
able to explain them to my grandmother. But after decades of electronics
hobby and carreer, I see that I do not understand them at all.


Build a crystal radio, attach antenna and ground, get it working, tune in
a station and listen... then remove the coil and capacitor. What should
happen in theory? In theory, we have removed the bandpass filter. As a
result, all strong stations should be heard in the little earphone. What
happens in practice? Silence.

Ever since being a kid I was unaware of any explanation for this effect.

Here finally is a possibility: it's the "energy sucking" effect in action.
The incoming EM causes an oscillation to build up in the antenna (after
all, the antenna is directly connected to the resonator.) The strong
field on the antenna acts to cancel some of the EM energy in the
surroundings (this is EM energy coming from the distant AM transmitter to
which the LC tuning circuit is set.) The device intercepts far more
energy than a small wire antenna ever could. As a result, the device is
able to drive the earphone. Without the LC circuit, the antenna stays
"electrically small" and cannot gather enough energy.

I had always believed that the LC circuit was a bandpass filter which
removes unwanted stations. This cannot be the case, since the behavior of
a bandpass filter doesn't match the evidence. Instead the tuned circuit
might be part of an active antenna-resonating system: an "energy sucker".
Check out fig. 2 in my article. It's a capacitor-mode energy gathering
device. All it needs is a crystal diode. Its purpose is to transmit a
high-frequency e-field into the nearfield region of the wire antenna, so
that the power throughput may be enhanced and the "effective area" of the
antenna may be increased.

((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
Seattle, WA 206-781-3320 freenrg-L taoshum-L vortex-L webhead-L