Unfortunately, I think you have this one wrong. The crystal is a solid
state diode that rectifies the radio-frequency a.c. voltage on the antenna.
(Most kits sold today actually use a diode.) Like all solid-state diodes it
requires a certain forward voltage before it conducts. Without the LC
circuit, the voltage at the crystal never gets high enough for it to
conduct, so you don't hear anything (the response of the headphones is so
slow that you hear only the modulated envelope of the RF signal when the
radio is working).
What the LC circuit actually does is present to the antenna a very high
impedance at the frequency of interest, this causes the RF voltage to be
high enough to drive the crystal.
Mark
-----Original Message-----
From: William Beaty [mailto:billb@ESKIMO.COM]
Sent: Thursday, August 26, 1999 2:40 PM
To: PHYS-L@lists.nau.edu
Subject: 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.