Re: How do crystal radios really work

[William Beaty <billb@ESKIMO.COM>]

On Thu, 26 Aug 1999, Shapiro, Mark wrote:

> Bill,
>
> 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.

Yep, a very important point.  (I've seen old crystal radio schematics
where they include a tiny battery and a resistor in order to keep the
diode "turned on" and improve the receiver's sensitivity.)

> 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.

Ah, but if you remove the LC circuit, it's impedance becomes infinite.  I
agree that when the LC circuit is in place, then off-channel stations are
essentially shorted out.  Only the desired station is not "pulled down" by
the reactive load of the incorrectly-tuned LC coil.  This is how I always
thought that the bandpass filter works.

However, this suggests that if we remove the LC circuit, then all nearby
radio stations should pass from the antenna and go into the earphone.  We
might expect to hear a jabbering of simultaneous audio tracks and music.
But when we actually try this, we don't hear all stations.  Instead we
hear nothing at all.  I always found this vaguely confusing, and
attributed it to the fact that the diode must pass DC, and therefor it
would charge up the antenna and make the diode go nonconducting.  This is
a good explanation, but now I see that there is a better one which
connects with other facets of science (see the Bohren paper I mentioned.)

Here's a way to experiment with this phenomenon.  (I haven't tried this
myself as yet, however it is a well-known effect in RF electronics.)  Get
two AM radios.  Turn one on and tune it to a very weak station.  Place the
second radio next to the first, but don't turn it on, but tune it to the
same frequency as the operating radio (follow the number on the dial since
you obviously cannot tell when an unpowered radio is tuned correctly) .
When you hit just the right frequency on the unpowered radio's dial, the
sound of the weak station being received by the powered radio should
change.  The EM oscillation produced by the LC circuit in the unpowered
radio interferes with the reception process in the powered one.  In other
words, the unpowered radio reaches out and absorbs energy in the space
around itself, and acts like an "invisible umbrella" which partially
shields the powered radio from the weak signals it's trying to receive.
This same process explains how tiny atoms can intercept signals of
relatively large wavelength.  An LC circuit is like a "non-quantized"
version of a gigantic atom.  When it is in resonance with propagating
radiation, the "giant atom" presents a greatly enhanced collision cross
section, and it absorbs much more energy than such a tiny object possibly
could.


((((((((((((((((((((( ( (  (   (    (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