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RE: Variable Capacitance and Inductance
Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>
>The vast majority of receivers in the world don't use crystals for the
local oscillators. The commercial transmitters that use them and the tiny
number of military receivers that do, keep them in temperature-controlled
environments, because even a crystal's frequency IS temperature sensitive
Every military radio I have sold (about 100 in the past year) had crystals
in them. I haven't heard of a military radio without crystals. Not that
they couldn't exist, but the military is very picky about frequency drift.
I have a Motorola Micom-1 base station right behind me. It is one of the
older (1970s) military FM base stations and it has crystals. I've got
several older military frequency generators upstairs and every one of them
has crystals in them.
As I mentioned to Dave P., the inductance used in the oscillators of radios
is far smaller than the inductance used in Tesla's coils. And most
inductances in radio antennas that I know of use ferrite cores, which help
to stabilize the inductance. In older radio oscillators most of the
oscillator inductance is in the wires between the vacuum tubes, and the
oscillations are maintained mostly with RC circuits.
In radios using only air core inductors in the antenna circuit, constant
tuning is necessary throughout the day. Dave P. mentioned he turned on the
radio to one station, and the next day the same station was there with no
drift. This is not surprising. To check for radio drift according to the
diurnal effect Tesla mentioned one would have to monitor the same radio
station throughout an entire 24 hour period to notice any drift.
I have 4 old AM tube radios. The stations come in fine during the day but
just after the sun sets and just before it rises the radio stations drift
When I had my 100 amp amplifier connected to my computer audio output last
winter, the hum in the speakers would modulate at about 7:30pm every night
for about 20 minutes and I would hear a radio broadcast from St. Louis come
over my computer speakers. There wasn't any radio circuitry per se in my
amplifier or computer.
I know stranger things have happened. Ham operators tell me strange stories
all the time. But there is no doubt that the earth's position with respect
to the sun can alter circuitry such that there are frequency changes. And
if frequency changes, that can only mean the inductance and or capacitance
have changed (in a LC resonant circuit.)