Re: Dip Meter TC Tuner

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <teslalist-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
 >
 > Original poster: "Matthew Smith by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>
 >
 > Hi Steve/All
 >
 > >Might have to cannibalize one out of a cheap AM radio.  If you know of
 > >sources, please post it or email me directly - they are becoming scarce
 > >items.
 >
 > Can't speak for anywhere but Australia, but I have seen ex-military
 > variable caps on Yahoo auctions over here.  But you're right, neither
 > Farnell nor RS have them listed (I haven't tracked down a Digikey Australia
 > yet).

	I'll bet there are plenty of ham radio swapmeets there (Australian hams
are very active),

 > Old wireless sets should be plentiful in junk shops, so that may be the 
answer.

	Lately such things seem to have become "collectibles" with
fantastically inflated prices.

 > This does raise the question: how do people build radios now if you can't
 > get variable capacitors? (Apart from trimmers.)  I'm sure that they don't
 > use variable inductors and I didn't think that RC oscillators were suitable
 > for high frequencies; forgive me for being out of date...
 >
 > Cheers
 >
 > M

	Variable capacitors are still used in some designs, air variable in
high-quality RF equipment, semiconductor varicaps,polystyrene insulated
variables (in cheapie transistor radios), etc.  There have been some
very high-quality RF oscillators (both receiver local oscillators and
transmitter frequency sources) using precision variable inductors.  For
a long time Collins used "permeability tuned oscillators" for their best
frequency generators. There was a period in which most automobile radios
used variable inductors (magnetic cores sliding inside of fixed coil
forms), but all but the cheapest radios today use frequency
synthesizers, which in themselves are very cheap now.

Ed

Ed