SparkySKO wrote:
Hello All,

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]

I'm interested in getting my feet wet in building a receiver, but don't
want to buy a 'kit'. I'd like to build a radio that has a knob I can
somewhat tune it with. I've been really looking at regenerative radios
and superheterodynes, although a simple radio is more important than
how well it really performs.

One common theme in almost every 'simple' radio design I can find, is
that they *ALL* use Air variable capacitors. While I'm sure these were
everywhere 20 years ago, going to every major electronics supply house
in Portland, Oregon has yielded me only with small trimmer capacitors,
which AFAIK, will not do for a 'tuner'. I'm aware I can order online
for about 10-20$ and purchase an air variable, but I'd like to see if
any of you have any suggestions on what might work as an alternative?

Varactor diode seemed like a neat idea, although I guess they suffer
from temperature fading. Would this make it impratical for a shortwave
or AM receiver? Can you even make a regenerative radio out of a
varactor? (Actually, I'll try ghetto rigging a red LED for a varactor,
since I don't really care about it being nicely linear, I just want
something with a knob that does something!).

Can anyone recommend any type of reasonably simple radio (simple mostly
as in reasonably low parts count, i don't need an LCD readout) that
could possibly be done with parts that can be had at a reasonably
stocked electronics store. (Much more inventory than Fry's). A knob is
important, I don't want to build one of those ghetto radios with the
coil and a metal bead that I slide along the coil to 'tune' it. Any
other way to use a potentiometer to tune a radio?

If you want to build a radio circuit without much circuitry you really
have to put up with either a variable cap or a variable coil. I'd say
you either have to scrounge something or buy one -- they're really not
that spendy.

Yes, you can go the varactor route, and folks have made it work well.
But you have to add more components, it'll never be the same, and you'll
have to tinker with it to keep it from drifting all over the place. For
a 1st-time radio it may not be a bad thing, but after that I'd go with a
real variable element.

As far as your complaints about 'ghetto' radios -- your ability to build
a high-quality, good looking tunable element is a matter of the talent
that you were born with and the effort you take to use it. If you want
to build something without many components then you're going to need to
work at it anyway.

Having said that I wouldn't build -- I'd buy. Have you checked with
Cascade Surplus Electronics in North Portland? It's been ages since
I've been in there, so I can't guarantee anything. You may also want to
try R 5-D 3 Electronic Surplus out in Southeast. He's got lots of old
stuff there, if he doesn't have a variable cap he'll have some piece of
crud that you can buy and harvest the caps.

After those two, consider going to yard and garage sales, buying old
radios, and tearing out their capacitors. Stereo tuners from the 70's
and 80's are best -- if it's big and has a knob, chances are it has a
cap. If it receives FM and it's big and has a knob , chances are the
cap will be a good size for shortwave. You may even find that it has a
cap with a built-in reduction drive which is cool if you want to tune in CW.

Good luck.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html