Klop wrote:
. . .
Also
http://www.hanssummers.com/radio/varicap/index.htm
for G0UPL's testing of several common diodes as varicaps.
Red LED's came out top!
Does anyone else have info on using them as varicaps for tuning?

It's too bad he didn't think of trying zener diodes. By using zeners,
you can choose a wide range of capacitances -- low voltage zeners have
the most C and higher voltage zeners have less, high power zeners more
than low power ones. I've used them for decades for such things as RIT
adjustment. A reverse-biased transistor emitter-base junction is also
basically a zener, with typical breakdown voltage of around 5 volts, so
this works fine as a varicap when a fairly large amount of C is needed.

Any reverse biased diode will behave like a varicap, although as G0UPL
found out, small signal diodes (intentionally) have a very small amount
of C. The diodes sold as varicaps are doped to provide a particular and
repeatable C-vs-V characteristic, but otherwise are the same as other
diodes.

The real problem with using varicaps of any kind for tuning is the
relatively high and nonlinear temperature coefficient.

Roy Lewallen, W7EL