"Tim Wescott" wrote in message
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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.

Varactors are not that tough, and don't mistake the "never be the same".
Good varactor rigs are much more pleasant than anything with a variable cap.
(Although there are plenty of bad varactor designs out there!) Yes,
sometimes there are a couple more components, but not many more. It does
take some work to get a varactor oscillator close to drift-free, but five
minutes with a calculator will get you close enough for SWLing. Oh by the
way, getting a capacitor oscillator close to drift free can be a challenge,
especially if you don't have hard-to-get air inductors, and microphonics in
that case can be a challenge, too. The larger size of the variable cap/air
inductor makes mechanical construction details a lot more important. There's
really no free lunch.

Varactors do have some temperature coefficient, and they are often coupled
with toroid inductors, which also have some considerable temperature
coefficient. Most of the designs you see out there are for CW rigs in the
ham bands, where temperature stability is extremely important. The maze of
capacitors around the varactor are there to balance the temperature
coefficients. Usually there is a polystyrene capacitor which has a
temperature coefficient opposite to the toroid and varactor, but you can
never get exactly the right value for that, so it is a question of getting
the right combination of positive and negative temperature coefficients AND
the right value of capacitance. For AM in the broadcast band, you can
probably come close enough with one or two capacitors. For a rig with a 200
Hz CW filter at 15 meters, it can be a real bear keeping the frequency to
within the 0.0001% that you need for comfortable operation.

That being said, it is undoubtedly easier to use a variable cap, and caps
can be cannabalized from old radios easily and cheaply. Leon also mentions
the possibility of making a variable inductor. Actually, from time to time
articles have appeared about making variable caps, too. In either case,
though, you will need considerable mechanical skills.

One thing that makes projects hard, especially your first projects, is this
idea that you need exactly the right part. With a little help from your
trusty calculator, you can play pretty fast and loose with parts values in
most projects. In the case of a tuning cap, if you can find something on
the right planet it typically isn't too tough to make adjustments to
accomodate the different part. Study your circuit and understand what each
part is doing, especially around the tuning circuits. It is usually pretty
simple to make some adjustments, especially if, as you suggest, getting some
precise tuning range isn't critical.

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