Air or Vacuum?

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Air or Vacuum?

Postby W2WAS » Fri Jan 25, 2008 8:22 am

For the tank circuit of the Class E RF amplifier...

Assuming that cost is not one of the factors in the decision, is it worth the expense using a vacuum variable for the series tuning and/or loading capacitor?

Why do you prefer one over the other?
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Re: Air or Vacuum?

Postby kf1z » Fri Jan 25, 2008 8:56 am

I used a vacuum variable ONLY because it was 1/4 the physical size of an air variable of similar capacity and voltage rating.

I have used both in this transmitter, with no noticeable difference in operation.

That was for the series tuning cap...

As for the loading cap....

You really can't use just any old broadcast multi-gang capacitor, but at the same time, you don't need a vacuum cap.

The loading cap needs to be something that can handle a little current... (not a lot of voltage)...
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Re: Air or Vacuum?

Postby KD3CN » Fri Jan 25, 2008 1:52 pm

I second Bruce's comments. Both vacuum and air variables have their plusses and minuses, and both will work.

Let me add though that if you decide on an air variable tuning cap, you may find that one with enough C and KV rating difficult to find at a reasonable price. An alternative option is a smaller C air cap with high KV rating, padded by a fixed capacitance vacuum cap. I go from one end of 75m to the other end of 80m phone without very much change in the tuning cap adjustment. Also, fixed vacuum caps can still be found at reasonable prices.

Just some thoughts...

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