LF: Capacitance switch box design - what is the simplest?

[Roger Lapthorn <rogerlapthorn@gmail.com>]

These may be stupid questions (indeed I may have asked them already months ago) but here goes....

To match my simple loop antenna on TX on 136 and 500kHz I've been using an old laboratory capacitance decade box. At the very low powers I have been using, the voltage ratings of the capacitors within the box have been adequate and results predictable and repeatable. However, as I increase the RF power I'm beginning to see the capacitors suffering and values change over a minute or so because the components are being used beyond their (voltage or heat I assume) ratings. With a hi-Q loop I guess the voltage across the capacitance can be very many times larger than that at 50 ohms.

So, I need to build a capacitance selection box with values from around 0-30nF with sufficient resolution to fine tune a hi-Q loop. I will need the capacitors to be rated sufficiently high voltage to cope with any power level I am likely to put into the loop antenna (almost certainly never more than 20-30W though).

My questions are these:

(a) what is the best choice of capacitors - type, voltage working, pulse behaviour, accuracy, price?
(b) what is the simplest arrangement to give me the adjustment accuracy and range I need with the fewest parts and fewest switches?

Any advice would be appreciated.

73s
Roger G3XBM
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