TRABEM wrote in message
...
What caps should I use for a resonant loop antenna for VLF? I have
some antenna plans and need .2 uf total capacitance to resonate a
loop
antenna at 60 Khz.

I know I should avoid electrolytic and tantalum due to their poor
temperature stability.

Disc ceramics are so poor thermally that they are out of the
question,
especially since the caps are going to be outdoors.

Silver Mica's are horribly expensive these days, and aren't
available
much past .01uf anyway.

I have a Mouser catalog and plan to order from them soon, but it's
not
clear which type of cap I should order.

Any suggestions for 50,000 pF caps that don't cost a fortune?

Note that this is a receive only antenna so voltage rating and
current
carrying capacity are not an issue.

Thanks.

T
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Avoid electrolytics and ceramics like the plague!

Since whatever you do, 90 percent of losses will be in the coil
resistance you have a good choice of capacitor types. Any type of
plastic film insulation will do fine. You may wish to have a 2000pF
air-spaced variable in parallel to cover a small frequency range.

0.2 uF = 10 times 0.02 uF. In any case you will need a number of
small value components in parallel for exact tuning of a high-Q
circuit.

The normal 5 or 10 percent tolerance means that you will have to
measure and select values from a larger batch of inexpensive
capacitors.

Don't forget you will have to select from a small number of standard
values such as 0.22, 0.1, 0.047, 0.033, etc.

Specially manufactured close tolerance capacitors will cost the Earth.

Temperature coefficients don't matter very much but if you have a
choice then select those with the lowest coefficient. But TC is seldom
specified by manufacturers. You would need a very good capacitance
bridge to measure the small TCs involved although it is easy to make
TC measurements.

To reduce size of the capacitor just increase the number of coil
turns. You will notice little or no difference in operation.

The most efficient loop has a single turn of very thick wire.
The ONLY reason for multiple coil turns at VLF is to avoid impractical
values of capacitance. Receiving sensitivity does not depend on the
number of turns, only on the area enclosed by the loop.

A change in the number of coil turns involves only a change in how the
loop is Z-matched to the receiver. With a single-turn coupling loop
no changes are needed.

You may find program RJELOOP3 useful. It covers multi-turn square
loops and other regular shapes of the same enclosed area.

Download program from website below in a few seconds and run
immediately.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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