Dear Roger, LF Group,
It is worth thinking a bit about the requirements before handing over the
money, though...
With a small loop, the loss resistance might be of the order of an ohm. From
your choice of capacitance range, you seem to be tuning a loop of several
10s of uH inductance, consistent with a loop perimiter totalling several 10s
of metres. So you might have a loop Q of 50 or so at 136k, and probably
somewhat more at 500k. With a resistance of about 1 ohm, 30W TX power would
give you 5.5A of loop current, so the capacitors and switches have to be
good for this. The voltage will be current x reactance roughly; a few
hundred volts at 136k, and rather more at 500k, so the capacitors and
switches need to be good for this too.
The way polypropylene or similar capacitors are rated for high frequencies
needs some working out. If you download manufacturers' data, there is
usually a DC voltage rating and a low-frequency AC voltage rating (often
less than half the DC rating for high voltage types). You will find graphs
showing the peak voltage rating is the same as the low-frequency AC rating
up to some frequency in the kilohertz range, above which the allowable
voltage is derated approximately inversely proportional to frequency, i.e.
the current (causing IsquaredR heating) is the limiting factor rather than
the voltage puncturing the dielectric. These limits are specified for a
particular ambient temperature, which often seems to be 85degrees C, and a
maximum allowable internal temperature rise above that, usually 10degrees C,
i.e. the max internal temperature is about 95degrees C. This leads to a very
conservative rating for an antenna tuner, since the ambient temperature is
never likely to be 85degrees C. So for a reasonably well-ventilated tuner,
with max ambient temp of 35degrees C, a 60degrees internal temperature rise
should be allowable, allowing something more than double the current at the
lower ambient temperature, provided the low-frequency voltage rating is not
exceeded.
The allowable HF current for individual capacitors from the datasheet graphs
varies according to the capacitance and DC voltage rating, but not all that
much - so if you double the capacitance, you only get a relatively small
increase in allowable current and the same goes for the DC voltage rating. I
guess the temperature rise is more connected with the surface area of the
case, and the thermal resistance of the insides, rather than the thickness
or area of the dielectric or electrodes. This all boils down to RF current
ratings of a few amps for capacitors of some nanofarads and VDC ratings of
1 - 2kV.
So in your tuner, you would want to ensure that combinations 2 - 3
capacitors in parallel are used, in order to cope with the highest
currents... or limit the loop current to a few amps. This applies to both
the larger capacitances at 136k and the smaller capacitances at 500k, since
the currents will be fairly similar. I guess 1600VDC/630VAC rated capacitors
would probably be suitable for your power level and loop inductance. Like
Alan says, you would probably want a variable capacitor for fine tuning,
since the antenna Q will be quite high.
Cheers, Jim Moritz
73 de M0BMU
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