Dear Jim
Many thanks for taking the trouble to give me such a detailed reply. Yet again
I am so grateful to the good and helpful folk on this group.
If I may summarise, I think the message is design the tuner with very generous
ratings both with regard to current and voltages which may be far greater than
the relative QRP system might suggest.
73s
Roger G3XBM
Sent from my iPod Touch 4g
On 16 Jan 2011, at 18:10, "James Moritz" <[email protected]> wrote:
> 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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