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From: Roger Lapthorn <rogerlapthorn@gmail.com>
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Date: Sun, 16 Jan 2011 18:53:18 +0000
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
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Subject: Re: LF: Re: Capacitance switch box design - what is the simplest?
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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 gene=
rous ratings both with regard to current and voltages which may be far gre=
ater 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" <james.moritz@btopenworld.com> wr=
ote:

> Dear Roger, LF Group,
>=20
> It is worth thinking a bit about the requirements before handing over th=
e money, though...
>=20
> 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 se=
veral 10s of uH inductance, consistent with a loop perimiter totalling sev=
eral 10s of metres. So you might have a loop Q of 50 or so at 136k, and pr=
obably somewhat more at 500k. With a resistance of about 1 ohm, 30W TX pow=
er would give you 5.5A of loop current, so the capacitors and switches hav=
e 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.
>=20
> The way polypropylene or similar capacitors are rated for high frequenci=
es 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 graph=
s showing the peak voltage rating is the same as the low-frequency AC rati=
ng 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 th=
an the voltage puncturing the dielectric. These limits are specified for=
 a particular ambient temperature, which often seems to be 85degrees C, an=
d a maximum allowable internal temperature rise above that, usually 10degr=
ees 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 tem=
perature is never likely to be 85degrees C. So for a reasonably well-venti=
lated tuner, with max ambient temp of 35degrees C, a 60degrees internal te=
mperature rise should be allowable, allowing something more than double th=
e current at the lower ambient temperature, provided the low-frequency vol=
tage rating is not exceeded.
>=20
> The allowable HF current for individual capacitors from the datasheet gr=
aphs varies according to the capacitance and DC voltage rating, but not al=
l 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 dow=
n to RF current ratings of a few amps for capacitors of some nanofarads an=
d VDC ratings of 1 - 2kV.
>=20
> So in your tuner, you would want to ensure that combinations 2 - 3 capac=
itors 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 curr=
ents 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.
>=20
> Cheers, Jim Moritz
> 73 de M0BMU=20
>=20