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Message-ID: <A96F14DD72BA4A65A24B3E0D5A386324@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <AANLkTimNE73xJS9tHvpE2X7Afd33mWsQm7n-ZdfvG_9+@mail.gmail.com><532F7D4648B74CCF825F0D0E596655AF@home2361108df7><006901cbb595$e50b51f0$4001a8c0@lark> <AANLkTin39mU1Q_FBqY5L8bm9Ae0aoD3saG8K-RNamLQ=@mail.gmail.com>
In-Reply-To: <AANLkTin39mU1Q_FBqY5L8bm9Ae0aoD3saG8K-RNamLQ=@mail.gmail.com>
Date: Sun, 16 Jan 2011 18:10:10 -0000
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Subject: LF: Re: Capacitance switch box design - what is the simplest?
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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