TopBand: Capacitors for Shunt Feed

Sat, 1 Nov 1997 20:37:12 +0000

Hi Guys,

> I had the exact same happen. I used some ceramic caps in the match and the
> SWR changed.

The problem is ESR (equiv series resistance), not voltage. Ceramic 
single layer caps (especially HV types) have high series resistance 
and also have poor temperature stability so they drift.

> the tower and they work fine. It is interesting how much current is flowing
> in the match with 1500 watts, many amps !!

If the shunt tap is positioned correctly, Q is reduced.  Lower Q 
reduces the undesirable circulating currents that unnecessarily 
heat parts while making the bandwidth narrower than a simple gamma.

If that was a simple single capacitor gamma adjusted for 1:1 SWR, 
current through the cap would be only sqrt of P/R.  A mere  5.5 
amps at 1500 watts, and bandwidth would be wider! With an omega 
match it can be almost anything more than, and never less than 5.5 
amperes..

That's the price we pay for less tower work! ;-)

73, Tom W8JI

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