TopBand: Re: Gamma Cap Voltage Ratings

Earl W Cunningham k6se@juno.com
Wed, 24 Dec 1997 12:44:10 EST


Bob, K6ZZ wrote:
>Yet another quick question for the gang... I'm getting ready to purchase
a
>motorized vacuum variable cap for the Gamma Match on my shunt fed tower.
>Looks like about 50-150 PF will be needed for a match.  The higher the
>voltage rating, the better.  Just curious though, would a 10-1000 pF
vacuum
>cap with a 3 KV rating be sufficient for the task, assuming maximum
legal
>transmit power is used (1500W)?  I've run the calcs 2 different ways and
get
>different answers each time!  Thanks again.
====================
If you know the required value of capacitance, this formula will yield a
close approximation for 1500 watts applied to the antenna:

             Volts across cap = 475,000/capacity in pF.

              For 1000 watts, use 388,000/capacity.

A shunt-fed vertical on 160m can require a gamma capacitance value of
anywhere from about 75 to 600 pF.  If the structure is electrically
1/4-wave, the largest value of capacitance is needed (least voltage). 
Less capacity (larger voltages) is required when the structure is
electrically longer (up to 1/2-wave) or shorter than 1/4-wave.

73, Merry Xmas, de Earl, K6SE


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