TopBand: Re: Gamma Cap Voltage Ratings

[Earl W Cunningham]

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


--
FAQ on WWW:               http://www.contesting.com/topband.html
Submissions:              topband@contesting.com
Administrative requests:  topband-REQUEST@contesting.com
Problems:                 owner-topband@contesting.com