Topband: alternative feedpoint capacitor

[Tom W8JI]

> My top-loaded vertical needs a series 260pF capacitor at the feedpoint to
> match the 53 + j335 impedance (1.825 MHz) to my coax feed.
>
> I have a few different air-variables I could use, but I was wondering what
> sort of fixed capacitors would be suitable in this application.
>
> Max power is 1500w.  I have doorknob caps but they don't have current
> ratings marked on them.


Steve,

You had some suggestions on capacitor types.

Finding the actual voltage and current on a series capacitor like this is 
pretty easy.

Voltage is the square root of the power times reactance. That is RMS, so you 
have to consider the peak plus a safety factor.  So you have the square root 
of 1500*335 or 708 volts RMS across the cap.

That's about 1kV peak. 2kV would give you good safety factor.

Current is just square root of the power over the load resistance.

sqrt of 1500/ 53   = 5.32 amps.

Exceeding voltage is an instant failure because it will arc. You want good 
headroom.

Current failures are heat and take a little time, so the heat averages over 
a short period of time. A physically large part can be somewhat abused in 
Ham duty for current.

73 Tom