ORIGINAL MESSAGE: (may be snipped)
On Wed, 4 Dec 2013 09:16:06 -0500 (EST), K5GW wrote:
>
>The voltage rating is not the problem; after all there is less than 300v
>rms across a 50 ohm load with 1500 watts power.
REPLY:
Capacitors don't arc at the RMS voltage. They arc at the peak of the RF
cycle. For 1500 watts into 50 ohms, the peak is about 387 VAC. And that's
with a 1:1 SWR.
A high SWR can cause voltage nodes many times the normal voltage to appear
on the coax, and if the coax is just the wrong length, one of those nodes
may appear right at your load cap. Have you ever transmitted into the wrong
antenna?
IMO, padder caps rated at 5 or 6 kV are NOT overkill.
Once a capacitor arcs, even if it survives, little blisters form at the
point of the arc and, due to corona effect, are prone to arc again but at
even lower voltage. It is always best to prevent the arc in the first place.
High voltage caps are your friend.
73, Bill W6WRT
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