On Tue, 17 Feb 2009 17:33:41 -0800 (PST), Rick Karlquist wrote:
>I use WWII surplus "transmitting mica" caps. If you want to make
>the antenna tunable, you can switch in various fixed caps. A vacuum
>variable typically doesn't have enough capacitance, and in any
>event, you don't need the high voltage rating.
Right. The most important specification is the equivalent series
resistance -- this is a fairly high current point, so the capacitor
will be carrying a lot of RF current. A lossy capacitor will let the
smoke out very quickly when you put some power into it. :)
I don't have any of those old WWII caps, but there's an electronics
surplus house in my area that stocks a bunch of good RF capacitors,
and sells them cheap, so I'm using a series/parallel combination of
those capacitors. Most of the time I'm running 1.2kW on 160, and
during a contest I'm transmitting a lot. :) Once I found low loss
caps, they run cool. It's pretty easy to do that -- you put the cap(s)
in series, transmit for a while, and then feel them to see of they're
hot (or if they're still there). :)
Rick and I are nearly neighbors and compete with each other in
contests. I can vouch for his antennas -- he often does nearly as well
with 100 watts as I do with a kW!
73,
Jim Brown K9YC
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