[TowerTalk] 43' vertical mod for true resonance on 80M

[Jim Brown]

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