[TowerTalk] feedpoint capacitors

[David Gilbert]

If you see thermal drift in the doorknobs caps due to high SWR, wait 
until you see what it does to the ferrite core in a common mode choke 
(see p.s. note below).  Put the L-C match between the antenna and the choke.

By the way, there are all type of "Russian HV Doorknob" caps.  You may 
just need bigger ones than the ones you're using.  I recently built a 
very high power homebrew antenna tuner (it takes up about five cubic 
feet) and it would be easy to mistake the Russian doorknobs I used for 
skateboard wheels.

I've had good luck with this guy when he has the values I needed. Good 
prices and reliable shipping:

https://tubes-store.com/index.php?cPath=35_40&osCsid=e8gal5st2tvuc8rkhsab3l21p4

73,
Dave   AB7E

p.s.  I realize that the voltage across the cap in a tuned circuit 
(depending upon the circuit) can be higher than the voltage across a 
choke, but I'm also pretty certain that the core of a choke is less 
forgiving of whatever high voltage it may see.  I once had a 160m choke 
built per K9YC's cookbook at the feedpoint of an Inverted-L that had a 
pretty high SWR (I tuned it out at the shack).  High power very quickly 
made the choke hot and the SWR at the shack went out of control.  
Reworking the Inverted-L to give a reasonable SWR fixed the problem with 
the same choke.


On 4/25/2021 9:04 AM, John Keating wrote:
> Looking for advice on best type capacitors for legal limit L-C match at wire
> dipole feedpoint. Am using Russian HV doorknob type currently, apparently
> high TCC since SWR exhibits thermal drift.
>
>   
>
> Also wondering what are the pros and cons of putting the L-C match on one
> side or the other of the choke at the feedpoint.
>
>   
>
> John
>
> AI6LY
>
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