[TowerTalk] feedpoint capacitors

[K9MA]

I've long been using some ceramic doorknob capacitors in my 160 shunt 
feed. It seems only the N750 TC is available in large values, and they 
do drift when they warm up. I ended up putting two smaller ones in 
parallel, which solved the problem. They just need to be derated to 50 
percent or less of rated current. The loading capacitor in my Ameritron 
amplifier also drifts as it warms up, no doubt for the same reason.

73,
Scott K9MA


On 4/25/2021 2:55 PM, David Gilbert wrote:
>
>
> 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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-- 
Scott  K9MA

k9ma at sdellington.us