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@sdellington.us
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