Yes, there is a great solution, I'm using it for almost 10 years. The
issue
with capacitors in 160m is the dielectric an most capacitors get hot and
change the capacitance. I used 19 x 62 pFNPO 3KV capacitor to get 1200pF.
Never had a failure. Just keep them apart , don't let two capacitor touch
This is an age old problem highlighted with amplifiers. The problem with
higher value ceramic doorknob capacitors is temperature coefficient, not so
much the heat. It is impossible to find NP0 (negative positive zero) in a
thick dielectric ceramic. Typically the transmitting doorknob cap has to be
less than 100pF to get an NP0. A 200 pF 5 or 7.5 kV was typically measured
in the N1000-2000 range, more often than not worse than advertised. 170 pF 5
kV's of better selection could get into the N150-300 range.
Most of the reason I used to parallel multiple transmitting doorknobs was to
keep the temperature coefficient reasonable, not for heat.
Multi-layer transmitting chip ceramics and some disk capacitors are
available in NP0 or the equivalent.
Mica capacitors are generally very stable, so the old surplus WWII screw
terminal block micas work pretty well and are often just a few bucks.
We should remember coaxial cables are transmission lines. Because of that,
they have pronounced standing waves. With any significant electrical length
in degrees the capacitance will NOT be what we calculate for capacitance per
foot. At just a few degrees they deviate from calculated because they start
to have significant open stub effect. This also results in higher voltage at
the open end than we have applied across the feed end. With longer stubs
(even though we might imagine them capacitors) the voltage step up can be
significant. This was a problem in more than one amateur antenna using coax
to make cheap capacitors, the Unihat vertical being one of them that comes
to mind.
There are many cheap alternatives, but there are few cheap universally good
alternatives except perhaps a multitude of lower value temperature stable
caps in parallel, or mica or air insulated caps.
73 Tom
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Topband Reflector
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