Topband: alternative to vacuum variables

[Tom W8JI]

> 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