[Antennas] Doorknob Capacitors

[Barry L. Ornitz]

Michael Harley asked about "doorknob" capacitors and how 
their construction differed from other capacitors.  John 
Tait, EI7BA, gave an excellent answer to which I would like 
to add just a few points.  As he notes:

> They are used where large RF currents and voltages are
> present.  High voltage disc ceramics etc may be able to 
> handle high voltages OK, but are not up to coping with 
> high RF currents.

The key here is a stable, very low loss dielectric.  
Typically high grade mica is used in the HF to VHF range.
The design of the capacitor is for an extremely low 
internal inductance such that the self-resonant frequency 
of the capacitor is generally well above the actual working 
frequency.

The dielectrics used in many disc ceramic capacitors are 
usually variations on barium titanate.  This material has a 
very high dielectric constant, but its dielectric losses 
are large enough that with high RF currents flowing through 
the capacitor, it can heat excessively.  Disc ceramic 
capacitors have low internal inductances, so they are quite 
suitable for use in most RF circuits - but at LOW POWER 
only.

You can often find older vintage transmitting mica 
capacitors that look like the big cousins of "postage 
stamp" mica capacitors.  These generally have wide tab 
leads and may be as large as several inches on a side.  
They generally have more internal inductance than do 
"doorknob" capacitors so their use should be limited to the 
lower HF bands.  These capacitors usually date to before 
1950 and they may have absorbed moisture over the years.
This can lead to high losses and voltage breakdown at much 
less than their rated voltage so beware when using them.

        73,  Barry L. Ornitz   WA4VZQ          ornitz@tricon.net