Topband: 200pF HV-capacitor for shunt feeding

[Pete Smith]

This raises an interesting (to me) question.  My current omega match uses a 
couple of identical capacitors that I got from Henry Radio in one of their 
surplus parts sales -- I forget exactly, but think they are rated 3000 volts.

If I were to begin running something close to the legal limit on 160, I've 
been assuming that the voltages at the bottom of the shunt feed would 
exceed this rating.  My question is whether I would likely need vacuum 
variables in both positions or only in place of the series 
capacitor?  Since the value of shunt capacity is quite small, could I just 
use RG-213 for the shunt cap, where the length (and resistance) would be low?

73, Pete N4ZR

  At 08:47 PM 12/14/2004, John Kaufmann wrote:

> > Does anybody know where to get a 200pF HV-capacitor for shunt feeding a
> > tower on 160m? Is it possible to use RG213-cable as capacitor?
>
>RG-213 generally makes a poor choice for a capacitor.  The issue is RF loss,
>which appears as a resistance in series with the desired capacitance (or
>inductance).  The longer the coax, the larger the resistance tends to be.
>For example, one coax stub I measured with ~100 ohms of reactance also had
>something like 10 ohms of series resistance!  This represents a Q of about
>10, which is extraordinarily poor for a capacitor or inductor.  Put that at
>the feedpoint of a low-impedance antenna and you'll suffer measurable gain
>loss because of the loss resistance.
>
>Use an air-variable or vacuum-variable capacitor and you won't have these
>problems.  There are always a number of these capacitors for sale on eBay.
>
>73, John W1FV
>
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