Topband: 200pF HV-capacitor for shunt feeding

[John Kaufmann]

> 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