Topband: de n7dd

[Paul Christensen]

> does anyone know of a HV (20KV) capacitor with a value of approximately
500 pf that has a NPO value?

Once you get above about 100pF, NP0 options are very limited.  Be mindful of
the RF current magnitude in the application.  Small doorknobs can heat and
change in value.  In fact, I am dealing with this issue now on a 160m T
vertical.   My hope was that a paralleled pair of HEC doorknobs would work
in a NEMA 4x4x2 enclosure at the base.  The fit is fine, but the caps are
overheating in FT8/RTTY modes.  The fix will involve replacement with a
vacuum cap in a larger enclosure.

MaxGain Systems has a surplus Jennngs 500pF/20KV cap for about $350.  Also
consider transmitting mica capacitors.  Both will be quite large but will
meet your NP0 requirement.  Kintronic Labs sells a new 500pF/20KV mica for
about $650.  These are typically used in AM broadcast installations.  In
time, you can likely find these at lower prices.  The most common used mica
will be manufactured by Sangamo.  If you're in a rush, be prepared to pay
more.

Paul, W9AC