Hello Paul,
I am dealing with the same overheating problem. I need a 475 pf
capacitor to shunt feed my 160 tower. I just happened to have a 500 pf
15 KV doorknob almost the size of my fist. It did work for awhile,
(several years) but now seems to heat up and the VSWR climbs to 2:1.
The unit is definitely heating. I measured about 95 degrees when the
outside air temp was 12 F. I tried a single red Sprague plastic
doorknob and it starts to run away with 500 watts of power. My next plan
was to put four NEC caps together to get 475 pf: 2X 200 PF, 1X 50 PF,
and 1X 25 PF. It sounds like maybe this is not going to work either. I
guess I need a bigger box and a vacuum capacitor. My amplifier is
capable of 1500 watts output. There is not a lot of voltage as it is a
50 ohm input. 6 amps at 300 volts RMS would be 1800 watts at 50 ohms.
What about an air variable in a wx proof box?
Dave K1WHS
On 1/9/2020 3:24 PM, Paul Christensen wrote:
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
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