Topband: de n7dd

[David Olean]

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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