Topband: de n7dd

[Paul Christensen]

You can try several paralleled doorknob caps to lower ESR, thus minimize cap heating.  In my case, one vacuum variable will replace the HEC doorknob pair.   

In the alternative, an air variable in a WX-proof enclosure should work fine.  You can compute the peak voltage across the cap as well as average current in TLW software that's included in recent editions of the ARRL Antenna Book.  Once the peak operating voltage is known, you can obtain an air-variable cap with adequate plate spacing.

Polycarbonate tool boxes from Lowes or Home Depot make great weatherproof enclosures when base-feeding an antenna.  The side walls are sturdy and many models have a gasketed rubber seal that forms around the lid's perimeter.   
 
Paul, W9AC  

-----Original Message-----
From: David Olean <k1whs at metrocast.net> 
Sent: Thursday, January 9, 2020 4:15 PM
To: Paul Christensen <w9ac at arrl.net>; 'topband' <topband at contesting.com>
Subject: Re: Topband: de n7dd

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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