[Milsurplus] OK Smart People: When is a Cap NOT a Cap?

[Ray Fantini]

Doorknob style capacitors work best in RF circuits, Especially when dealing with vintage equipment. Here is a whole flock of them for $12,

http://www.ebay.com/itm/Set-of-Doorknob-Capacitors-with-screw-56pf-68pf-100pf-6kV-10-/261200609019?pt=LH_DefaultDomain_0&hash=item3cd0c4fefb

Don't know if it's a power factor issue or what but you have to admit that they would look better in line with the output then a disk ceramic. Figuring the difference in impedance across the capacitor in the output circuit has to wonder what the peak voltage is across it anyway.

Ray F

----Original Message-----
From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of David Stinson
Sent: Tuesday, April 30, 2013 9:26 AM
To: milsurplus at mailman.qth.net; ARC-5 Yahoo
Subject: [Milsurplus] OK Smart People: When is a Cap NOT a Cap?

As I wrote earlier, I'm working on the GF transmitter tuning units.
Getting them to load a 50-ohm antenna requires a series capacitor.
The 75-meter unit uses 150 pFd.  Initially, I dug a couple of micas out of the junkbox and they worked, delivering 12 watts to the dummy load.  A junkbox 150 pFd ceramic worked as well.
But I had no idea of the voltage rating of these caps or their long-term prospects.
So I went to Digikey and searched for some
150 pFd, 2KV ceramic caps.  I settled on:

http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&KeyWords=490-4271-ND

Got them in and installed one.
The max power I can get out is about 5 Watts.
The cap heats, too.  It's eating power.
The whole bunch is useless.

Tell me what I did wrong.
When is a 150 pFd, 2 KV ceramic capacitor.... not one?
And can your recommend a better solution that isn't $20 a pop?

73 DE Dave AB5S

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