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

[mstangelo at comcast.net]

Dave 

Safety Capacitors have wide temperature coefficients and may not pass a large amount of RF current.

I'd recommend using Mica capacitors. Here is a selection:

http://www.talonix.com/shop/category.aspx?catid=115

If the RF cirrent is high I'd hook uo three 51 pf caps in parallel.

Mike N2MS

----- Original Message -----
From: David Stinson <arc5 at ix.netcom.com>
To: milsurplus at mailman.qth.net, ARC-5 Yahoo <ARC-5radio at yahoogroups.com>
Sent: Tue, 30 Apr 2013 13:26:14 -0000 (UTC)
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?