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

Tue Apr 30 12:14:07 EDT 2013

Got a Schismatic we can look at?? 

Bry Carling
http://www.af4k.com
http://alltheaboveband.com

----- Reply message -----
From: "David Stinson" <arc5 at ix.netcom.com>
To: <milsurplus at mailman.qth.net>, "ARC-5 Yahoo" <ARC-5radio at yahoogroups.com>
Subject: [Milsurplus] OK Smart People:  When is a Cap NOT a Cap?
Date: Tue, Apr 30, 2013 9:26 am

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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