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

[J. Forster]

You need to use a "transmitting" capacitor, designed to carry RF current.
They are designed with low lead inductance and resistance, and probably
low dielectric losses.

They are still made by companies like ATC (American Technical Ceramics ?).

For your purposes, one of the brownish, WW II, sort of octagonal puppies
w/ screw terminal should work fine. I've also used small air variables.

Best,

-John

==================





> 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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