Hi Jeffrey,
I cannot imagine why you'd need a high voltage capacitor in the "loading"
position of an output network for a 100W amplifier. Presumably, in that
position, the capacitor is across a 50 Ohm load, which means at 100W the
voltage would never exceed about 100Vpk under normal conditions -- maybe
141Vpk with a 2:1 VSWR assuming the Z is 100 Ohms.
I've had good luck with NPO chips from ATC (American Technical Ceramics),
and the small values are not expensive; however, I've also had good luck in
your specific application using Unelco (et al) J602 miniature mica chip
caps. RF Parts sells these in small quantities for $1.40/ea per their
latest catalog (you might try 27pf, 30 pf, 32 pf, etc...if you buy more than
10 pcs, the unit price comes down to $1.25/ea.)
Last time I checked SRF on a 27pF J602 miniature mica chip it was above 400
MHz.
WB2WIK/6
"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill
> -----Original Message-----
> From: Jeffrey Madore [SMTP:K1LE@arrl.net]
> Sent: Sunday, March 16, 2003 10:01 PM
> To: AMPS@contesting.com
> Subject: [Amps] Need a Capacitor- help
>
> I have a 100w, 144Mhz solid state PA board that I am repairing. I
> back-burnered it for a while but really want to get it completed. I only
> need one component; an smt capacitor that is in the "antenna load"
> position of the pi network.
>
> The original was less than 1/8" square. I was able to obtain a 47pf cap
> which worked, but was about 10pf to high. Unfortunately, after soldering
> and unsoldering it a couple times I killed it.
>
> I tried a 500v rated silver mica cap with the leads cut very short and it
> worked, but heated up quickly.
>
> Then I tried your recommendation, Steve, and shaved the silver mica cap
> down to a chip. Yes, I ruined the first one but the second one came out
> fairly decent. Great idea! After shaving it down, I tested it and all
> seemed ok. Though it was physically much larger, I was able to solder it
> in place. The circuit would then not resonate at all (there is a 15pf
> trimmer in parallel with the cap) and showed no sign of coming near
> resonance as I tweaked. I'm not sure whether the end bands of the cap
> increased the L or the physical size caused additional capacitive coupling
> to ground. I did try standing it up on the board and there was no change.
> The tank circuit L consists of a U shape piece of 1/4" flat stock roughly
> 3/4" high and 1 1/2" long. Just looking at the physical make-up of the
> inductor tells me that the inductance is very small and the additional
> inductance might have a significant effect on the overall L.
>
> I then tried a small 27pf ceramic 1Kv rated cap and obtained full output
> with the trimmer plates fully meshed. The capacitor however, showed signs
> of heating.
>
> So it appears that I need a very high Q, small smt chip capacitor rated at
> 500v to 1000v, and I probably should get a couple different values so that
> I can select the right one. I would guess that something around 37pf would
> give me what I need.
>
> Can anyone help me obtain these? All sources that I have checked sell smt
> caps with much lower voltage ratings. Sources that had what I need
> required that I buy a significant quantity. I really don't know what to do
> and would appreciate any suggestions.
>
> Thank you,
>
> Jeff - K1LE - CT - ><>
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