[Lowfer] Question on building homebrew variable caps...

Mon, 11 Nov 2002 20:10:56 +1300

Mike
 Perhaps you could use an ex ARC-5 tx cap- about the right capacity and
plate spacing and certainly will handle the current-in fact just about any
tx variable with adequate spacing will do for up to a few amps-provided the
wipers are up to it.
If you want to, it's quite in order at LF to use a piece of copper braid
wrapped twice (or so) round the shaft and solidly soldered at each end
(rotor and stator) to bypass the usual wiper arrangement-which is generally
the weak point in all standard variable caps used this way.

I use a 2000pf variable for loop fine tuning (conventional air spaced job)
with approx 3mm plate spacing. It's connected in parallel with the main
capacitor (semi-fixed) which comprises 102  x  4700 pf 500 volt polystyrenes
in series parallel. I have series strings of 6 to get the voltage rating up
enough and 17 strings in parallel to keep the current below about 1.5 - 2
amps in the individual caps in the series strings. Otherwise they blow.
Coarse tuning is done by changing the number of series strings in use, but
it's never less than 13.

73
 Dave
ZL3FJ

----- Original Message -----
From: "WE0H" <[email protected]>
To: <[email protected]>
Sent: Monday, November 11, 2002 5:27 PM
Subject: RE: [Lowfer] Question on building homebrew variable caps...

> Dang, I never thought of that. Thanks, now that cap should be easier to
> build.
>
> Mike>WE0H
> http://www.we0h.us/lf.html
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]On
> Behalf Of Bill Ashlock
> Sent: Sunday, November 10, 2002 9:55 PM
> To: lowfer
> Subject: Re: [Lowfer] Question on building homebrew variable caps...
>
> Mike,
>
> >I would like to know if anyone has built variable capacitors for
> >transmitters or tuners. I would like to build a variable capacitor that
is
> >at least 100pf and can handle 40 amps of RF current.
>
> How are you figuring 40A in a 100pf capacitor? If this trimmer is part of
> 7000pf total resonant capacity for the loop the current would be divided
in
> proportion to the capacity. 100pf/7000pf x 40A = .57A.
>
> BillGet more from the Web.  FREE MSN Explorer download :
> http://explorer.msn.com
>
>
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