EMRFD Message Archive 1997

Message Date From Subject
1997 2008-08-30 17:52:36 bkopski HOME BREW PISTON CAPACITOR
I've been playing with a 400 mHz filter and needed a low value piston
(or similar) cap for tuning purposes. I found that I could home brew
a satisfactory one with some simple parts and supplies. The
accompanying posted photos (see the K3NHI folder) illustrate this.

Basically, I'm using a 1/2" length of 1/4" brass tubing (Hobby Shop)
attached with a #20 wire to the end of a heavy wire inductor. A 1"
brass screw runs thru a brass hex nut soldered to the business side
of the pc board. The screw runs about in the middle of the open-air

The white nylon nut you see on top of the soldered brass nut is a
jamb nut finger tightened against the brass nut - does a great job of
friction-fitting to the screw to hold it where its been set - and I
can run the tuning screw in and out without loosening the
nylon "friction nut" - defies imagination. Serendipitous! (Doesn't
work with a metal nut.)

The only thing wrong with all this is the tubing depends on its
attachment to the inductor for support - I could not find a way to
anchor it to the pc board chassis without messing up the Qu. One
photo shows the initial results of the resonator frequency sweep. I
guess it's hard to argue with air dielectric!

Cordially and 73,

2010 2008-09-06 09:43:57 rcbuckiii Re: HOME BREW PISTON CAPACITOR
Bob, interesting configuration. I'm curious as to what you are using
the filter for. I put together a 400MHz PLL controlled VCO a couple
of weeks ago. I thought about working at 100MHz and trying to build
some kind of 400 MHz filter to pass the 4th harmonic. Your filter
might do that.