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 tubing. 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, Bob K3NHI
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. Ray AB7HE

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
tubing.

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,

Bob K3NHI