[Microwave] 0.5pf "feedthru" capacitor?

[David VanHorn]

At 05:23 PM 5/4/2002 -0700, Scott Townley wrote:
>I have the 4th Ed. of the VHF/UHF Manual (Jessop, RSGB) and there are
>several interesting 2m lumped-element bandpass filter examples within.
>The most interesting uses a 0.5pF "feedthru" type capacitor...feedthru
>because it penetrates a shield wall between resonators (not shunting to
>ground).
>I have never seen such a small-valued feedthru cap--the smallest I see in
>looking about the web is a 10pF.
>Any hints/ideas?  I have a feeling I am going to be making this myself, but
>at one time this beast must have existed...or it's a misprint.
>TIA,

A very good one could be built with concentric brass tubing, with a bit of 
glass tubing as insulator, so that it will stand the heat of soldering.
Shouldn't take much length to get to half a puff. Just use the surface area 
of the inner conductor for plate area calculations.