Topband: Homebrew capacitors

[nekvter at hushmail.com]

>> Could have saved more money with a Piece of RF-9913 or RG8 coax
cable about 25pF per foot plenty voltage rating.

Per my original post, "I'm working on a project that will require 16
variable high-voltage transmitting capacitors with a maximum value of
200pF." I don't know how one would make coax-based capacitors variable
and having to place four 8-ft lengths of coax into a box would require
a large enclosure, indeed, unless one used (expensive) small-diameter
coax like RG303.

KK1L noted that there must be either an overlap or a gap in the sleeve
that wraps the inner tube. A gap would create a "hot spot" where the
breakdown voltage would be reduced, while an overlap could cause the
inner tube to bind. This could be avoided if one could obtain
dielectric tubing of the required dimensions or if one could "weld"
the dielectric to seal the gap. I don't anticipate having a problem
with the possibility of a small gap in my application, but two
possible solutions to this condition come to mind.

First, one could wrap the dielectric material twice around the inner
conducting tube. While there would still be a gap, it would not be
entirely an air gap, which would increase the breakdown voltage. Of
course, one would also get 1/2 the capacitance per unit length, since
the dielectric would be twice as thick - unless material half the
original thickness could be obtained.

Second, one could cut a slot down the length of the outer tube and
center the gap in the dielectric material in the slot. In this way,
the potential at the gap would be reduced according to the width of
the slot. Here, again, the capacitance per unit length would be
reduced, but less so than in the first solution.

K1LI