In article ,
John Ferrell wrote:

I have been successful at modeling and then constructing transmitter
matching circuits at low power with common sizes of capacitors. Now I
want to move up to the 1 KW power level. Inductors are easy enough to
homebrew but capacitors are both expensive and scarce. Anyone have
ideas or maybe experience in this area?

DigiKey has a good selection of High Voltage units (3 KV Plus) at
modest cost but considering their size and the losses expected in the
capacitors I don't think they can dissipate the heat...

People working on "small magnetic loop" transmitting antennas have
developed some homebrew approaches. These antennas present a highly
inductive feedpoint impedance, and must be tuned to a high-Q match
using a variable capacitor which can stand off very high voltages.

I've seen at least three types described:

(1) "Trombone" capacitors. These typically consist of a U-shaped
piece of copper pipe (adjustable element), whose ends slide into
two longer pieces of larger-diameter pipe that act as the
connection terminals for the capacitor. The sliding pipe is
insulated from the outer pipe - its ends are wrapped in sheets of
a suitable dielectric (Teflon or Kapton sheet is often used). The
cap can be adjusted by sliding the U in or out of the terminal
pieces... often done via a worm-gear motor drive, attached to the
center of the bottom of the U.

(2) "Butterfly" capacitors... two stator plates, one completely-
insulated rotor plate, and suitable dielectric insulation sheets
between them.

(3) "Sliding glass sheet" capacitors... plate glass dielectric, with a
fixed metal sheet sitting below (as one terminal) and a metal
sheet glued to the top of the glass (as the other terminal).
Slide the glass sheet back and forth on the bottom metal sheet to
vary the capacitance. Researching the dielectric and high-voltage
properties of leaded window glass, borosilicate glass (Pyrex and
similar), acrylic, polycarbonate, and Teflon sheets might be
worthwhile.

Other (non-homebrew, but available-surplus) choices are large-gap air
variable caps, and "doorknob" fixed-value transmitting capacitors. The
former are often known as "cheese slicers"... and I've actually seen
some people home-brew them, using either sheet aluminum or brass, or
copper-clad PCB material for the stator and rotor plates.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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