Hi,

I'm currently having to build some high-voltage capacitors for an auto-ATU
that I'm building. The capacitors are made from interleaving brass shim
with some kind of dielectric. It was suggested that the dielectric was made
from a triple layer of polyethylene sheet (ie freezer bag) stuck together
with DC4 grease to exclude any air. However, this seems a messy approach
and some engineer friends suggested that instead I use overhead projector
transparency. I found that a double layer of this transparency worked to
give me reasonable capacitor values with a reasonable number of plates per
capacitor.

Now, as the project requires, I fiddled the caps into a binary addition
series (1pF, 2pF, 4pF, ... , 1024pF) using a capacitance bridge with a 1kHz
oscillator. I was careful to measure the caps in situ so as to take into
account any stray capacitance on the circuit board. However, the tuner
doesn't appear to be matching as expected (its an L-match). Does anyone
have any views on using this dielectric? Is it possible that it has a
frequency-dependent dielectric constant so that the cap values as measured
at 1kHz are different at HF?

Any comments/help etc gratefully received!

73

Chris 2E0AYO


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