W8NF submitted excellent discussion on ceramic capacitors for RF.
Thanks Dave! Those were very useful tips on how to test caps with
real circuitry, instead of just trusting some datasheet written for
another application. I believe that a lot of people, hams included,
don't pay attention the Z5U and X7R losses, and have component
failures when the RF current is high.
I have used DuPont Pyralux (copper clad Kapton polyimide film)
capacitors and had mixed results. As bypasses for filament around
tube sockets, such as what you can now purchase from Eimac in the
SK-350 socket, it is great. As a bypass for the HT lead into a
cavity, where there was some residual RF on the inside wall (maybe
even harmonics coming through the choke in a class C amp), they
lasted about a year in the field and then self-destructed. The edges
of the foil had corona which eventually ate the Kapton and weakened
the dielectric. Handling of the edges seems to be a problem in such
designs. If you look at the books on dielectrics, its field
enhancement, where the copper ends and the dielectric continues. If
you solder a copper tubing around the edge (Kapton takes the heat OK)
you still have a problem where the radius of the tubing rolls away
from the film surface. The tiny air gap here has enhanced electric
field and is stressed. With partial discharge, it will erode the
film. One way to improve this is to derate the design to the extreme.
Much beyond what a DC hipot test would suggest.
Kapton is only available up to 5 mil (thousandths of inch) thickness
in single sheets. Teflon can be made much thicker, and make good
capacitors also. But mechanically it isn't great. Polyflon company in
New Jersey has a business of making copper clad Teflon, Mylar, and
Kapton capacitors for high power Rf.
When I was employed at E.I. DuPont and Co., I measured the loss
tangent for various Kapton films with adhesives, used for Pyralux, at
2500 MHz. It was 'usable' but not as great as Teflon or quartz.
Raytheon and NASA were wondering as they considered it for roll-up
antennas on satellites. The real need for lowest loss was when they
were considering it for Rectennas, direct rectifying antennas with
shottky diodes embedded in them. They were used for microwave power
transmission experiments. Remember the flying craft (with electric
motor driven propellor) about the size of a lawnmower that they flew
over a focused microwave beam?
73
John
K5PRO
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