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Copper tubing capacitors
I'm thinking about how I can make matching networks for the center of a
40 foot doublet for field day. I have a couple of those 20 footer fiberglass poles and a center mount and want to be able to match on 20m, 15m, and 10m for now, and there are lots of ways I could do this, ladderline to the base and a manual or auto tuner, autotuner at the feedpoint, etc, etc, but what I'd really like to do is have switchable fixed-tuned networks as my intent for this antenna is to be set up in the middle of a field in the clear at the same height all the time, and I want to have FAST bandswitching. An autotuner really isn't in the cards financially, and needs power. So, inductors are easy. I've got 500 feet of #10 copper wire. I need capacitors. Preferably very very cheap ones, and I guess I need a number of values. So it seems like 3/4" copper tubing (type L) slipped into 5/8" copper tubing with an air dielectric has a capacitance of 30pF per inch and a breakdown voltage of 1300V, ballpark in the *ideal* case. Two questions: It seems like having a small gap in a cylindrical capacitor like this probably makes it act like more of a lumped-element capacitor than, say, a length of RG-58. I guess the latter is more like 30pF per FOOT, and so you need to use a much longer line in terms of a wavelength to get the desired capacitance. Sound right to you folks? Second, I know the actual breakdown voltage of such an object is going to be much less than 30kV/cm*the gap width. Anyone using copper tubing caps like this (I saw a magloop "trombone" unit some years back like this) and what are the dimensions and actual breakdown voltages of real units? I guess polishing the tubes, making the cut edges rounded and so forth will help some, but maintaining the gap will be tough. I can easily derate for concentricity errors, but what about the ends? What is a good overlap of the two sections? Is it better to have the tubes be exactly the same length, flush with each other, or maybe have one longer than the other one so that the ends of one are far away from the ends of the other, or is it likely no difference? I can always go to a bigger gap; 0.035in diametral difference is probably pushing it for 100W use but I haven't yet calculated likely voltages across these things, so here I'll take any general comments on making coaxial caps that stand high voltage. I'd probably prefer air dielectric to teflon heat-shrink or something, I don't know how available that is. 73, Dan N3OX |
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