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#1
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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 |
#2
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Copper tubing capacitors
wrote in message oups.com... 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 Seems like you want something that once it is adjusted to the right capacitance it will probably never have to be tweaked again. I would go with brass plates. You can make up a nice little "brick" and then tweak it by grinding parts of it off or drilling holes in it. Sounds a little crude but really worked quite nicely for an antenna I built a long time ago. It did prove a little impractical though, Capacitance really seemed to change a bit with humidity. I never really solved this problem or even tried because I came by one of the old Heathkit antenna tuners that was a motor driven cap that you put in seies with a long wire antenna. Placing the brick in plastic box would have probably been a viable fix. Also consider that your linear caps may have quite a bit of inductance, just something else to figure into it all. |
#3
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Copper tubing capacitors
Jimmie, I like the idea of drilling some holes to tweak the
capacitance. Good idea for a set-and-forget.... nice. I've also used PC board caps. I don't know what the Q of those is like; I guess it depends heavily on the dielectric... I'm sure the duroid microwave board is much better than regular G10 or whatever. Never really used them with power though. There was a good tip in QST last? month about using big teflon heat shrink tubing to make caps that were inherent parts of vertical antennas, but I checked some prices of teflon heat shrink in large sizes and it's not cheap. 73, Dan |
#4
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Copper tubing capacitors
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#5
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Copper tubing capacitors
The short answer is because I like building antennas.
The more detailed answer is that I've got a forty foot long rotatable doublet made from some of those 20' crappie poles, and I want to match that. Making a fan dipole out of it would not really be possible. I'm already going to do that for 40/80 for FD, but the doublet will work well for the higher bands. I'm eventually looking to have a 40-10m inclusive extremely lightweight rotatable antenna with good efficiency on an easily erected mast. On 40m/30m I already have a loading and matching coil to make it a "shorty 40" style shortened dipole. For 20 and up I don't need loading I just need to match it. It's going to be nice because it is tree independent. I could set it up in a parking lot on a light mast if I needed to. Simple is not my style for field day. In a pinch I'll throw up simple dipoles. In a real emergency, I'm not going to go fancy, I'm going to go simple and resonant. For field day, though, I like to put weird things up. I built a 3 element 15m yagi one year just for FD. The year before, I built an aluminum tubing trap dipole for 10/15m. If 15 and 10 are dead as I expect they might be, I might even turn the crappie pole antenna into a delta loop for 20 only. 73, Dan N3OX |
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