Homebrew Hi-voltage Variable Capacitor is Taking Shape

I mentioned in a post a while back that I successfully built a variable capacitor out of copper pipe and polyethylene sheet as a dielectric. The PE sheet has a dielectric strength of 22 kV/mm and the capacitor I have tested has a gap of almost 2.5mm! The problem that I have been working on is how to adjust this on a 40m magnetic loop antenna that I am testing for EMCOMM NVIS use. My initial tries at this either would not work mechanically or require quite a bit of structure to build. So I sat on this project awhile hoping to find a solution. I was sitting down at church last Sunday and just before the service started…BAM! the answer just popped into my head!

The secret is soldering the SO-239 connectors to the end of the pipe that form the capacitor. In this way, I not only minimize the losses but I can make the capacitor part of the mounting structure. A quick visit to Home Depot during the week yielded various PVC fittings and hardware. Here is how the prototype is looking thus far:


The PVC cylinder in the center forms part of the structure. The screw eye will be used to hoist this up onto a suitable tree branch or other support. The bottom part of this fitting will take a 1/2″ PVC pipe as a spreader which will join two together in the center. The copper pipe is the capacitor. In this test of the mechanism, I have not soldered the SO-239’s in place or have the PE sheet on the smaller tube. The block on the end of the smaller copper pipe has a couple of lock nuts sandwiching a delrin block forming a crude bearing. The other end of the threaded rod attached to a nut force fit into the PVC support. As the threaded rod is turned the smaller pipe will move in and out of the larger pipe giving me variable adjustment. The mechanism works so now I have to figure out how to solder the connectors in place, wrap the small pipe with the 45″ long piece of 4 mil PE sheet and epoxy down the parts that need to stay fixed. I have figured that for a magnetic loop that only works on the 40m band I need an adjustment range of 5pF which is about an inch of slide. The target capacitance range is 37 to 42pF. Each turn of the screw will work out to about 1/4pF so plenty sensitive to adjustment.

Stay tuned as I get out the oven mitt and the butane torch and try and do some heavy duty soldering.


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