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DIY Transmitting Air Variable Capacitors

17 Aug

http://www.eham.net/articles/15975

Very detailed article on building your own Air Variable Capacitor using Acrylic & Phenolic

Exerpt (Click On Image For Complete Article)


By: David Hammack, N4DFP

As I was building my transmatch, I became increasingly dissatisfied with my choice of capacitors. They were too small for more than about a hundred watts, and the ganged capacitor was in pretty sad shape. The frame was rusted and I could not remove all of the oxidation from the plates. I had a very nice Hammerlund 450pf that would do well for the Input Tuning, but I didn’t have an equivalent ganged capacitor to use for Output Tuning. I am temporarily disabled and have no income at the moment, so it looked as if my transmatch would either be constructed of inferior parts or put on hold indefinitely. As I was looking over the Web, I came across an article by DL5DBM, Anwar von Sroka on building your own capacitors. It looked pretty simple, but I was concerned that the materials would strain my very limited budget. I did some thinking and looked around my local hardware store to check the price of materials. I found that the heavier gauge sheet metal was beyond my price range, but a 10′ roll of aluminum roof flashing was only about $4.00. I figured I could manage that, so I looked for the other materials. A 3′ stick of 1/4″ all-thread was only $0.99. I couldn’t find the spacers, and did not have a tubing cutter to make them with. I knew from experimentation that the spacing on the Hammerlund Capacitor’s plates was the same as a 1/4″ nut, so I decided to use nuts as spacers. I didn’t feel the flashing would be suitable to make the contact spring for the rotor, so I found a small compression spring to use for tensioning the rotor to the contact plate. I also decided to use nylon filled stop nuts for securing and adjusting the rotor. The total bill of materials came to less than $15.00, and all materials were found at my local hardware store. I had some Lexan on hand to use for endplates, but any good non-conductive material such as PlexiGlass, Teflon, Nylon, Lucite, or phenolic would work. Usable scraps are usually available at reasonable prices at most glass shops or plastics suppliers. I would recommend not less than 1/4″ thickness.

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Posted by on August 17, 2007 in Acrylic, DIY, Fabrication, Industrial Plastics

 

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