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Aluminum foil capacitance hat
MGFoster wrote:
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 What do you mean "maintaining good contact to it"? The copper tape adheres well & doesn't move. The foil is secured in place by the packing tape (it completely covers all of the foil surface and copper tape connections). The tubes propeller a bit in the wind, but I don't think that causes any disruptive flexing of the foil; at least not enough to cause the soldered/taped connection to break/discontinue. I couldn't xmt on 40M cause of the high SWR, but I could rcv &, except for the noisy conditions, it sounded fine. Thanks for your comments. The instant aluminum comes in contact with air, an oxide layer forms on it. The oxide layer is a hard, brittle ceramic which is an excellent insulator. Making electrical contact to aluminum requires breaking through the oxide to the metal underneath. This causes broken pieces of the oxide to dig into the aluminum. If pressure is maintained and no air ever strikes the aluminum again, the contact will remain sound. That is, in fact, how aluminum house wiring is (properly) handled -- a high pressure contact, with a generous amount of grease to prevent contact with air. If your contact jiggles a bit in the wind or for any other reason, the physical contact in various places will be broken at times. Each time, more oxide is formed and, when pressure is reapplied, it breaks and gets pushed into the aluminum. Eventually, you have a surface that's mostly shards of oxide and little or no metallic aluminum. (This is also why tin plated contacts always inevitably fail.) In addition, copper and aluminum form an electrolytic cell. So if any moisture is present, corrosion of the joint will occur and contact will eventually become bad or lost altogether. That's what I mean by the difficulty of "maintaining good contact". Your system will work fine. For a while. Roy Lewallen, W7EL |
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