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#11
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Aluminum foil capacitance hat
MGFoster wrote:
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Michael Coslo wrote: What is the antenna length? Now as for Al foil as an antenna construction material: It works electrically, but isn't very strong. Your cardboard tubes aren't terribly weatherproof either. You might be able to shellac them heavily. But still it isn't a very good choice. Making contact with the aluminum could be an issue. Tubular aluminum is used on antennas, but the connection is usually helped by corrosion inhibitors, and nuts/bolts with washers to help "dig in" to the AL. Your thin foil will deform at best, and more likely rip with that sort of treatment. The antenna is abt 34 ft (1/4 wave for 40 meters). Okay. Assuming that is 34 feet per leg, you might be cut a little long for 40 meters. (Let me make sure I'm correct here - this is a linear loaded dipole, yes?) What would be good to know is the frequency where you are at your lowest SWR. Can you beg borrow or steal an antenna analyzer? Your antenna is probably cut for somewhere outside of where you can transmit with your rig. That would be the quick way to find out. Otherwise you should take SWR measurements across the band to see if that gives you an idea about how the antenna is cut. Another thing is to try a measurement without the capacity hat - see where that puts you. It might just get you a lot closer to tuned. The cardboard tubing was for weight - this ant. was to be a portable ant. and would be deployed only in good wx. "Treatment" - that's why I wrapped the tubes/foil in clear plastic wrapping tape. Barring me accidentally smashing them or some discombobulated squirrel "having at 'em" I'd expect them to last for abt a year or 2. Well, you can give it a try, homebrewing is a big part of the fun in Ham radio. - 73 de Mike N3LI - |
#12
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Aluminum foil capacitance hat
David G. Nagel wrote:
Roy Lewallen wrote: Richard Clark wrote: Hi Roy, If I recall history and physics correctly (and simultaneously), house wiring failed through cold flow, a property of aluminum to relax under a one-time application of pressure (impulse rather than sustained pressure) and thus the joint failed. The failure raised resistance, resistance raised temperature, temperature raised flames.... no more common use of aluminum in house wiring. When I put in a new service entrance in 1979, it was after the problems with ordinary aluminum house wiring were discovered and it was no longer being used. But the service entrance cable I got was aluminum. The service box connectors are marked as approved for both aluminum and copper wiring, and have a compression fitting which doesn't give the aluminum anywhere to flow. Generous application of NoAlOx or a similar grease was also required for proper installation, so that's what I did. It's been completely trouble-free for nearly 30 years now. So it was, and is, possible to use aluminum wire. But it has to be done correctly, with due regard to its vagaries. Roy Lewallen, W7EL Roy; The problem with AL house wire is that the pure Al that the wire is made of will flow under pressure. When it was used with Cu wire switches and outlets the attachment points could not maintain the pressure to insure adequate contact. there's a bit more to it than that.. and, there's a whole industry that has sprung up to do wiring retrofits, adapters, inspect your house, etc., as well as to litigate claims of one sort or another. Tough to find unbiased information on the web. It has to do with which alloy of aluminum was used, and the casual use of the wrong alloy with connection methods not suitable for it (i.e. screw terminals). When the copper prices rose and the economy sank in the late 60s early 70s, they used the same aluminum wire (EC1350?) as had been used (successfully) for overhead and underground feeders for decades. The wire companies will make wire out of any alloy you want, so folks said, give me that stuff I've been getting in 2/0, except in AWG14 or AWG12. Bad choice, as it happens. An alloy that is good for big ol' fat feeders supporting their own weight on a 100 ft span, especially bare ones, going into big (aluminum) compression clamps, is probably not a good choice for electricians working in tight quarters, stripping insulation with a penknife, and then wrapping it around a screw made of steel that's installed into a threaded hole made of brass. Even worse, the quickwiring "push it in the hole" type connectors. Another problem was the change from brass to steel screws in the terminations, also as a result of cost. The Coefficient of Thermal Expansion (CTE) mismatch between brass and aluminum isn't as big as between steel and aluminum. And, issues with Steel screws staying tight in brass threads, etc. I don't know that cold flow is actually an issue. After all, most power lines and the feeder to your house is done with old style aluminum wires, into a compression screw termination, and failures are very rare, even with the connector untouched for 50 years. It's more the gradual loosening from temperature cycling, which is self aggravating: small gap opens, contact area reduces, localized heating causes more of a gap, etc. Modern aluminum wiring uses another alloy (AA-8000 series or ACM), which has better mechanical properties: it's less brittle, so nicks and tight turns don't cause problems, it's also mechanically weaker, but that's not an issue, unlike with overhead power lines, which need to support their own weight. In addition, the termination device (the screws or clamps) are specifically designed to work with the aluminum (or copper) wire. Things like little ridges and grooves in the plate you tighten down against. Some interesting photos at: http://www.qpi-inspect.com/aluminum06-02-04.pdf |
#13
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Aluminum foil capacitance hat
MGFoster wrote:
Roy Lewallen wrote: SNIP If pressure is maintained and no air ever strikes the aluminum again, The contact will remain sound. Wouldn't the complete enclosure of the Al by the packing tape reduce, or prevent, air contact. I'm not saying I did that good a job of wrapping the foil, I was just concerned w/ protecting the foil from physical wear & tear. It should help, but I honestly can't say how much. The tape should help keep moisture out, anyway. SNIP That's what I mean by the difficulty of "maintaining good contact". Your system will work fine. For a while. I was going to use the antenna just for portable ops, and I was hoping it would last longer than "a while." Surely the larger the surface area of the contact, the longer before it fails to make contact somewhere. There's no reason not to use it, but be on the lookout for intermod and harmonics or noise in the receiver when the wind blows and jiggles it, and don't be surprised when it fails. Roy Lewallen, W7EL |
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