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Aluminum foil capacitance hat
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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. -- MGFoster:::mgf00 at earthlink decimal-point net Oakland, CA (USA) KI6OFN Roy Lewallen wrote: Aluminum foil is fine for the purpose, electrically, but I think you'll have a dickens of the time maintaining good contact to it. You might even end up creating a rectifying junction which could generate harmonics when transmitting and intermod when receiving. -----BEGIN PGP SIGNATURE----- Version: PGP for Personal Privacy 5.0 Charset: noconv iQA/AwUBR6/dr4echKqOuFEgEQI8kwCgwEtTPufV3CntPuijUrZlyUQVsBYAn 0+U 98QGrHvI1noU4MSeDJm9Dwvx =lgr+ -----END PGP SIGNATURE----- |
#2
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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 |
#3
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Aluminum foil capacitance hat
On Mon, 11 Feb 2008 03:01:38 -0800, Roy Lewallen
wrote: 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. 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. 73's Richard Clark, KB7QHC |
#4
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Aluminum foil capacitance hat
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 |
#5
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Aluminum foil capacitance hat
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. At work one day the personnel department secretary requested one of the company two way radios for her desk. I installed a Motorola HT220 in a desk charger and plugged it into the outlet next to her desk. That night the mobile home trailer that we used for personnel burnt to the ground. To this day I don't know if there was a connection. ;^)... However, the radio was a mess. The trailer was manufactured during the copper shortage that necessitated the use of Al wire. Dave Nagel WD9BDZ |
#6
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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 |
#7
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Aluminum foil capacitance hat
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Hash: SHA1 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. 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." Oh well, back to the ... etc. Thanks, Roy 73 -- MGFoster:::mgf00 at earthlink decimal-point net Oakland, CA (USA) KI6OFN -----BEGIN PGP SIGNATURE----- Version: PGP for Personal Privacy 5.0 Charset: noconv iQA/AwUBR7C6SYechKqOuFEgEQIhIACg1s/RhwuFvUR+9nzAtKayoWb2rjIAn2Mo 3GdiQhJXuEr6RmICnaAfCJfC =aF9x -----END PGP SIGNATURE----- |
#8
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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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