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#1
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Aluminum foil capacitance hat
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Hash: SHA1 I've made some Al foil cap. hats for a 34ish foot linearly loaded dipole (based on NN0F's antenna in the ARRL Antenna book, 21st ed, p. 6-35 - he used 6ft wires as cap. hats and 450 ohm ladder wire as the radiator, w/ coax feed). I was wondering if Al foil has enough "umpffh" to act as cap. hat material? Cap hat construction: 45 inch cardboard rolls wrapped in 1-1/2 layers of Al foil (shiny side out). 300 ohm twin-lead (the ant. radiator) leads soldered to copper tape, which was stuck to the Al foil (I check for continuity between the foil & the copper tape & between leads & Al foil they were OK). Final version wrapped in clear packing tape. I attached this ant. to my Elecraft K1 & got 4.7:1 SWR on 80 meters (where I expected it to resonate). On 15 meters got 1.2:1 SWR. Ant. is abt 45 ft above ground, fed by RG58 coax. Ground is concrete. Actually, the ant. is on my apartment bldg roof. That roof is covered in a silver paint or metallic weather-proofing (I don't know). The ground surrounding the apartment bldg is concrete and asphalt. I always select "Very poor - cities, industrial" as a ground description when using the EZNEC pgm. Thanks for any comments. -- MGFoster:::mgf00 at earthlink decimal-point net Oakland, CA (USA) ** Respond only to this newsgroup. I DO NOT respond to emails ** -----BEGIN PGP SIGNATURE----- Version: PGP for Personal Privacy 5.0 Charset: noconv iQA/AwUBR65zgIechKqOuFEgEQIMuQCgrOJElbBHN76ZhqGRp8yZO9 QB4ZcAoJeR fhsycR874iMwUFrSFk3Mmb39 =0zts -----END PGP SIGNATURE----- |
#2
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Aluminum foil capacitance hat
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. Roy Lewallen, W7EL MGFoster wrote: -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I've made some Al foil cap. hats for a 34ish foot linearly loaded dipole (based on NN0F's antenna in the ARRL Antenna book, 21st ed, p. 6-35 - he used 6ft wires as cap. hats and 450 ohm ladder wire as the radiator, w/ coax feed). I was wondering if Al foil has enough "umpffh" to act as cap. hat material? Cap hat construction: 45 inch cardboard rolls wrapped in 1-1/2 layers of Al foil (shiny side out). 300 ohm twin-lead (the ant. radiator) leads soldered to copper tape, which was stuck to the Al foil (I check for continuity between the foil & the copper tape & between leads & Al foil they were OK). Final version wrapped in clear packing tape. I attached this ant. to my Elecraft K1 & got 4.7:1 SWR on 80 meters (where I expected it to resonate). On 15 meters got 1.2:1 SWR. Ant. is abt 45 ft above ground, fed by RG58 coax. Ground is concrete. Actually, the ant. is on my apartment bldg roof. That roof is covered in a silver paint or metallic weather-proofing (I don't know). The ground surrounding the apartment bldg is concrete and asphalt. I always select "Very poor - cities, industrial" as a ground description when using the EZNEC pgm. Thanks for any comments. |
#3
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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----- |
#4
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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 |
#5
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Aluminum foil capacitance hat
MGFoster wrote:
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I've made some Al foil cap. hats for a 34ish foot linearly loaded dipole (based on NN0F's antenna in the ARRL Antenna book, 21st ed, p. 6-35 - he used 6ft wires as cap. hats and 450 ohm ladder wire as the radiator, w/ coax feed). I was wondering if Al foil has enough "umpffh" to act as cap. hat material? Cap hat construction: 45 inch cardboard rolls wrapped in 1-1/2 layers of Al foil (shiny side out). 300 ohm twin-lead (the ant. radiator) leads soldered to copper tape, which was stuck to the Al foil (I check for continuity between the foil & the copper tape & between leads & Al foil they were OK). Final version wrapped in clear packing tape. I attached this ant. to my Elecraft K1 & got 4.7:1 SWR on 80 meters (where I expected it to resonate). On 15 meters got 1.2:1 SWR. Ant. is abt 45 ft above ground, fed by RG58 coax. Ground is concrete. Actually, the ant. is on my apartment bldg roof. That roof is covered in a silver paint or metallic weather-proofing (I don't know). The ground surrounding the apartment bldg is concrete and asphalt. I always select "Very poor - cities, industrial" as a ground description when using the EZNEC pgm. 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. - 73 de Mike N3LI - |
#6
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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 |
#7
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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 |
#8
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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----- |
#9
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Aluminum foil capacitance hat
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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). 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. Regards & 73s, -- MGFoster:::mgf00 at earthlink decimal-point net Oakland, CA (USA) KI6OFN -----BEGIN PGP SIGNATURE----- Version: PGP for Personal Privacy 5.0 Charset: noconv iQA/AwUBR7C744echKqOuFEgEQIWpgCfbHwps+8RAVBLgtYGf7uu/UywXPkAmwXI iL1XU4yOSzyPugTPgpEZKxYU =d9Cc -----END PGP SIGNATURE----- |
#10
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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 |
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