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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 **

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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:
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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.

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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.



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MGFoster wrote:
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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

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

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

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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

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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

MGFoster wrote:
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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 -

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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

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