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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MGFoster wrote:
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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).

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 -