I designed and built a rotatable dipole for 40 meters that was
about what you want. I used ~35' elements (about full size 20
meter length), with 6' diameter 8 spoke capacity hats with
a perimeter wire. The elements were built according to the
20 meter taper schedule in the Antenna Book, heavy duty version.
The hats were made from .177" aluminum rod - four 6' pieces.
I sourced the tubing and rod from Texas Towers - great guys!
I drilled through a short (4") piece of aluminum tubing that
was sized to slip over the element tip in four places. Then
I slipped each rod through to the middle, and brazed them
in place using a propane torch and some of the new low temp.
aluminum brazing (they call it "welding") rod. Then I brazed
aluminum wire to the ends of the rods as a perimeter wire.
It played great! I later built a 2 element yagi using similar
techniques, but longer elements (~50'). That yagi (which is
still up), uses 4 spoke capacity hats with no perimeter wire.
You want about a 24 foot boom, for decent feed impedance. Use
a hairpin coil to step it up to 50 ohms - it will be less due
to the short elements.
GL es 73,
Remember the California QSO Party, in October!
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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