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W7AV in WA has put up a half square (2-element 40m)
that fits his small city lot and has been an outstanding DX
performer. He has been kind enough
to share photos of his installation. The antenna is built
with guyed, aluminum tubing verticals.
The first vertical is on the left, and the top of the
second vertical can be seen above the roofline near
the tree. The elements are made from telescoping tubing
sections, starting at 1.25 inch diameter. They are 32 feet long.
Element spacing is 68.5 feet. Allowing for wire sag, the length
of the horizontal wire is about 70 feet. |
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The L/C matching network is protected from the weather by a plastic
box. 1/4 inch copper tubing is used for the coil, and an RG-213
coax capacitor tunes it to resonance at 7.020 MHz. The network
sits atop the counterpoise, which is made from scrap copper
wire and tubing, and connection to the base of the vertical is
made with a short, horizontal length of wire. The tubing element
is clamped to a 2" by 4" wood insulator, which is clamped in turn
to a pipe set in the ground in concrete.
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5 inch diameter plastic drainage tubing is
used to insulate the first 10 feet of each
element for safety. When transmitting, very high RF voltage
is present- any possible contact by humans, animals,
or plants must be prevented.
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An inside view of the network box shows the coax capacitor
and the copper tubing coil. Strips of epoxy cement hold the
turns in place.
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