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Bobtails and Half Squares

These antennas offer big gun DX performance on the low bands,
yet they are surprisingly simple to build. Try one- you will like it!

W7AV 40m Half Square

  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.

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.  

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.

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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