Friday, June 20, 2014

Vertical antenna



This type of antenna exhibits an omni-directional pattern, with a low radiation angle. The length of the radiator is calculated by using the formula 234/Frequency in Mhz, for feet, or 71.5/Frequency in Mhz, for meters to make a 1/4 wavelength, at the desired frequency. The radiator can be made entirely from 1" aluminum tubing, but can also be made from several sections of tubing of different sizes (Below 20M it is not appropriate to use 1" tubing). These could be fastened together using pipe clamps, after splitting the lower section about 1", across the circumference, along the diameter to facilitate clamping of the upper section.

The radiator is mounted on a reasonable length square wooden post which is buried in the ground or fastened to the roof. Large diameter pipe clamps are used to fasten the radiator to the wooden pole. At about 1/2" from the base of the radiator a hole 1/8" should be drilled into the aluminum pipe. This hole is used for a bolt onto which the coax center conductor is fastened.

The radials should be slightly longer than the radiator. To facilitate multi-band operation at least four radials should be used for each band you wish to operate in, the radiator being cut for the lowest band. All the radials are then fastened, or soldered together, and a connection is made to the coax cable shield. The radials should be buried a few inches into the ground, and ideally spread out in a circle. An antenna matching unit is used for multi-band operation. Alternatively, in place of aluminum tubing, a PVC pipe could be used, with a 1/4 wavelength wire positioned inside as the radiator. Note: For this antenna the Antenna Matching Unit should be at the base of the antenna, for best performance, and can be remotely controlled.

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