Helical dipoles are manufactured to specific frequencies, but may require fine tuning after
installation. To enable this the dipole elements have an adjustable length tip to allow fine
tuning for optimum VSWR during installation. Install the antenna in its final position and
check the VSWR on each of the frequencies that the antenna was manufactured for.
Should the VSWR be greater than 1.5:1 the antenna will require adjustment. If a tunable
transmitter is available, determine on each frequency the helical dipole was manufactured
for, at what frequency the best VSWR is obtained. If this occurs at a frequency below the
required frequency then the tips will have to be shortened on the pair of elements
corresponding to that frequency. If the best VSWR occurs on a frequency higher than the
required frequency then the tips will have to be lengthened. Adjust both ends by an equal
amount and repeat the above sequence until an optimum VSWR is obtained. If a tunable
transmitter is not available use a method of trial and error to adjust the length of the tips, a
little at a time, until an optimum VSWR is obtained. Remember always adjust each pair of
elements by the same amount at each adjustment.
Hub detail
3 frequency helical dipole
BARRETT 950 HF TRANSCEIVER
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