w4cng wrote:
>There are DB Products antennas that are available with factory supplied
>downtilt...
>Downtilt is accomplished by special spacing of the dipoles on the mast.
>
>Steve W4CNG PHD., The Repeater Engineering Co.
>
Steve,
Not sure what engineering company you work for, but you might want to
revisit the theory behind Downtilt with exposed dipole antennas (Binary
Fed) (Corporate Fed). Vertical spacing of the elements on the
supporting mast has little or nothing to do with the vertical beam
pattern. Horizontal spacing of the elements from the supporting mast
will vary the terminal impedance of that bay.
The phasing harness is what creates tilt of the vertical beam pattern.
The harness legs are cut so each successive bay is fed slightly more out
of phase with respect to the one below or above it, depending on which
direction you want to skew the pattern (up or down) and how much (in
degrees).
Also realize that Downtilt Cut antenna only performs to its
specifications on the frequency in which it was made for. Since
frequency is the determining factor of how many degrees of variance in
cable lengths perform the operation of a specific downtilt number,
changing the applied frequency also changes the amount of downtilt.
I commonly use 460.xxx MHz. fiberglass antennas (stationmaster type) on
my high mountain sites for UHF Ham repeaters. They 'automatically' give
downtilt when used on a 440 repeater pair.
Kevin Custer
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