Laryn is absolutely correct. For any antenna to have gain and produce a
clean pattern, the elements must be directly above one another. Why?
As the wavefront hits the elements from the side, the disparity in
horizontal distance will create an out of phase condition. Obviously,
the degree of this becomes worse as frequencies increase; given the same
amount of distance displaced horizontally, and is why it is acceptable
on the lower bands.
I did a test once with Frank, N3FLR a member of this list frog
Greensburg PA. When I erected a UHF repeater at the famed Seven Springs
site, the first generation of antenna was a Cushcraft AFM-44DA, a 4 pole
dipole array. The array was side mounted on the tower, however the
tower is very open being a fire tower. At first, I had the antenna in a
cardioid pattern headed for Pittsburgh which was directly off the face
of the tower. This proved to work very well in the Pittsburgh,
Greensburg direction, and wasn't too bad even in the null direction. I
decided to change the pattern to omni to see if the signal would improve
in the null direction as that direction was toward my house. I was
totally surprised as the signal didn't really change very much in the
original null direction, and Frank consistently had lost 20 "S" units in
the original peak direction while mobiling about the countryside. This
proved that single bay dipole antennas don't provide as much gain in an
omni-directional pattern as their fiberglass counterparts, especially on
UHF.
One of the better working 2M repeaters in the Pittsburgh area uses a 4
bay exposed dipole array, side mounted, with all elements directed
*toward* the tower. This has proven to give the best omni directional
coverage from a side mounted array at that particular site.
Kevin Custer
Laryn Lohman wrote:
>Mike, don't you think that this might produce a strange pattern? DB
>Products does not recommend taking their DB411 and 413 models, for
>example, and moving the dipoles around the support pole for this
>reason. The horizontal displacement of the dipoles in terms of
>wavelength becomes significant at 70cm. At 2m this displacement is
>not significant so moving the dipoles around the pole for omni
>coverage is OK.
>
>At 6m and below, it might work pretty well to space the dipoles around
>the tower, depending on the cross-section of the tower. DB offers
>this method as an option for it's low band dipoles.
>
>Laryn K8TVZ
>
>
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