kc4fwc wrote:
> Hello Group,
> Have a dual antenna system for a 2M repeater. Decibel DB-222 top
> mounted, clears the tower and is oriented for omni-directional
> coverage. TX antenna is directly below and is side mounted on the
> southwest side of the tower. The antenna is a Celwave PD-340-3 with
> dipoles oriented around the mast (omni). It is mounted on the corner
> (not the face) of Rohn 25-G and is a factory mount and top sway brace
> from Decibel Products and they recommend this for side mounting DB-
> 224's. The mount lets the antenna extend around 22 inches from the
> leg of the tower.
>
> I am noticing VERY significant deficiencies in TX coverage back
> through the tower, even what I would estimate 8 to 10 dB. In other
> directions, it seems as if the coverage is as good as can be
> predicted.
>
> I have heard someone mention that taking all four dipoles on the same
> side of the mast and directing the elements straight into the tower
> would create more of an omni pattern. I can't remember if this was
> for UHF or VHF. I would like to eliminate any deep nulls, no more
> than 3 dB down in any direction if possible. Will turning all
> elements towards the tower help my situation?
The issue is how far the antenna is from the tower. You need to be about
one wavelength away from the tower to get close to an omni pattern from
a side-mount. Doesn't matter if it's VHF, UHF, SHF, whatever.
Pointing the elements into the tower will give you something funky,
depending on the shape of the tower and so on.
For a 2M repeater, push the antenna out about 5-6 feet from the tower.
--
Jim Barbour
WD8CHL
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