Comments woven...
--- In [email protected], bradley glen
<[EMAIL PROTECTED]> wrote:
> Hi All
>
> Some time back I did do that same-upside down
> collinear-my theory was with the slight uptilt given
> to general collieas I tried it upside down and it
> worked very well.
> Loss of main RF radiation in the horizon is in essance
> the same as the majority of the radiation is from the
> first radiating element ie: the base of the collinea.
> The second theory is by using an antenna slightly off
> frequency that in itself will cause slight up-or-down
> tilt by a small degree.
That's true for series-fed type gain antennas like most fiberglass
styles. Not true for stacked dipole parallel-fed types.
> The good thing with mounting it upside down is that in
> "most "repeater installations there is always a less
> important direction that does not need full gain so
> the offset mounting becomes in your favour as in the
> case of stacked dipole arrays.
>
> There is of course no harm in mounting two collinears
> via wilkonson divider -first in up right and the
> second on the upside down position.
Actually, mounting one upright and the other upside-down while fed in-
phase with a divider would cause cancellation of the RF field at the
horizon. You would need to phase them properly by adding an
electrical half-wave to one of the feeders.
Commenting on other parts of this thread:
Back in the early 70s, one of the local 2M repeaters used separate
antennas for tx and rx. They were Moseley DI-2, basically a 5/8 wave
ground plane. They mounted the top rx antenna upright and the bottom
tx antenna upside down about 50ft apart for better isolation. Later
on, when they acquired a duplexer, they used the top antenna only.
When using the top antenna it was very evident that the tx signal was
slightly weaker in the normal coverage area but ultimate tx range was
slightly greater. The upside-down tx antenna put more signal toward
the closer-in areas, while the top upright antenna had a higher angle
of radiation, so didn't cover the close-in areas quite as well, while
putting a little more signal at the horizon. This was at a site
about 300ft HAAT. Keep in mind that these were small ground plane
antennas that didn't have good low-angle radiation. Inverting a good
higher gain antenna like a DB224 would probably not change a thing,
RF-wise.
Laryn K8TVZ
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