Thanks for saving me all the typing ;-)
Chuck
WB2EDV
Kevin Custer wrote:
I think what Chuck was getting at was the
'automatic' beamtilt of a
vertical omni collinear (usually fiberglass) when
it is run outside of
its specified bandwidth. As a function of the
element length in a
coaxial collinear as compared to the applied
frequency, the vertical
beam pattern will change with applied frequency.
If a coaxial collinear
is fed with a signal that is exactly on its design
frequency, the
vertical beam pattern will be centered about the
antenna, and the
antenna will be at its highest radiating
efficiency. If a coaxial
collinear is fed with a signal that is 2% lower
than its design, the
antenna will exhibit a vertical beam downtilt of
approximately 3 degrees
and suffer approximately 10% loss in overall gain.
If a signal that is
2% higher than the antenna design is fed into a
coaxial collinear,
vertical beam uptilt of approximately 3 degrees
will occur, and again a
loss of overall gain.
These instances are not the case with binary or
corporate fed dipole
arrays, as the phasing harness predominantly
controls the vertical beam
pattern. Beam Tilt and efficiency doesn't change
very much with applied
frequency, and is one reason that the exposed
dipole array is a better
choice where wide band operation is required.
Kevin Custer
skipp025 wrote:
Kind of loaded question/statement/answer really.
All
antennas have both horizontal and vertical
beamwidth.
Depending on what you think is beam-tilt... one
could
and some do say all antennas have a beam tilt and
or
a beam width. Others combine the description...
In the more commercial world of antennas, we now
see
vertical omni repeater site antennas with
adjustable
beam tilt.
But I'm not sure if I'd say they have to be made
with
fiberglass radomes (covers). There's more than
one method
used by the various mfgrs to adjust the beam tilt
- beam
width. For the most part we only see some models
with
adjustable setting in some vertical omni models
with
composite radomes.
... and you pay serious money for the adjustable
beam
tilt models. If you pay attention to the specs,
you'll
see values for the horizontal, vertitcal beam
width and
where needed, the/any adjustable beam tilt
values.
Your results will probably vary...
cheers,
skipp
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