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... 


Chuck Kelsey <[EMAIL PROTECTED]> wrote:
I believe the down tilt issue only happens with the 
fiberglass antennas.

wa9ba wrote:
Our new antenna is 1.:1 on 
145.490 and the coverage has increased about 10 or 20 miles. 
Apparently we had some downtilt that we no longer have. 
I would recommend buying a DB224 cut for the ham bands first, save 
yourself trouble.
Bill WA9BA

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