Probably mounting it further away from the tower will reduce the nulls. Turning
the dipoles all one direction into the tower has the effect of directionalizing
the pattern and pointing it into the null. When you directionalize, you get
about 9 db gain toward the direction of the dipoles and loose 3 db the other
way (you still have an average of 6 db). So if you turn the dipoles all toward
the tower, you should reduce the null by 3 db but you also reduce coverage in
the other direction by 3 db, assuming everything else remains the same. The
gain off the sides will not change much and just interpolate in between. I
would try it because such a big difference between transmit and receive can be
very annoying.
For short towers (less than 500 feet or so), I prefer to use the top antenna
for receive since it is more irritating to be able to hear the repeater and not
get in than vice-versa.
Dave Baughn
Director of Engineering
The University of Alabama
Center for Public Television and Radio
Box 870150
Tuscaloosa, Alabama 35487
205.348.8622 cell 205-310-8798
NEW EMAIL [EMAIL PROTECTED]
>>> [EMAIL PROTECTED] 08/12/05 04:08PM >>>
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?
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