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[TowerTalk] Structure-Antenna Interactions

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Subject: [TowerTalk] Structure-Antenna Interactions
From: (Pat Barthelow)
Date: Sat, 13 Sep 1997 08:08:31 -0700 (PDT)
Regarding dipoles, yagis, effective heights on buildings....
We at N6IJ have a log periodic, 40 thru 10, that is mounted on an
(apparently) standard Military MARS station design platform, which
consists of 3 ea  70 ft wooden poles,  set eqilaterally 18 feet apart,
with a wooden platform supporting the antenna, (Hy-Gain LP 1017)
and rotator (Hy-Gain 3501).  The MARS standard design structure
apparently is required to have an 8 guage bare copper wire (grounded 
at the base) and running
up each of the 3 poles, and has a perimeter wire at the top, forming a 
grounded wire triangle, 18 ft on a side, about 3 feet below the log 
          My question is, does this ground wire system with top
perimeter wire constitute much of a reflective surface, adversely
affecting the launch angle or pattern (though broad, and poor as it is..) of 
the LP, which is otherwise 70 
feet above ground?  I understand that for low launch angles, the ground 
involved in the bounce re-inforcement is a long ways off horizontally 
from the antenna.  If I were to disconnect the perimeter wire at the 
top, leaving the vertical ground wires, would that give the LP a better
"effective" height above ground?  The design obviously has origins
in providing reasonable lightning protection in those areas of the
world that need it, but we, in Monterey CA rarely have lightning.
          BTW, do any of you MARS types know of this structure, and if
there are in MARS archives any structural engineering Docs on it wrt wind
loads, etc?  We would like to have drawings on such a structure, if
they exist.
Tnx, 73, DX de Pat, AA6EG/N6IJ

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