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
periodic.
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
aa6eg@tmx.com
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