20m will likely benefit slightly from the extra height, however 10m will
likely worsen, especially when conditions improve in a few years.
To: towertalk@contesting.com
Subject: [TowerTalk] Antenna Height
Date: Wed, 18 Apr 2007 13:58:20 -400
TT'ians,
I am planning to replace a cushcraft A3WS that has a failed trap this
spring.
It is mounted 10 feet above a 50 foot 25g tower along with an A3S just above
the tower top. As I got to thinking about it, since I would be doing some
antenna work, might as well replace the A3S with something without traps and
perhaps a bit more punch while there so I don't have to do another
replacement
in the future. Well, that then led to the thought of maybe I should go up
to
70 feet with an added set of top guys and the guy bracket in addition to the
two HBUTVRO brackets which are at about 10 and 30 feet to get a little
higher
off the ground. The lower house brackets is bolted to a floor plate, the
other
is through bolted to a wall with lumber on the inside spanning several
studs.
So, the current thinking is a F12 WARC 7 at 8 - 10 feet over the tower top
with
a Skyhawk just over the tower top at 70 feet. That's 13.4 sq ft of wind
load
plus mast. Since the added set of guys is non-standard for a bracketed
tower,
I see nothing in the Rohn book addressing it.
Seems to me I would have the equivalent of a 70 foot 25g tower "effectively"
guyed in three spots with the two house brackets and a top set of guys. I
am
wondering if this would withstand the wind loads and twisting or I would
need
to go with something bigger like 45g? I am in a 75mph wind zone from what I
can tell and the installation is surrounded on all sides by 60 to 70 foot
trees
that shelter it from the wind a bit.
Secondarily, am I likely to see much improvement in performance for the
extra
20 feet of antenna height, particularly on 20m?
I would appreciate the thoughts of the experts out there on this
Thanks/73
Tim
KK9T
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