At this time I am only in the early stages of planning a tower installation at
a new QTH. The local planning board has changed their tower ordinance, so I
will need to request a variance (called administrative permit $883) for
anything above 45 feet. The building department requires an engineers stamp to
certify that the structure will meet the UBC with wind rating of 85 mph and
exposure C.
Most of the crank-up towers are advertised with 70 mph ratings, but it is
difficult for me to translate that to 85 mph, as I understand that the wind
loading goes up exponentially. I would like to be able to handle about 15 sf
of antenna on a self-supporting crank-up tower with 10 foot mast extended about
6 feet above the tower.
With Tashjian Towers, for example, I can use their LM-354 HDSP which will get
the yagi to 60 feet (54' tower plus mast). The only other option they would
have would be the DX-86 with the top section removed, thereby bringing the
tower down to 70 feet and the yagi to 76 feet. Using my calculations (not a
formal bid), the higher tower would cost an additional $3000. (Their LM-470
wouldn't be able to handle a 15 sf load at 85 mph/exposure C).
How much more effectiveness, in terms of working dx, can I expect in going from
an antenna at 60 versus 76 feet? I've played around with EZNEC and can
demonstrate a decrease take off angle, but the difference doesn't appear
dramatic. Hmmm.... is it worth the extra money??
Thanks for your thoughts,
Craig, AE7I
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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