----- Original Message -----
From: "Craig Sande" <craig.sande@sbcglobal.net>
To: <TowerTalk@contesting.com>
Sent: Saturday, March 27, 2004 11:09 PM
Subject: [TowerTalk] Bang for the buck?
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??
-------------------------------------
You can't extrapolate from 70mph exposure B to 85mph
exposure C
without engineering calculations. I know that Tashjian
can do
this, and provide the stamped drawings, for a fee of
about $200
(as of a few years ago).
Going from 60 to 76 feet, you will gain about 1.8 dB at
the lowest
angles. This is transmit and receive gain, on each
band. Whether
it is worth it depends on what you want to achieve, and
how
much money you have available.
If you don't really need a crankup, I'm sure AN
Wireless can
fix you up with a self-supporting tower that will
handle the
load at a decent height. It will probably be a lot
cheaper
than a crankup.
Dave Hachadorian, K6LL
Yuma, AZ
_______________________________________________
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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