[TowerTalk] Bang for the buck?

[Craig Sande]

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