[TowerTalk] Antenna Height

trife at sirus.com trife at sirus.com
Wed Apr 18 13:58:20 EDT 2007


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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