In a message dated 2/7/03 11:54:05 AM Pacific Standard Time,
RCARIELLO@si.rr.com writes:
> Is there a rule of thumb to follow?
No - that's why many ham masts bend. There are formulas.
>
> Tower rated:
> 15 Square feet 1 foot above tower top.
>
> Moving the antenna to 15 feet above tower top.
>
> How large in square feet can the antenna be without exceeding the tower
> limits?
The first thing you need to know is your county windspeed rating. You can
look it up at <A HREF="http://www.championradio.com";>www.championradio.com
</A> under Tech Notes.
There are calculations that'll help you determine the bending moment of
the load you're proposing based on the windspeed pressure you use. Dr. Dave
Leeson, W6NL, ex-W6QHS, covers it all in his book "Physical Design of Yagi
Antennas" (I understand it's out of print).
I offer a little program called MARC - the Mast, Antenna, and Rotator
Calculator. you can get it at the aforementioned website for ten bucks.
The thing to remember with your above example is that an antenna 15 feet
up the mast exerts 15 times as much bending moment than it does when mounted
at one foot above the tower. The forces can add up quickly.
The rule of thumb is "do the calculations".
Cheers,
Steve K7LXC
TOWER TECH
CHAMPION RADIO PRODUCTS
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