In a message dated 5/12/2010 5:18:40 P.M. Pacific Daylight Time,
towertalk-request@contesting.com writes:
> Actually, I have read and re-read "Up The Tower"
several times. It actually doesn't give specifics about where on the mast
to
go. I don't understand why they sell 15' reinforced masts if they want you
to put the beam right on top of the mast. Only a collection of very light
weight 2 meter and above antennas would qualify. Also not sure why they
pick
a tower height of 55' which is an odd height if comparing wavelengths of
amateur bands, being more than a half wavelength at 20 and less than a half
wavelength at 40.
Antenna location on a mast is user determined, that's why there's
nothing in UP THE TOWER on it. It's a balancing act between antenna load and
safety margin. Not being an engineer, tower calculations and capacities are
based on loads at the top of the tower. Obviously a segmented antenna load
on a tall mast will have some effect on the tower that is typically not part
of the manufacturer's specs. Since hams do this all the time, is this a
dangerous condition? Potentially. But virtually all of the towers configured
like this have proven to be reliable with no problems over many years of
use, which shows how much abuse a tower can take. In my experience the
incidences of bent masts far exceeds the incidences of bent towers.
The bending moment force on the mast is simple physics - force times
distance. With an antenna 1 foot above the top of the tower, the force
multiplier is 1. The effect of a load 10 feet up the mast is 10 times - a BIG
difference. That's why it's so important to get the right mast for the job.
As far as a big mast in a crank-up (or any tower), while there isn't
any manufacturer's spec for it (an engineer can calculate it for you) you
can be pretty confident that the tower will take the load. Obviously you
should err on the side of being conservative (e.g. lowering the tower when
it's
not in use or when a storm is predicted) but I don't see much of a
potential fatal problem with your proposed installation.
Cheers,
Steve K7LXC
TOWER TECH
and
_www.championradio.com_ (http://www.championradio.com)
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