Practical_Antenna_Handbook_0071639586

660 P a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s
Although the preceding calculation is grossly oversimplified, it provides a basic
understanding of the key structural issues germane to communications towers. Clearly,
larger antennas and/or taller towers result in larger compressive forces at the base of
the tower. Starting from an understanding of the basic lever action that is involved, we
can now more easily visualize three ways to improve the antenna-Âcarrying capacity or
increase the height of our antenna support:
• Use thicker tower legs of the same material, or make the tower members out of
stronger materials. (Both approaches are important contributors to the strength
of freestanding rotating poles.)
• Increase the distance R from each leg to the centerline of the tower, especially
toward the base of the tower. (This is the principle behind tapered self-Â
supporting towers such as the Vesto, the Rohn SSV series, some rotating poles,
and most crank-Âup towers such as the EZ-ÂWay products.)
• Reduce the moment resulting from wind pressure on the antenna and tower
members with the judicious addition of guy wires that counteract the force of
the wind. (This is the principle behind all guyed towers.)
Caution The preceding numerical calculations are overly simplified in order to emphasize
basic structural strength issues in towers. No attempt has been made to include the effects
of wind gusts, sudden shifts in wind direction, uneven terrain, updrafts and downdrafts,
icing, mechanical resonances, and a host of other effects that a thorough design must take
into account. Any new tower installation project should be conducted in accordance with
the determinations of a professional engineer (PE) licensed to practice in your area. In
many municipalities, certain paragraphs of the local building code may apply and periodic
inspections may be required during the course of the project. But many, if not most, building
code enforcement offices will accept preexisting standard installation drawings and calculations
provided by commercial tower manufacturers if they have been signed and stamped by
locally licensed PEs, so that avenue should be explored before incurring the expense of designing
a tower installation from scratch.
Guyed Towers
One way to minimize the moment arm at the base of a tower from wind blowing against
an antenna at the top of the tower is to directly resist the force of that wind through
some other physical mechanism. The most efficient way is to directly oppose the wind by
pushing on the antenna in a direction opposite the wind or by attaching a guy wire or
guy line to a firmly fixed support on the windward side of the antenna. If the guy wire
is adjusted to be taut with no wind present, then any increase in force on the antenna
when the wind comes up will be met with an increase in tension in the guy wire, with
the result that the moment arm at the base of the tower is substantially reduced or
eliminated.
In the ideal installation, we would attach the far end of the guy wire to a support at
the same height as the antenna so that the guy wire was perfectly horizontal. But if such
a support existed, we might as well attach the antenna itself to it and eliminate the
tower entirely!