Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 665
the top half of the tower is cranked down, four guy wires, uniformly spaced at 90-Âdegree
intervals, are required. A waist-Âhigh winch spools aircraft cable in or out to raise or
lower the upper part of the tower between its fully upright and foldover positions. A
safety bolt at the lower end (or farthest point) on the counterbalance boom secures the
tilt assembly once erected, thus minimizing end-Âof-Âtravel “slop” and eliminating the
need to rely on the cable to keep the upper portion perfectly vertical when fully erected.
At one time Rohn sold foldover towers in both 25G and 45G ratings, but they are no
longer in their catalog. Occasionally one can be found on the used market. Alternatively,
if the special hinged section and counterbalance boom can be procured, the remainder
of the tower is conventional Rohn product that is still available new.
The advantage of a foldover, of course, is that the top of the tower can be brought
close to the ground and all work on antennas, rotator, cables, etc., can be done while
standing on the ground or a stepladder. However, there are some disadvantages to this
system that are not present in a conventional guyed tower:
• During the process of raising and lowering the tilting portion of the tower, the
winch cable and upper tower sections experience maximum stress when the tilt
assembly is horizontal. This puts a severe weight limit on what can be installed
on top (in addition to any wind load limit) because side load limits for triangular
lattice tower sections are much lower than their longitudinal compressive and
tensile limits. In addition, the winch cable itself has a safe working limit that
must not be exceeded.
• The manual winch represents a potential safety hazard. With a full load at the
top of the tower, a person of normal build may have difficulty turning the crank
smoothly.
• If the winch assembly gets rusty or gummed up such that the safety catch does
not properly engage the teeth on the crank assembly, the winch handle can
freewheel if the operator’s hand slips while cranking the antennas up or down.
Even if the safety catch does engage, any lag in the catching mechanism can
result in a lot of weight on a very large moment arm coming to an abrupt stop,
with the possibility of damaging parts of the tower or, worse yet, snapping the
aircraft cable and seriously injuring the operator or helpers.
• As marketed, the Rohn foldover had no guy wires at the top. There is merit to
adding a set, depending on how many sections have been installed above the
hinge and the total wind load of the antennas. However, the owner needs to
also factor their added weight into the maximum weight calculations for the tilt
process.
• Some antenna rotators—especially those with internal ball bearings—may not
take kindly to being tilted partially upside down, depending on internal play
and retainer spacings.
Crank-Âup Towers
Figure 29.5 shows a crank-Âup tower. In the example shown, a wide base section supports
a smaller moveable upper section. Commercially available models usually have two to
five sections. In all of these, a manual or a motorized winch is used to lift the upper
section(s) into fully extended position. Typical two-Âsection towers can be raised from a