Practical_Antenna_Handbook_0071639586

666 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
low position of 20 ft to a height of 35 ft or so. When the tower is fully extended, some
models incorporate provisions for locking the sections in place with steel bolts, steel
bars, or other mechanisms, but these options are by no means universal. A lot of shear
force is applied to these bolts, so it is wise to use several very hard stainless steel bolts
for the lockdown.
Caution There is a serious danger inherent in the design
of the crank-Âup tower: the guillotine effect. If a cable supporting
extended sections breaks while you are working on or
near the tower, one or more sections will come plummeting
down the “elevator shaft” formed by the section(s) below it
and shear off any arms, legs, or other body parts that get in
the way. Whenever you work on this form of tower in its
extended position—either fully or partially—use steel fence
posts (or similar pieces of metal) as a safety measure (Fig.
29.6); at least two should be used, and both should be attached
securely at both ends with rope so they can’t be
knocked out accidentally. These pipes are used in addition to,
not instead of, the normal bolt fasteners that keep the antenna
tower erect.
Winch
Shown
cut-away
Some crank-Âup towers are motorized so that the
tower can be raised or lowered without anyone having
to stand at the base. Some crank-Âups also tilt over, so
only a stepladder is needed for installing, removing, or
adjusting a typical HF Yagi. Between these two optional
features, quite a few of the potential hazards
Âassociated with crank-Âup towers are eliminated or substantially
reduced. Of course, crank-Âup towers with
either or both of these features cost quite a bit more.
All towers require periodic maintenance and examination,
but crank-Âup towers as a category require
more than most others. In particular, the cable(s) that
perform the raising and lowering functions must be
frequently inspected and lubricated with compounds
specified by the manufacturer of the tower.
In the author’s opinion, there is also a major practical
inconvenience with crank-Âup towers. For many
models, the specified maximum wind load when the
tower is fully extended is so small as to effectively preclude
the installation of even a modestly sized HF
beam. Some crank-Âup owners will respond, “No problem!
I just crank it down whenever I’m not using it or
whenever the wind blows.” That may be an acceptable
Figure 29.5 Crank-Âup tower.