Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 677
Caution Always wear a hard hat when installing a tower or its antennas. This is especially
true for the ground crew because even a small stainless steel bolt or hand tool can cause great
injury when dropped from above. But it is also true for any tower climbers, as well, since it’s
extremely easy to crack your skull on the underside of a guy bracket or boom-Âto-Âmast bracket
while ascending or while wrestling an antenna into place. Just ask the author, who is a
graduate of the school of “hard knocks” (so to speak)!
At some point you may decide that your best course of action is to have your tower
installed by professionals. The boom in cellular tower installations over the past few
decades has resulted in no lack of highly competent tower installation companies
throughout the world. There are also companies with particular expertise in tower and
antenna installations for radio amateurs in addition to the work they do for commercial
and institutional customers. An example in the northeastern United States is XX Towers,
Inc., headquartered in New Hampshire. Often the tower suppliers listed in App. B
can provide a list of reputable installers.
Rotators, Masts, and Thrust Bearings
Many, if not most, antennas placed atop a tower are intended to be rotated because their
radiation patterns are highly directional and the desired direction of communication is
subject to frequent change. This is true of almost all Yagi and cubical quad installations
for not only HF but VHF and UHF as well. The typical installation of such antennas
atop a guyed tower is shown in Fig. 29.11: An antenna rotator mounted to a shelf a few
feet below the top of the tower turns a
strong mast, which is often prevented
from tilting sideways and binding by a
thrust bearing at the very top of the tower.
Note particularly the rotator loop in
the coaxial cable that goes past the rotator.
One end attaches to the rotating assembly
(usually by being taped to the
mast, just above the top of the tower or
rotator, whichever is higher) and the
other end attaches to the highest practical
nonrotating tower surface (usually by
being taped to one tower leg). Leave
enough slack to allow full rotation of the
antenna(s) in both directions without
putting any tension on the cable. Dress
the loop carefully so that it does not catch
on the top of the tower, and check its operation
by having a helper rotate the antennas
while you stay just below the top
of the tower to watch how the cable coils
Figure 29.11 Tower top showing rotator on shelf,
mast, thrust bearing, and rotator bypass loop in
feedlines.