Practical_Antenna_Handbook_0071639586

668 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
the top of the tower and typical amateur rotators, masts, and thrust bearings will usually
entail custom metalworking and welding at a local shop.
One problem with tapered self-Âsupporting towers is the relative difficulty of climbing
them. The climbing spikes commonly seen attached to one leg of the tower are usually
too small a diameter to be comfortably grasped, and if you’ve just walked across a
muddy field in your climbing boots to get to the tower, you’ll find you’re trying to hang
on to mud-Âcovered spikes when you descend. However, often a matching metal ladder
is available for purchase.
AN Wireless Tower Company is another manufacturer of tapered self-Âsupporting
towers for the amateur radio community. Like Rohn, their product consists of a family
of sections with some heavy-Âduty options possible. AN Wireless’s sections are 10 ft
long, and the maximum height currently marketed is 120 ft.
A lighter-Âduty tapered tower family of 8-Âft sections, originally offered by Spaulding
and later by Rohn, can occasionally be found on eBay or amateur used equipment bulletin
boards. Although inexpensive, these have some disadvantages a prospective
buyer needs to be aware of:
• The side rails are made of U-Âshaped steel channel instead of cylindrical tubes.
Consequently, they are very painful to grip while climbing or when working at
the top of the tower.
• The cross-Âbracing is comprised of diagonal steel strips on edge. Since there are
no horizontal surfaces to stand on, climbing and working on one of these towers
is rough on the feet.
• Invariably the used tower has rust on it. If so, it absolutely must be sandblasted
and regalvanized or painted after being inspected for soundness.
• All hardware securing the cross-Âbraces to the side rails must be removed,
discarded, and replaced with new high-Âstrength nuts and bolts.
• These tower sections also came in a heavy-Âduty version (the HDBX series). This
is the only version that is truly adequate for even small amateur HF Yagis; the
standard-Âduty model is suitable only for VHF and TV antennas.
Rotating Poles
Another form of self-Âsupporting tower is the rotating pole. As compared to almost any
other kind of tower, the rotating pole provides the greatest degree of flexibility in placing
multiple high-Âgain antennas (notably long-Âboom Yagis) at specific heights on the
tower for optimum stack spacing and full 360-Âdegree (or more) rotation. A typical rotating
pole “sits” on a giant ball bearing below ground level and is rotated by chain drive
at ground level with a prop pitch or similar motor mounted on the same concrete pad
that holds the tube in which the bearing is located.
Today the most well-Âknown of the rotating poles is the Big Bertha, available through
Array Solutions. The name is derived from the original 115-Âft rotating pole offered in
the 1960s by now-Âdefunct Telrex Antenna Systems. The current Big Bertha design is
available in heights up to 142 ft, and at least one such tower in the northeastern United
States a few years ago sported a bevy of HF Yagis totaling more than 150 ft 2 of equivalent
wind load! A typical antenna complement for the “satisfied user” is a three-Âelement
80-Âm Yagi at the top, a pair of four-Âelement 40-Âm Yagis at the midpoint and near the top,