Practical_Antenna_Handbook_0071639586

678 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
and uncoils. For minimum required loop length, determine where either end of rotation
is and attach the cable below the rotator on the leg of the tower that is most nearly directly
opposite the end-Âof-Ârotation point. An alternative approach involves coiling a
turn or two of the cable around the mast just above the top of the tower so that it coils
and uncoils just like a spring as the antennas turn. Most hardline and some kinds of
coaxial cable are not meant to be flexed frequently; they should not be used to form the
rotator loop.
Although some commercial amateur VHF and UHF beams still come with predrilled
boom-Âto-Âmast brackets and mounting hardware for a 1.5-Âin mast, the standard
mast outside diameter (OD) for all but the largest amateur Yagis today is 2 in. Often it
is possible to modify older brackets for use with a 2-Âin mast by simply drilling two new
holes in the bracket and replacing the U-Âbolts.
The best mast material is galvanized seamless steel tubing designed for this purpose.
Rohn sells excellent 10-Âft masts, and specialty metals suppliers can provide even
longer ones. Note that the galvanizing process results in a worst-Âcase OD of slightly
more than 2 in, so rotator clamps, thrust bearings, and other support tubes should be
sized somewhat larger than 2.0 in. The mast clamp provided with the MFJ/CDE Ham-ÂM
and TailTwister rotators can handle diameters up to 2 1 ⁄8 in.
Some rotator manufacturers, including MFJ/CDE, make a lower bracket available
for mounting the rotator at the very top of a mast, but this type of installation should be
used only for very small VHF and UHF antennas with minimal wind load and torque
specifications. Even then, only a short (1 or 2 ft long) mast should be used. Most tower
manufacturers sell an accessory rotator shelf to fit inside their towers or tower sections;
typically one of these plates is mounted about 2 ft below the top of the tower. A second
plate is sometimes used to provide a thrust bearing or clamp support just above the top
of the rotator so that if maintenance or repair work on the rotator is required, it can be
removed from the tower while the mast and antennas remain clamped in place.
Although some amateurs have successfully used thick-Âwalled aluminum tubing for
mast material, this should be limited to Yagis with short booms and elements—i.e., installations
that apply only limited torque to the mast and rotator. Aluminum of any
thickness is no match for steel, and the teeth of a typical rotator mast clamp or the
threads of a pinning bolt can eventually chew through even solid aluminum tubing!
(Once again, just ask the author!)
For small and medium-Âsize HF Yagis and virtually all VHF and UHF beams, a simple
piece of hardwood can serve as a perfectly adequate thrust bearing. Use an inexpensive
circle cutter set to the next available diameter greater than 2 in.
The tops of most commercial towers and tower sections sold into the amateur radio
market have been predrilled or prepunched at the factory with standard hole patterns
for thrust bearings and commonly available rotators.
While some amateurs in areas with low wind loading characteristics have had success
using some of the larger TV rotators, those devices should not be used to rotate
anything larger than small (i.e., short-Âboom) HF beams—such as a three-Âelement triÂ
band Yagi or two-Âelement quad—in a benign environment. Rotators, such as the MFJ/
CDE TR-Â44, in this class were originally intended to turn nothing larger than long-Â
boom VHF deep fringe TV antennas; they are sometimes adequate for turning Yagis
such as the Cushcraft A-Â3S, Force 12 C-Â31, Mosley TA-Â33, or equivalent.
For larger tribanders (Cushcraft A-Â4S, Force 12 C-Â32) and small monobanders, the
venerable MFJ/CDE Ham-ÂM or others of equivalent torque and braking capability are
appropriate.