Practical_Antenna_Handbook_0071639586

680 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
Another feature found in certain rotators (and their controllers) is provision for rotation
in excess of 360 degrees—in some cases an additional 180 degrees! This is a great
convenience for users such as DXers who may be “following propagation” with the
heading of their Yagis or quads. However, the owner of one of these devices must be
careful to provide an extra-Âlong rotator loop in all coaxial cables that pass the rotator on
their way up the mast.
Each different type of antenna rotator imposes different requirements on the rotator
cable that runs between the control box and the tower unit. For years, the standard in
amateur rotators was the CDE Ham-ÂM, which requires an eight-Âwire cable in a conventional
installation. However, two of those wires have the high solenoid current of the
brake wedge in them and require larger-Âdiameter wire than the other six wires for any
given distance between the controller and the top of the tower. One manufacturer,
Belden Wire, offers a jacketed cable (their catalog #9405) with two #16 and six #18 conductors
that is designed specifically for moderate runs to a Ham-ÂM or TailTwister rotator.
There is nothing magic about the wire, however, so any way the owner of a rotator
wants to put together eight cables of sufficient gauge for the length involved is fine.
Keep in mind:
• The required wire size depends on the total voltage drop of the rotator control
signals over the total length of the run. CDE has converted this to a specification
on the maximum allowable resistance for each individual wire; this information
can be found in the manuals for their rotators.
• Make sure the cable or individual wires you select are suitable for outdoor and/
or burial applications. Protecting your rotator cables by running them inside
PVC or other conduit material (with proper drainage for condensation) is
desirable.
• Some brands of wire insulation and cable jackets appear to have a chemical
makeup that is more attractive than others to field mice, squirrels, and other
“critters”. If you feel you have a disproportionately high degree of difficulty
with chewed cables, you may wish to try different makes of cable (or route
them through conduit with screening at the ends).
One trick for reducing the number of wires running between a CDE rotator and its
controller is to replace the motor start capacitor in the controller with one of similar capacitance
and voltage rating across terminals 4 and 8 on the bottom of the tower unit.
This will eliminate the need for two of the eight wires coming from the controller. Keep
in mind that motor start capacitors have a pretty sloppy temperature coefficient, so one
mounted outside at the rotator or tower base may lose as much as 50 percent of its rated
capacitance during cold winters, leading to some potential for sluggishness compared
to its summer performance.
Cabling requirements for the Orion and M 2 rotators and controllers are quite different.
Beam heading information is obtained from a pulse-Âcounting system in those units,
whereas CDE rotator control boxes get headings by reading the dc voltage on the wiper
of a wire-Âwound resistor (rheostat) located inside the upper housing of the tower unit.
Further, Orion and M 2 don’t employ a brake wedge, so there is no brake solenoid to
power.
Many users of the Orion and M 2 rotators have reported that it is very important to
run a completely separate cable for the pulse-Âcounting position indication system. If