Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 669
and three long-Âboom six-Âelement 20-Âm Yagis spaced appropriately for maximum stack
performance!
No longer in production, the Tri-ÂEx Sky Needle was another rotating pole, available
in 70-Â and 90-Âft heights. Sometimes one of these appears on the used market. Of course,
dismantling and transporting one of these beasts can cost as much as three or four
guyed towers!
A guyed rotating pole is substantially less expensive than a Big Bertha (but still quite
a bit rougher on the wallet than a conventional guyed tower). In some cases, enterprising
amateurs erect their own tower(s), using standard Rohn 45, 55, or 65 sections and
guy ring assemblies purchased separately. Others buy the complete tower (and often its
installation, as well) from Array Solutions, K0XG Systems, and a few other system vendors.
Compared with a self-Âsupporting rotating pole, the major advantages of a guyed
rotating pole are:
• It is far less expensive.
• It can be erected to greater heights.
The major disadvantages are:
• Guy wire attachment points may interfere with optimum placement of antennas.
• The guy rings can be a “challenging” maintenance item.
A hybrid guyed rotating pole consists of a fixed lower tower portion with a rotating
portion above. This is sometimes done to reduce the overall cost of the installation
when the height of the lowest rotatable antenna on the tower is a substantial percentage
of total tower height. The disadvantage of this approach is that the rotator and associated
drive system (e.g., chain) now must be located at the junction of the fixed and rotating
segments, many feet above ground, making rotator maintenance more difficult.
Tower Bases
Regardless of its exact configuration, a communications tower is typically installed atop
a buried pedestal made of concrete (Fig. 29.7). The base serves multiple purposes:
• It keeps the tower from sinking into the ground.
• It keeps the bottom of the tower from “walking” around.
• In conjunction with a layer of stone beneath, it allows proper drainage of tower
legs.
• For a self-Âsupporting tower, it resists any overturning moment.
• It provides a physical barrier between the tower legs and corrosive soils.
• It may provide part of the lightning ground system for the tower.
Base construction must follow any relevant local codes, but there are some general
considerations to keep in mind. The surface area of the top of the pedestal should be as
specified by the manufacturer, but the smallest practical or useful base is probably at
least 3 ft by 3 ft. If a backhoe makes the hole, the base is likely to be square, but augers
can also be used, especially if a circular hole is desired or necessary.