Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 655
• Height optimization (at HF in particular, “higher is not always better”)
• Intended purpose (e.g., local community coverage for a small business)
• Fear of heights
• Feedline losses
• Proximity to airports and heliports (i.e., FAA limitations)
• Local zoning and building code restrictions
The last two factors are discussed in more detail in Chap. 31, new in this edition.
Types of Towers
Several different forms of tower construction are used by amateurs and other services.
By far the vast majority in the United States consist of lattice structures with a triangular
cross section, although a few employing a square cross section (called windmill towers),
such as the Vesto self-Âsupporting towers of the 1950s, are in use. A small but structurally
impressive segment of the tower market consists of hollow tube, solid surface poles
with square or circular cross section. At least one commercial model, Big Bertha—available
in heights up to 142 ft—is a thick-Âwalled round pipe that is driven by an electric
motor at the base. In this model, the entire tower (called a rotating pole) is turned via
chain drive from the motor.
Ignoring roof-Âmount tripods and house-Âbracketed towers, both of which are severely
limited in maximum height, the following list of common tower types is arranged
in order of increasing cost for towers in the 70-Â to 100-Âft range:
• Guyed triangular lattice sections
• Guyed triangular lattice section foldover
• Triangular lattice section crank-Âup (some with tilt-Âover option)
• Freestanding nonrotating pole (mall parking lot lamppost style)
• Freestanding tapered triangular lattice sections
• Guyed triangular lattice section rotating tower (or a hybrid)
• Freestanding rotating pole
Tower Fundamentals
Before we look at the each type of tower in more detail, a brief review of tower fundamentals
is in order.
If the wind never blew here on earth, a tower’s role would be limited to supporting
the deadweight of antenna(s), rotator, and mast at the desired height. But for most antenna
and tower installations, deadweight is a secondary concern compared to the
forces that result when strong winds act on the tower, antenna(s), rotator, mast, cables,
and other accessories located above ground level.
Consider the freestanding tower of Fig. 29.1. The purpose of this tower is to support
and allow rotation of the Yagi beam at a desired height H in the presence of a wind. In