Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 671
pedestal should extend below the local frost line—even if local law permits less; in
northern parts of the United States, this will put it at least 4 or 4½ ft below the surface.
Beneath the base it is very important to have a layer of stones for proper drainage; layer
thickness will be a function of the type of soil encountered. If the tower base consists of
a partial section of tower extending all the way through and out the bottom of the concrete,
the stones should be large enough so that they can’t become wedged in the tower
legs and block condensate drainage.
Most manufacturers specify that the concrete pedestal include rebar reinforcing
steel; refer to the manufacturer’s drawings or Up the Tower by Steve Morris, K7LXC,
before pouring the concrete. Also, make sure you know exactly what needs to be embedded
in the concrete as part of the tower base before summoning the concrete truck.
There are many forms of tower bases and at least an equal number of custom inserts for
the concrete. Examples:
• Short section of tower (typically used for constant-Âdiameter guyed triangular
lattice towers)
• Pier pin (also used for constant-Âdiameter guyed triangular lattice towers)
• Hinged baseplate (often used for guyed triangular lattice towers up to 40 ft)
• Tilted flanges (for each leg of a tapered self-Âsupporting tower such as Rohn
SSV)
• Custom leg brackets (often used for smaller self-Âsupporting towers)
• Single I-Âbeam or mounting post (for crank-Âup towers)
• Special bolts supplied by the tower vendor
Some professional engineers and tower installers believe the base and its reinforcing
metal should be part of the tower grounding system for diverting lightning energy
into the earth. Bases designed for this are said to provide a Ufer ground. Other professionals
prefer the traditional approach of multiple ground rods surrounding the tower
base in all directions, with heavy-Âgauge ground cables running between the bottom of
the tower and the rods. See Chap. 30 (“Grounding for Safety and Performance”) for additional
discussion of grounding techniques.
Standard concrete reaches 99 percent of full strength about a month after it is
poured. To the prospective tower owner, that month can seem like an eternity. However,
the cure rate is an exponential curve, so many experienced contractors will begin to
attach hardware within a week of the pour, making sure no extreme stresses are applied
to the concrete pad—either directly or through the potent lever arm of installed tower
sections. Commonly, the first 20 ft of tower is held plumb with temporary guy wires
during the pour and cure cycles.
Guy Wires and Anchors
If you are installing a guyed tower, you may also be using concrete pedestals for the guy
anchors—a minimum of three, but possibly four (if you’re putting up a foldover) or
even six (if the tower is very tall and/or you want some redundancy in your guying
system). Basically, there are only three reasonable guy anchoring approaches to choose
from: