Practical_Antenna_Handbook_0071639586

628 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
Figure 28.2 Mast-Âmounted vertical antenna.
Roof
Antenna
(18 to 26 ft.)
TV-type
mast
(12 to 30 ft.)
24" Ground pin
is low enough for vehicles, people, or animals to
come in contact with it, injury and damage can result.
In the United States, even when the person is a
trespasser (or even a burglar!), the courts may hold a
homeowner liable for injuries caused to the intruder
by an inappropriately designed and installed antenna.
Take care for not only your own safety but also
that of others.
Consider a typical scenario involving a four-Â or
five-Âband trap vertical antenna (Fig. 28.2). It will be 18
to 26 ft tall, judging from the advertisements in magazines,
and will be mounted on a roof or mast 12 to 30
ft off the ground. The total height above ground will
be the sum of the two lengths—perhaps 30 to 60 ft
overall. You must select a location at which a 30-Â to
60-Âft metal pole can be erected—and can fall—safely.
This requirement limits the selection of locations for
the antenna. In particular, if the drop wire from the
power pole to your house attaches to the house high
on an end wall rather than coming in underground,
you probably will not be able to find a proper location
for a house-Âbracketed or roof-Âmounted antenna of
that overall height.
When installing a vertical antenna, especially one that is not ground-Âmounted,
make sure that you have help. It takes at least two people to safely install a standard
HF vertical antenna, and more may be needed for especially tall or heavy designs.
Wrenched backs, smashed antennas, crunched house parts, dented vehicles, and other
calamities are rare with a well-Âorganized work party that has sufficient hands to do the
job safely.
Wire Antenna Construction
Volumes have been written on the electromagnetic design and analysis of wire antennas,
which are primarily employed in the MF and HF regions of the radio spectrum.
What often seems to be missing, however, are practical details of the mechanical aspects
of wire antenna construction and installation, so let’s take a look at some of the
basics.
Wire antennas come in many forms, but good techniques for construction and
support are pretty much the same regardless of the wire configuration, so we shall
use our old friend the half-Âwavelength horizontal dipole for our examples. In its most
common configuration (Fig. 28.3), the HF l/2 dipole is insulated from other conducting
materials at each end and supported by ropes, cables, or other wires attached to
trees, buildings, masts, or any other structures of suitable strength and height in fixed
positions.