Practical_Antenna_Handbook_0071639586

110 P a r t I I : F u n d a m e n t a l s
To summarize: The geometry of almost all transmission lines has been designed to
minimize the tendency of the lines to act as antennas and radiate on their own. In contrast,
the geometry of an antenna has been designed to maximize its tendency to radiate!
If we know the geometry of the conductors in a transmission line, as well as dielectric
properties of the material that occupies the space between them, we can use equations
found later in this chapter to calculate the characteristic impedance, or Z 0 , of that
line. While it is possible to construct transmission lines of any desired Z 0 over a very
wide range of impedances, certain favored impedances have evolved over the years
and today represent almost the entirety of commercial transmission line production.
Selection of a characteristic impedance, also known as the surge impedance, for a system
is a very important decision that should be made early in the design of that system.
Certain standard choices exist today: Most amateur, CB, and marine communications
installations utilize a system impedance of 50 to 52 Ω (ohms). Most cable television
systems (in the United States, at least) have standardized on 75 Ω. There are also historical
and practical reasons for 300-, 450-, and 600-Ω lines, too. And a few transmission
lines for specialized applications have more than two conductors.
Types of Transmission Lines
One of the earliest types of transmission lines was a single copper wire connected between
the transmitter and some point on a horizontal antenna. Then, as now, the return
path for this type of line was the earth or other ground path between the antenna and
the transmitter. In almost all installations, this form of transmission line exhibits many
undesirable characteristics, and we shall not dwell on it because it has, for the most
part, been supplanted by far superior two-conductor lines with controlled geometries.
Parallel Conductor Lines
Next to appear were parallel conductor lines, whose general shape is shown in Figs. 4.1A
through 4.1E. Figure 4.1A shows a cross-sectional, or end, view of a typical parallel
conductor transmission line. Two conductors, both of diameter d, are separated by a
dielectric (which might be air) at a spacing
S. (These designations will be used
in calculations later.) At first, all parallel
lines were open-wire line (OWL), shown
in Fig. 4.1B. Here, the wire conductors
are separated by an air dielectric; spac-
Conductors
S
d
(End view)
d
Insulator
Tie
Figure 4.1A Parallel line transmission line (end
view).
Figure 4.1B Parallel open-wire line construction
details.