When shown an early radio station, the viewer was
heard to remark, “Why do they
call it wireless? I’ve never seen
so many wires in all my life!”
The fact that “wireless” requires
a lot of wires is undeniable to
those of us who have ever built
or maintained any kind of radio
facility — from a simple CB
base station to a full-power
broadcast facility. Most of the
“wires” you’ll find behind the
equipment are more than
simple strands, though. If they
are conveying radio frequency
(RF) signals from place to
place, they are really
transmission lines. In this article, we’ll
talk about a basic element of
transmission lines — the standing
wave ratio, or SWR — and why it’s
important and how to measure it.
Transmission Line
Overview
Any conductor carrying an AC
current can be treated as a
transmission line, such as those
overhead giants distributing AC utility
power across the landscape.
Incorporating all the different forms
of transmission lines would fall
considerably outside the scope of
this article, so we’ll limit the
discussion to frequencies from about
1 MHz to 1 GHz, and to two
common types of line: coaxial (or
“coax”) and parallel-conductor (a.k.a.,
open-wire, window line, ladder line,
or twin-lead as we’ll call it) as shown
in Figure 1.
Current flows along the surface
of the conductors (see the sidebar
on “Skin Effect”) in opposite
directions. Surprisingly, the RF energy
flowing along the line doesn’t really
flow in the conductors where the
current is. It travels as an
electromagnetic (EM) wave in the
space between and around the
conductors. Figure 1 indicates where
the field is located in both coax and
twin-lead. For coax, the field is
completely contained within the
dielectric between the center
conductor and shield. For twin-lead,
though, the field is strongest around
and between the conductors but
without a surrounding shield, some
of the field extends into the space
around the line.
This is why coax is so popular —
it doesn’t allow the signals inside to
interact with signals and conductors
outside the line. Twin-lead, on the
other hand, has to be kept well away
(a few line widths is sufficient) from
other feed lines and any kind of
metal surface. Why use twin-lead? It
generally has lower losses than coax,
so is a better choice when signal loss
is an important consideration.
Characteristic
Impedance
Both kinds of transmission lines
are specified as having a
characteristic impedance, represented
by Z0. For example, popular RG- 58
cable is designated to be a 50Ω
cable, RG- 6 is a 75Ω cable, and so
on. If you measure the cable with an
ohmmeter, you’ll just get a reading of
a few ohms. What’s going on?
Z0 applies to how EM waves flow
through the cable, and it depends on
the size of the conductors, the
relative placement of the conductors,
and the type of dielectric between
them. (Formulas for Z0 can be found
online and in most RF engineering
references.) Sometimes referred to as
surge impedance, the characteristic
impedance determines how the EM
wave’s energy is allocated between
the electric and magnetic field as it
travels along the cable.
You can experience acoustic
characteristic impedance for yourself
with a common soft drink straw and
hwardsil@gmail.com
■ BY WARD SILVER N0AX THE HAM’S WIRELESS WORKBENCH
Transmission Lines and SWR
What’s going on in that cable, anyway?
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
January 2016 15
FIGURE 1. Coaxial cable
(A) consists of a solid or
stranded center conductor
surrounded by an insulating
plastic or air dielectric and
a tubular shield that is
either solid or woven wire
braid. A plastic jacket
surrounds the shield to
protect the conductors.
Twin-lead (B) consists of a
pair of parallel solid or
stranded wires. The wires
are held in place by either
molded plastic (window
line, twin-lead) or by
ceramic or plastic
insulators (ladder line).
