CHAPTER 3
INTRODUCTION TO
TRANSMISSION LINES AND WAVEGUIDES
A TRANSMISSION LINE is a device designed
to guide electrical energy from one point to another.
It is used, for example, to transfer the output rf energy
of a transmitter to an antenna. This energy will not
travel through normal electrical wire without great
losses.
Although the antenna can be connected
directly to the transmitter, the antenna is usually
located some distance away from the transmitter. On
board ship, the transmitter is located inside a radio
room, and its associated antenna is mounted on a mast.
A transmission line is used to connect the transmitter
and the antenna.
The transmission line has a single purpose for both
the transmitter and the antenna. This purpose is to
transfer the energy output of the transmitter to the
antenna with the least possible power loss. How well
this is done depends on the special physical and
electrical characteristics (impedance and resistance)
of the transmission line.
TRANSMISSION LINE THEORY
The electrical characteristics of a two-wire
transmission line depend primarily on the construction
of the line.
The two-wire line acts like a long
capacitor. The change of its capacitive reactance is
noticeable as the frequency applied to it is changed.
Since the long conductors have a magnetic field about
them when electrical energy is being passed through
them, they also exhibit the properties of inductance.
The values of inductance and capacitance presented
depend on the various physical factors that we
discussed earlier. For example, the type of line used,
the dielectric in the line, and the length of the line
must be considered. The effects of the inductive and
capacitive reactance of the line depend on the
frequency applied.
Since no dielectric is perfect,
electrons manage to move from one conductor to the
other through the dielectric. Each type of two-wire
transmission line also has a conductance value. This
conductance value represents the value of the current
flow that may be expected through the insulation,
If the line is uniform (all values equal at each unit
length), then one small section of the line may
represent several feet. This illustration of a two-wire
transmission line will be used throughout the discussion
of transmission lines; but, keep in mind that the
principles presented apply to all transmission lines.
We will explain the theories using LUMPED CON-
STANTS and DISTRIBUTED CONSTANTS to further
simplify these principles.
LUMPED CONSTANTS
A transmission line has the properties of induc-
tance, capacitance, and resistance just as the more
conventional circuits have. Usually, however, the
constants in conventional circuits are lumped into a
single device or component. For example, a coil of
wire has the property of inductance. When a certain
amount of inductance is needed in a circuit, a coil of
the proper dimensions is inserted. The inductance
of the circuit is lumped into the one component. Two
metal plates separated by a small space, can be used
to supply the required capacitance for a circuit. In
such a case, most of the capacitance of the circuit is
lumped into this one component. Similarly, a fixed
resistor can be used to supply a certain value of circuit
resistance as a lumped sum. Ideally, a transmission
line would also have its constants of inductance,
capacitance, and resistance lumped together, as shown
in figure 3-1. Unfortunately, this is not the case.
Transmission line constants are as described in the
following paragraphs.
DISTRIBUTED CONSTANTS
Transmission line constants, called distributed
constants, are spread along the entire length of the
transmission line and cannot be distinguished sepa-
rately. The amount of inductance, capacitance, and
resistance depends on the length of the line, the size
of the conducting wires, the spacing between the
3-1
