1
Principles of
Electronic
Communication
Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 13
Transmission Lines
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Topics Covered in Chapter 13
13-1: Transmission-Line Basics
13-2: Standing Waves
13-3: Transmission Lines as Circuit Elements
13-4: The Smith Chart
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13-1: Transmission-Line
Basics
Transmission lines in communication carry telephone
signals, computer data in LANs, TV signals in cable TV
systems, and signals from a transmitter to an antenna or
from an antenna to a receiver.
Their electrical characteristics are critical and must be
matched to the equipment for successful
communication to take place.
Transmission lines are also circuits.
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13-1: Transmission-Line
Basics
The two primary requirements of a transmission line
are:
1. The line should introduce minimum attenuation to the
signal.
2. The line should not radiate any of the signal as radio
energy.
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13-1: Transmission-Line
Basics
Types of Transmission Lines
Parallel-wire line is made of two parallel conductors
separated by a space of ½ inch to several inches.
A variation of parallel line is the 300-Ω twin-lead. Spacing
between the wires is maintained by a continuous plastic
insulator.
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13-1: Transmission-Line
Basics
Types of Transmission Lines
The most widely used type of transmission line is the
coaxial cable. It consists of a solid center conductor
surrounded by a dielectric material, usually a plastic
insulator such as Teflon.
A second conducting shield made of fine wires covers the
insulator, and an outer plastic sheath insulates the braid.
Coaxial cable comes in sizes from ¼ inch to several inches
in diameter.
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13-1: Transmission-Line
Basics
Types of Transmission Lines
Twisted-pair cable uses two insulated solid copper wires
covered with insulation and loosely twisted together.
Two types of twisted-pair cable are
Unshielded twisted-pair (UTP) cable
Shielded twisted-pair (STP) cable
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13-1: Transmission-Line
Basics
Figure 13-1: Common types of transmission lines. (a) Open-wire line. (b) Open-wire
line called twin lead. (c) Coaxial cable (d) Twisted-pair cable.
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13-1: Transmission-Line
Basics
Balanced Versus Unbalanced Lines
Transmission lines can be balanced or unbalanced.
A balanced line is one in which neither wire is connected
to ground.
The signal on each wire is referenced to ground.
In an unbalanced line, one conductor is connected to
ground.
Open-wire line has a balanced configuration.
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13-1: Transmission-Line
Basics
Balanced Versus Unbalanced Lines
Balanced-line wires offer significant protection from noise
pickup and cross talk.
Coaxial cables are unbalanced lines.
Coaxial cable and shielded twisted-pair provide significant
but not complete protection from noise or cross talk.
Unshielded lines may pick up signals and cross talk and can
even radiate energy, resulting in an undesirable loss of
signal.
A device called a balun is used to convert from balanced
to unbalanced lines and vice versa.
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13-1: Transmission-Line
Basics
Figure 13-2: (a) Balanced line. (b) Unbalanced line.
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13-1: Transmission-Line
Basics
Wavelength of Cables
The electrical length of conductors is typically short
compared to 1 wavelength of the frequency they carry.
A pair of current-carrying conductors is not considered to
be a transmission line unless it is at least 0.1 λ long at the
signal frequency.
The distance represented by a wavelength in a given cable
depends on the type of cable.
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13-1: Transmission-Line
Basics
Connectors
Most transmission lines terminate in some kind of
connector, a device that connects the cable to a piece of
equipment or to another cable.
Connectors are a common failure point in many
applications.
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13-1: Transmission-Line
Basics
Connectors: Coaxial Cable Connectors
Coaxial cables are designed not only to provide a
convenient way to attach and disconnect equipment and
cables but also to maintain the physical integrity and
electrical properties of the cable.
The most common types are the PL-259 or UHF, BNC, F,
SMA, and N-type connectors.
The PL-259, also referred to as a UHF connector, can be
used up to low UHF frequencies (less than 500 MHz.)
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13-1: Transmission-Line
Basics
Figure 13-3: UHF connectors. (a) PL-259 male connector. (b) Internal construction and
connections for the PL-259. (c) SO-239 female chassis connector.
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13-1: Transmission-Line
Basics
Connectors: Coaxial Cable Connectors
BNC connectors are widely used on 0.25 inch coaxial
cables for attaching test equipment.
In BNC connectors the center conductor of the cable is
soldered or crimped to a male pin and the shield braid is
attached the body of the connector.
The least expensive coaxial connector is the F-type, which
is used for TV sets, VCRs, DVD players, and cable TV.
The RCA phonograph connector is used primarily in audio
equipment.
The best performing coaxial connector is the N-type,
which is used mainly on large coaxial cable at higher
frequencies.
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13-1: Transmission-Line
Basics
Figure 13-4: BNC connectors. (a) Male. (b) Female. (c) Barrel connector. (d) T
connector.
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13-1: Transmission-Line
Basics
Figure 13-6: The F connector used on TV sets, VCRs, and cable TV boxes.
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13-1: Transmission-Line
Basics
Figure 13-7: RCA phonograph connectors are sometimes used for RF connectors up
to VHF.
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13-1: Transmission-Line
Basics
Figure 13-8: N-type coaxial connector.
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Basics
Characteristic Impedance
When the length of transmission line is longer than several
wavelengths at the signal frequency, the two parallel
conductors of the transmission line appear as a complex
impedance.
An RF generator connected to a considerable length of
transmission line sees an impedance that is a function of
the inductance, resistance, and capacitance in the circuit
—the characteristic or surge impedance (Z0).
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13-1: Transmission-Line
Basics
Velocity Factor
The speed of the signal in the transmission line is slower
than the speed of a signal in free space.
The velocity of propagation of a signal in a cable is less
than the velocity of propagation of light in free space by a
fraction called the velocity factor (VF).
VF = Vp/Vc
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Basics
Time Delay
Because the velocity of propagation of a transmission line
is less than the velocity of propagation in free space, any
line will slow down or delay any signal applied to it.
A signal applied at one end of a line appears some time
later at the other end of the line.
This is called the time delay or transit time.
A transmission line used specifically for the purpose of
achieving delay is called a delay line.
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13-1: Transmission-Line
Basics
Figure 13-11: The effect of the time delay of a transmission line on signals. (a) Sine
wave delay causes a lagging phase shift. (b) Pulse delay.
© 2008 The McGraw-Hill Companies
