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ELECTRONIC COMMUNICATIONS
A SYSTEMS APPROACH
CHAPTER
12
Transmission Lines
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Types of Transmission Lines
• Two-Wire Open Line
 Transmission line between antenna and
transmitter or antenna and receiver.
 Simple construction.
• Twisted Pair
 Two insulated wires twisted to form
flexible line without use of spacers.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Types of Transmission Lines
• Unshielded Twisted Pair (UTP)
 Used for computer networking; CAT6
and CAT5e.
• See Table 12-1: T568A/T568B Wiring
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Table 12-1
T568A/T568B Wiring
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Types of Transmission Lines
• Unshielded Twisted Pair (UTP)
 Used for computer networking; CAT6
and CAT5e.
• See Table 12-2: Different Categories for
Twisted-Pair Cable
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Table 12-2
Cabl3
Different Categories for Twisted-Pair
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Types of Transmission Lines
• Shielded Pair
 Parallel conductors separated from each
other, and surrounded by solid
dielectric.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Types of Transmission Lines
• Coaxial Lines
 Rigid or air; flexible or solid.
• Balanced/Unbalanced Lines
 Unbalanced
• Amplitude of electrical signal by center
conductor in coaxial line measured with
respect to grounded outer conductor.
 Balanced
• Same current flows in each wire but
180° out of phase.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Electrical Characteristics of
Transmission Lines
• Two-Wire Transmission Line
 Electrical characteristics
• Construction of line.
• Characteristic Impedance
 Impedance measured at any point on
line would be the same.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Electrical Characteristics of
Transmission Lines
• Transmission Line Losses
 Copper
 Dielectric
 Radiation or induction
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Propagation of DC
Voltage Down a Line
• Physical Explanation of Propagation
 To understand characteristics of
transmission line with ac voltage
applied, infinitely long transmission line
analyzed with dc voltage applied.
• Velocity of Propagation
 Current moving down the line, its
electric and magnetic fields are
propagated down the line.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Propagation of DC
Voltage Down a Line
• Delay Line
 Delay signal by some specific amount of
time.
• Wavelength
 Distance traveled by wave during a
period of one cycle.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Nonresonant Line
• Traveling DC Waves
 Nonresonant line
• Line of infinite length or as one
terminated with resistive load equal in
ohmic value to characteristic impedance
of line.
 Traveling waves
• Voltage and current waves; move in
phase with one another from source to
load.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Nonresonant Line
• Traveling AC Waves
 Little difference between charging of
line when ac voltage applied to it and
when dc voltage applied.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Resonant Transmission
• DC Applied to an Open-Circuited Line
 Resonant line
• Transmission line terminated with
impedance not equal to characteristic
impedance.
 Because impedances are equal, applied
voltage divided equally between source
and line.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Resonant Transmission
• Incident and Reflected Waves
 Voltage on open-circuited wire equal to
source voltage, and current is zero.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Resonant Transmission
• DC Applied to a Short-Circuited Line
 Voltage reflection from open circuit is in
phase, while from short circuit it is out
of phase.
 Current reflection from open circuit is
out of phase, while from short circuit it
is in phase.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Resonant Transmission
• Standing Waves
 Open Line
 Mismatch
• Incident and reflected waves interact.
 Standing wave
• Remains in one position, varying only in
amplitude.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Resonant Transmission
• Standing Waves
 Shorted Line
 Out-of-phase reflection that occurs for
current on open line and voltage on
shorted line.
 Impedance
• Voltage divided by current.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Resonant Transmission
• Quarter-Wavelength Sections
 Phase inversion of voltage and current
every quarter wavelength.
 Cavity filter or selective cavity extends
idea of quarter-wavelength coaxial stub
filter to high-power operation.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Standing Wave Ratio
• Standing Wave
 Instantaneous vector addition of
incident-wave amplitude with that of
reflected wave as result of impedance
mismatch between transmission line
and load.
• Reflections
 Occur when impedance mismatch.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Standing Wave Ratio
• Degree of Mismatch
 Reflection coefficient, voltage standingwave ratio, return loss.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Standing Wave Ratio
• Voltage standing wave ratio (VSWR)
 Ratio of maximum voltage to minimum
of standing wave on a line.
• Standing wave ratio (SWR)
 Equal to ratio of maximum current to
minimum current.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Standing Wave Ratio
• Effect of Mismatch
 Flat line
• Perfect condition of no reflection when
load purely resistive and equal to Z0.
 Higher the VSWR, the greater is
mismatch on line.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Standing Wave Ratio
• Quarter-Wavelength Transformer
 Not physically a transformer; offers
property of impedance transformation.
• Electrical Length
 Line can be miles long physically and
electrically short at low frequencies.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
The Smith Chart
• Transmission Line Impedance
 Impedance
• Constantly changing along line and equal
to ratio of voltage to current at given
point.
 Smith chart presents solution to
impedance-matching problems posed by
complex sources and complex loads.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
The Smith Chart
• Transmission Line Impedance
 Smith chart impedance-matching tool
for transmission lines.
 Graphical Smith chart used to represent
network analyzer solutions.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
The Smith Chart
• Smith Chart
 Normalizing
• Dividing all impedances by characteristic
impedance of the line.
 Reciprocal of impedance defined as
admittance and reciprocal of reactance
is called susceptance.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
The Smith Chart
• Smith Chart
 Greatest utility as impedance-matching
calculator.
 Allows for simple conversion of
impedance to admittance, and vice
versa.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
The Smith Chart
• Smith Chart
 Spiral
• True standing wave representation on
Smith chart.
 Many calculations with transmission
lines pertain to matching load to line
and keeping VSWR as low as possible.
 Use of short-circuited stubs prevalent in
matching problems.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Transmission Line Applications
• Discrete Circuit Simulation
 Transmission line sections: used to
simulate inductance, capacitance, LC
resonance.
• Baluns
 Unbalanced-to-balanced transformer.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Transmission Line Applications
• Transmission Lines as Filters
 Quarter-wave section of transmission
line used as efficient filter or suppressor
of even harmonics.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Transmission Line Applications
• Slotted Lines
 Section of coaxial line with lengthwise
slot cut in outer conductor; pickup
probe inserted into slot.
 VSWR, generator frequency, unknown
load impedance.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Transmission Line Applications
• Time-Domain Reflectometry
 Short-duration pulse transmitted into a
line.
 Monitored with oscilloscope.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Impedance Matching and Network
Analysis
• Network analyzer
 Test equipment to characterize linear
impedance characteristics of devices
under test (DUTs).
• Scalar network analyzer
 Measures magnitude of DUT impedance.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Impedance Matching and Network
Analysis
• Vector network analyzer (VNA)
 Determines impedance magnitude and
phase characteristics of DUTs.
• Scattering or S-parameters
 Linear transmission and reflection
characteristics of signals applied to
DUTs.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Impedance Matching and Network
Analysis
• Scattering matrix
 Describes all voltages incident to and
reflected from all ports.
• Impedance analysis not confined to
microwave systems.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved
Impedance Matching and Network
Analysis
• Vector network analyzer
 System of three receivers and two-port
“test set” that separates forward and
reflected components of applied signals
through directional couplers.
Electronic Communications: A Systems Approach
Beasley | Hymer | Miller
Copyright © 2014 by Pearson Education, Inc.
All Rights Reserved