Lecture 1:
Introduction to Transmission
Media
-Classifications of Transmission Lines
-Electrical Parameters
-Propagation and Losses
-Smith Chart
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Transmission Media
A Transmission Medium is a material
substance that can propagate energy
waves
Transmission Media can be categorized as:
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Transmission Line
A Transmission Line is a metallic conductor
system that is used to guide or transfer
electrical energy from one point to another
Coaxial cable (coax)
Twin lead
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Transmission Line
CAT 5 cable
(twisted pair)
The two wires of the transmission line are twisted to reduce interference and
radiation from discontinuities.
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Electromagnetic Waves
Two basic kinds of waves:
1. Longitudinal Waves
2. Transverse Waves
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Longitudinal waves
• Longitudinal waves have vibrations moving in the same
direction that the wave is travelling in
• Examples of longitudinal waves are:
Sound waves
Shock waves
Transverse waves
• Most waves are transverse
• The energy is travelling in one direction, with the
vibrations moving at a 90o angle to the direction the wave
is travelling
Categories of Transmission Lines
1. BalancedTransmission Lines
- Impedance to ground from each of the two
wires is equal and this ensures that the
current in both wires are equal in
magnitudes but opposite in sign or travel in
opposite directions
Example:
Twin – lead , twisted pair
Categories of Transmission Lines
2. Unbalanced Transmission Lines
- One wire is at ground potential, while the
other is at signal potential
Example:
Coaxial Cable
Categories of Transmission Lines
Balun
- An electrical device used to connect a
balanced transmission line to an
unbalanced load
*Most common type of balun is a narrowband
balun
Parallel – Conductor Transmission Lines
Parallel – wire transmission lines are
comprised of two or more metallic
conductors (usually copper) separated by a
nonconductive insulating material
(dielectric)
Parallel – Conductor Transmission Lines
Open – wire transmission lines
-are two – wire parallel conductors
-consist simply of two parallel wires,
closely spaced and separated by air
Parallel – Conductor Transmission Lines
Twin Lead (Two – Wire Ribbon)
-commonly used to connect a television
receiving antenna to a home television set
Parallel – Conductor Transmission Lines
Twisted - Pair
-the line consists of two insulated wires
twisted together to form a flexible line
without the use of spacers
Coaxial Transmission Lines
Coaxial
-has an inner conductor surrounded by an
insulating layer(dielectric material), then
surrounded by a conducting shield and a
rubber protection outer jacket
Transmission Line Losses
 Copper Losses:
Example: I2 R loss
Whenever current flows through one of these
conductors, some energy is dissipated in the form of
heat (Power Loss)
 Dielectric Losses:
Result from the heating effect on the dielectric material
between the conductors
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Transmission Line Losses
 Radiation and Induction Losses
Radiation and Induction losses are similar. It is
caused by the fields surrounding the conductors.
 Coupling Loss:
Occurs whenever a connection is made to or from a
transmission line or when two separate pieces of transmission lines are connected together.
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Transmission Line Equivalent Circuit
R
L
+
Z0
G
C
...
-
z
4 per-unit-length parameters:
C = capacitance/length [F/m]
L = inductance/length [H/m]
R = resistance/length [/m]
G = conductance/length [ /m or S/m]
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Transmission Line: Primary Line
Constants
2 conductors
4 per-unit-length parameters:
C = capacitance/length [F/m]
L = inductance/length [H/m]
R = resistance/length [/m]

G = conductance/length [ /m or S/m]
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Transmission Line: Primary Line
Constants
where:
= Permeability of medium in H/m
= Permittivity of medium in F/m
S = center to center spacing between
conductors in m
d = diameter of conductor in m
For Coaxial Line
D = Diameter of outer conductor in m
d = diameter of inner conductor in m
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Transmission Line: Primary Line
Constants
 Example:
1. What is the capacitance of 55 miles #44 copper
wire spaced 18inches ? From wire tables, #44 wire
has a radius to 0.10215 ?
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Transmission Line: Secondary Line
Constants
 Characteristic Impedance ( Z0 )
Impedance seen looking at an infinitely long transmission
lines or the impedance seen into a finite length of line
that is terminated in a purely resistive load equal to the
characteristic impedance of the line
In terms of Primary Line Constant:
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Transmission Line: Secondary Line
Constants
In terms of Physical Dimensions:
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Transmission Line: Secondary Line
Constants
 Example:
1. The primary line constant for a coaxial cable at a
frequency of 10MHz were determine approximately
as follows: L = 234nH/m, C = 93.5pF/m, R =
0.568/m, G = 0. Determine the characteristic
impedance.
 Example:
2. A two – wire transmission line consists of N0. 12
wire AWG (81 mils). The distance between wire
centers is 10 inches. What is the characteristic
impedance of the line ?
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Transmission Line: Secondary Line
Constants
 Example:
3. A coaxial line with an outer diameter of 6mm has a
50 ohms characteristic impedance. If the dielectric
constant of the insulation is 1.60, calculate the inner
diameter.
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Transmission Line: Propagation Constant
()
 Propagation Constant
-a.k.a “Propagation Coefficient”
- use to determine the reduction in voltage or current
with distance as a TEM propagates down a transmission
lines
In terms of Series Impedance(Z) & Shunt Admittance(Y):
  ZY
2
In terms of Attenuation Coefficient() & Phase Shift
Constant():
    j
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Transmission Line: Attenuation
Coefficient () & Phase Shift Coefficient
 Attenuation Coefficient ()
 Phase Shift Coefficient ()
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Transmission Line: Attenuation
Coefficient () & Phase Shift Coefficient
 Example:
1. A signal will undergo a phase shift of how many
rad/m when propagating on a 25-m coaxial cable
with a velocity of 0.66c and operating at 5MHz.
 Example:
2. The primary line constant for a coaxial cable at a
frequency of 10MHz were determine approximately
as;
L = 234nH/m, C = 93.5pF/m, R = 0.568/m, G = 0.
Calculate the attenuation coefficient in dB/m.
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The END…
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