Table of Contents
1. INTRODUCTION……………………………………………………….…..2
2. Types of transmission line…………………………………………….…..….3
3. Coaxial cable………………………………………….…………………...….4
4. Transmission-line parameters R’,L’,G’,and C’ for three types of lines…..6
5. Equivalent Circuit of a Transmission Line……..……………….……...…..7
6. The Lossless transmission line……………………………….………………8
7. Components of Transmission Line.……......……….…….…….….……….9
8. Conclusion..……....……….…….....................................................….…..….10
- References ……………………….……………...…………………………….11
1
Introduction
Transmission lines are commonly used in power distribution (at low frequencies) and in
communications (at high frequencies) to transfer energy from one point to another (from a
source to a load). Various kinds of transmission lines such as the twisted-pair and coaxial
cables (thinnet and thicknet) are used in computer networks such as the Ethernet and
internet.
A transmission line basically consists of two or more parallel conductors used to connect a
source to a load. The source may be a hydroelectric generator, a transmitter, or an oscillator;
the load may be a factory, an antenna, or an oscilloscope, respectively. Typical transmission
lines include coaxial cable, a two-wire line, a parallel-plate.
Transmission line problems are usually solved using EM field theory and electric circuit
theory, the two major theories on which electrical engineering is based. In this report, we use
circuit theory because it is easier to deal with mathematically. The basic concepts of wave
propagation.
AC resistance of a conductor is always higher than its DC resistance due to the skin effect
forcing more current flow near the outer surface of the conductor. The higher the frequency of
current, the more noticeable skin effect would be.
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2. Types of transmission line
According to their propagation modes, The transmission lines may be classified into two
basic types are :
2.1 Transverse electromagnetic (TEM) transmission lines
The case of this call (TEM mode) because in this transmission lines the Waves propagation
along these lines are characterized by electric and magnetic fields that are entirely transverse to
the direction of propagation (i.e. They have no electric-or-magnetic field components along its
direction of propagation).
Figure.1 show different TEM structures , a good example for TEM mode is coaxial cable which
is shown in the (fig1-a) it's consist of two conducting cylindrical plane inner and outer and
dielectric material between them and as we see in the (fig1-a) the direction of energy flow is
along the length of the coaxial cable.
The other examples are a two- wire line (fig1-b) which consists of two parallel rods, a parallel –
plate line as shown in (fig1-c), an unbalanced line (fig1-d) that his structure is very simple with
one wire connected with +ve terminal of voltage source and the –ve terminal connected to the
ground and (fig1-e) show a micro strip line which consist of two strip the upper is smallest on
width than the lower and there are dielectric slab between them where the direction of energy
flow is along the strip.
▪
▪
All of these transmission lines can transfer energy with high efficiency.
A common feature among TEM lines is that they consist of two parallel conducting
surfaces.
a
b
c
d
e
Figure 1
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2.2 -Higher-order transmission lines:
Waves propagating along these lines have at least one significant field component in the
direction of propagation.
Hollow conducting waveguides, dielectric rods, and optical fibers belong to this class of lines.
3. Co-axial Cable:
Coaxial cable is called "coaxial" because it includes one physical channel that carries
the signal surrounded (after a layer of insulation) by another concentric physical channel, both
running along the same axis. The outer channel serves as a ground.
Many of these cables or pairs of coaxial tubes can be placed in a single outer sheathing and,
with repeaters, can carry information for a great distance.
Coaxial cable is the kind of copper cable used by cable TV companies between the
antenna and user homes and businesses. Coaxial cable is sometimes used by telephone
companies from their central office to the telephone poles near users.
(a) coaxial cable
(b) Twisted-pair cable
figure 2
The Coaxial cable carries signals of higher frequency ranges (higher Bandwidth) than
twisted pair.
The coaxial cable consists two conductors, Inner conductor carries signal and outer one
serves as shield against noise and as the second conductor, which completes the circuit.
Instead of having two wires as twisted-pair calbe(fig2-b), coax has a central core
conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which
is in turn, encased in an outer conductor of metal foil, braid, or a combination of the two.
The outer metallic wrapping serves both as a shield against noise and as the second
conductor, which completes the circuit, This outer conductor is also enclosed in an insulating
sheath, and the whole cable is protected by a plastic cover(PVC).
figure.3
We used a connector with coaxial cable , its called a BNC connector(short for British
Naval Connector) is mounted at each end of a cable(as shown in fig.3).
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General Rules for Coax:
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Attenuation or Loss
Raising the D/d ratio lowers attenuation
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Transmission-Line Input Impedance
The input impedance at a distance l from the load is:
When the load is a short circuit, Zi = jZo tan (l).
When the load is an open circuit, Zi = -jZo cot (l)
4. Transmission-line parameters R’,L’,G’,and C’ for three types of lines
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5. An Equivalent Circuit of a Transmission Line.
 Characteristics of a T.L. (uniformly distributed) are determined by:
Electrical proprieties: wire conductivity and insulator dielectric
Physical properties: wire diameter and conductor spacing
 Primary electric constants:
Series DC resistance R & Inductance L
Shunt Capacitance C & Conductance G
Circuit Model:

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z
6. The Lossless transmission line.
6.1 The Losses transmission line.
6.1.1 Conductor losses :

It is because current flows through a transmission line and a line has a finite
resistance there is an un-avoidable power loss.
This is sometimes called conductor loss or conductor heating loss and is simply a
power loss.

To reduce conductor loss simply shorten the transmission line or use a larger
diameter wire.
Conductor loss depends somewhat on frequency because of a phenomenon called
the skin effect.

6.1.2 Dielectric heating losses:




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A difference of potential between two conductors of a metallic transmission line
causes dielectric heating.
Heat is form of energy and must be taken from the energy propagating down the
line.
For air dielectric transmission lines the heating is negligible.
For solid core transmission lines dielectric heating loss increases with frequency.
7. Components of Transmission Line
1- Conductor
2- Earth wire
3-Insulator
4- Transmission Tower
5- Wave trap and other hardware (Clamp, Spacer, Vibration dampers, connectors etc.)
***The figure below shown the component of transmission line
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8. Conclusion

The transmission line is the important structure to establish any system.
fig. shows the system elements (a source, a transmission line and a load)
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References
- Ulaby,Fawwaz T.(Fawwaz Tayssir),Fundamentals of applied electromagnetic /Fawwaz T. Ulaby.
- http://searchnetworking.techtarget.com/definition/coaxial-cable-illustrated
- http://sparc.nfu.edu.tw/~lyc/intro_net/ch07.pdf
- www.qec.edu.sa
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