Lab 01

EXPERIMENT
NO
: 01
Transmission Lines and Measurement of their

TITLE : Characteristics-I
DATE OF
:
EXPERIMENT
INSTRUCTOR : Hamail Sultan
GROUP # : ______________________________________
: ___________________
GROUP MEMBERS
1. OBJECTIVES
a) To demonstrate that there is particular impedance, the characteristic impedance, which

absorbs the signal coming along the line.
b) To demonstrate that reflection occurs if the line is not correctly terminated. To prepare
the student for the theory of reflection at discontinuities.
c) To enable the student to observe partial reflections at a mismatched termination.
2. THEORY
There are many situations in which it is desired to connect a generator (or source of electrical
power) to a distant load (or power-absorbing device). The generator may be of high power, as in
a power station, or low power, as with a microphone; it may be of low frequency, again as in the
power station, or high, as in a radio transmitter. But in each case a pair (at least) of conductors is
required to convey the power from generator to Load. Such a pair of conductors is called a
'transmission line', or abbreviated to simply 'line' when convenient.
When a signal is applied to a transmission line at one end, the other end is not immediately
affected. Instead the signal travels along the line with a finite velocity, and reaches the load
somewhat later. The potential difference between the conductors is associated with an electric
field, and the current in the conductors is associated with a magnetic field. These fields are
sketched for two forms of line in fig 1.1. These fields interact with each other and with the line to
form a guided electromagnetic wave travelling along the line. The maximum speed such a wave
can have is the speed of light, 3 x 108 m/s; in lines having solid materials around the conductors
the speed of propagation can be much less.
408713L (Transmission Lines and Measurement of their Characteristics-I) Page 2

Fig 1.1 Fields associated with two–wire and coaxial lines
If a sinusoidal signal is applied to a line, different phases of the sine-wave will be distributed in
distance along the line owing to its travel, as shown in fig 1.2, for three successive instants. A
complete cycle of the wave occupies a distance along the line which is called the wavelength.
The wavelength is inversely proportional to the frequency f of the wave: they are related to the
propagation velocity v by the formula v=fλ
Fig 1.2
If the length of a pair of conductors is much shorter than wavelength for all frequencies of
interest, then usually the conductors need not be considered as a transmission line. The critical
distance is perhaps a few centimeters of television antenna feeder, a few hundred meters of
telephone line, and a few hundred kilometers for power supply frequencies.
The flow of current in the conductors' resistance gives rise to energy losses. Further losses arise
due to imperfections in the insulation between conductors, such as surface leaking across
insulators, or dielectric losses. In consequence if the power of a signal is W (watts) at the sending
end of the line, it may be reduced to 1/2W (watt) at some distance along the line. If a
transmission line is 'correctly terminated', i.e terminated in its characteristic impedance, all the
power sent down the line is absorbed at the termination and no signal returns.
Characteristic impedance
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission
line is the ratio of the amplitudes of voltage and current of a single wave propagating along the
line; that is, a wave travelling in one direction in the absence of reflections in the other direction.
Characteristic impedance is also known as natural impedance, and it refers to the equivalent
resistance of a transmission line if it were infinitely long, owing to distributed capacitance and

inductance as the voltage and current “waves” propagate along its length at a propagation
velocity.
If the line is open-circuit or short-circuit at the termination, complete reflection of the signal
occurs. In this assignment we shall investigate some typical cases where a finite impedance, not
equal to the characteristic impedance, is used to terminate the line.
A wave, travelling in a line of characteristic impedance Zo and incident on a termination of
impedance Zb gives rise to a reflected wave whose amplitude is that of the original wave
reaching the termination multiplied by a reflection coefficient. The reflection coefficient for the
voltage wave is
K = (Zl - Zo) / (Zl + Zo)
There is a large variety of terminations for RF lines. Each type of termination has a characteristic
effect on the standing waves on the line. From the nature of the standing waves, you can
determine the type of termination that produces the waves
TASKS
Equipment Required
- vernier caliper / screw gauge

- Sample Transmission lines
Procedure
Calculate the characteristic impedance (Zo) of different samples of transmission lines available
in the lab, by using vernier caliper / screw gauge.
Measurement technique and formula are given as under in figure 1.3
Observations and data manipulation
- Record separately the Zo value of each sample.
Discussions / Conclusions
- How does the Zo value is affected by varying the length of transmission line?

Fig 1.3

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EXPERIMENT

Transmission Lines and Measurement of their

INSTRUCTOR : Hamail Sultan

a) To demonstrate that there is particular impedance, the characteristic impedance, which

c) To enable the student to observe partial reflections at a mismatched termination.

408713L (Transmission Lines and Measurement of their Characteristics-I) Page 2

408713L (Transmission Lines and Measurement of their Characteristics-I) Page 3

- vernier caliper / screw gauge

Observations and data manipulation

- Record separately the Zo value of each sample.

408713L (Transmission Lines and Measurement of their Characteristics-I) Page 4

408713L (Transmission Lines and Measurement of their Characteristics-I) Page 5

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