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Transmission Lines and Waveguides - Transmission Line Theory - Important Short Questions and Answers: Transmission Line Theory

**TRANSMISSION LINE THEORY **

**1.Define
the line parameters?**

The parameters of a transmission line are:

Resistance (R)

Inductance (L)

Capacitance (C)

Conductance (G)

**Resistance
(R) **is defined as the loop resistance per unit
length of the wire. Its** **unit is
ohm/Km

**Inductance
(L) **is defined as the loop inductance per unit
length of the wire. Its** **unit is
Henry/Km

**Capacitance
(C) **is defined as the loop capacitance per unit
length of the wire. Its** **unit is
Farad/Km

**Conductance
(G) **is defined as the loop conductance per unit
length of the wire.** **Its unit is
mho/Km

**2. What
are the secondary constants of a line? Why the line parameters are called
distributed elements?**

The secondary constants of a line are:
Characteristic Impedance Propagation Constant

Since the line constants R, L, C, G are
distributed through the entire length of the line, they are called as
distributed elements. They are also called as primary constants.

**3.Define
Characteristic impedance**

Characteristic impedance is the impedance measured at the sending end of the line. It is given by

Z = R + jωL is the series impedance

Y = G + jωC is the shunt admittance

**4.
Define Propagation constant**

Propagation constant is defined as the natural
logarithm of the ratio of the sending end current or voltage to the receiving
end current or voltage of the line. It gives the manner in the wave is
propagated along a line and specifies the variation of voltage and current in
the line as a function of distance. Propagation constant is a complex quantity
and is expressed as

γ = α + j β

The real part is called the attenuation
constant α whereas the imaginary part of propagation constant is called the phase
constant

**5.What
is a finite line? Write down the significance of this line?**

A finite line is a line having a finite length
on the line. It is a line, which is terminated, in its characteristic impedance
(ZR=Z0), so the input impedance of the finite line is equal to the
characteristic impedance (Zs=Z0).

**6.What
is an infinite line?**

An infinite line is a line in which the length
of the transmission line is infinite.

A finite line, which is terminated in its
characteristic impedance, is termed as infinite line. So for an infinite line,
the input impedance is equivalent to the characteristic impedance.

**7.What
is wavelength of a line?**

The distance the wave travels along the line
while the phase angle is changing through 2_ radians is called a wavelength.

**8.What
are the types of line distortions?**

The distortions occurring in the transmission
line are called waveform distortion or line distortion. Waveform distortion is
of two types:

a)Frequency distortion

b)Phase or Delay Distortion.

**9.How
frequency distortion occurs in a line?**

When a signal having many frequency components
are transmitted along the line, all the frequencies will not have equal
attenuation and hence the received end waveform will not be identical with the
input waveform at the sending end because each frequency is having different
attenuation. This type of distortion is called frequency distortion.

**10.How
to avoid the frequency distortion that occurs in the line?**

In order to reduce frequency distortion
occurring in the line,

a)The attenuation constant α should be
made independent of frequency.

b) By using equalizers at the line terminals which
minimize the frequency distortion. Equalisers are networks whose frequency and
phase characteristics are adjusted to be inverse to those of the lines, which
result in a uniform frequency response over the desired frequency band, and
hence the attenuation is equal for all the frequencies.

**11.What
is delay distortion?**

When a signal having many frequency components
are transmitted along the

line, all the frequencies will not have same
time of transmission, some frequencies being delayed more than others. So the
received end waveform will not be identical with the input waveform at the
sending end because some frequency components will be delayed more than those
of other frequencies. This type of distortion is called phase or delay
distortion.

**12. How
to avoid the frequency distortion that occurs in the line?**

In order to reduce frequency distortion
occurring in the line,

a)The phase constant should be made
dependent of frequency.

b)The velocity of propagation is independent of
frequency.

c) By using equalizers at the line terminals which
minimize the frequency distortion. Equalizers are networks whose frequency and
phase characteristics are adjusted to be inverse to those of the lines, which
result in a uniform frequency response over the desired frequency band, and
hence the phase is equal for all the frequencies.

**13.What
is a distortion less line? What is the condition for a distortion less line?**

A line, which has neither frequency distortion
nor phase distortion is called a distortion less line. The condition for a
distortion less line is RC=LG. Also,

a)The attenuation constant a should be
made independent of frequency.

b)The phase constant β should be made
dependent of frequency.

d) The velocity of propagation is independent
of frequency.

**14.What
is the drawback of using ordinary telephone cables?**

In ordinary telephone cables, the wires are
insulated with paper and twisted in pairs, therefore there will not be flux
linkage between the wires, which results in negligible inductance, and
conductance. If this is the case, the there occurs frequency and phase
distortion in the line.

**15.How
the telephone line can be made a distortion less line?**

For the telephone cable to be distortion less
line, the inductance value should be increased by placing lumped inductors
along the line.

**16.What
is Loading?**

Loading is the process of increasing the
inductance value by placing lumped inductors at specific intervals along the
line, which avoids the distortion

**17.What
are the types of loading?**

a)Continuous loading

b)Patch loading

c) Lumped loading

**18.What
is continuous loading?**

Continuous loading is the process of increasing
the inductance value by placing a iron core or a magnetic tape over the
conductor of the line.

**19.What
is patch loading?**

It is the process of using sections of
continuously loaded cables separated by sections of unloaded cables which
increases the inductance value

**20.What
is lumped loading?**

Lumped loading is the process of increasing the
inductance value by placing lumped inductors at specific intervals along the
line, which avoids the distortion

**21.Define
reflection coefficient**

Reflection Coefficient can be defined as the
ratio of the reflected voltage to the incident voltage at the receiving end of
the line

Reflection Coefficient K=Reflected Voltage at
load /Incident voltage at the load K=Vr/Vi

**22.
Define reflection loss**

Reflection loss is defined as the number of
nepers or decibels by which the current in the load under image matched
conditions would exceed the current actually flowing in the load

**23.What
is Impedance matching?**

If the load impedance is not equal to the
source impedance, then all the power that are transmitted from the source will
not reach the load end and hence some power is wasted. This is called impedance
mismatch condition. So for proper maximum power transfer, the impedances in the
sending and receiving end are matched. This is called impedance matching.

**24.
Define the term insertion loss**

The insertion loss of a line or network is
defined as the number of nepers or decibels by which the current in the load is
changed by the insertion . Insertion loss=Current flowing in the load without insertion
of the network**/**Current flowing in
the load with insertion of the network

**25.When
reflection occurs in a line?**

Reflection occurs because of the following
cases:

1)when the load end is open circuited

2)when the load end is short-circuited

3)when the line is not terminated in its
characteristic impedance

When the line is either open or short
circuited, then there is not resistance at the receiving end to absorb all the
power transmitted from the source end. Hence all the power incident on the load
gets completely reflected back to the source causing reflections in the line.
When the line is terminated in its characteristic impedance, the load will
absorb some power and some will be reflected back thus producing reflections.

**26.What
are the conditions for a perfect line? What is a smooth line?**

For a perfect line, the resistance and the
leakage conductance value were neglected. The conditions for a perfect line are
R=G=0.

A smooth line is one in which the load is
terminated by its characteristic impedance and no reflections occur in such a
line. It is also called as flat line.

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Transmission Lines and Waveguides : Transmission Line Theory : Important Short Questions and Answers: Transmission Line Theory |

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