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 β

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.