Test: Transmission Lines- 2 - Electrical Engineering (EE) MCQ

Double stub matching eliminates standing waves on the source side of the left stub.

Characteristic impedance is given by

Given,
Z₀ = R₀
we have

and

Voltage standing wave ratio is given by

or,


(Using componendo and dividendo)
or,

Propagation constant of a transmission line is given by

Given
Z₀ = 300∠0º
and Z_L =150∠0ºΩ

Characteristic impedance,

or, 

(Since L/R = C/G for a distortionless line)


Hence, characteristic impedance of a distortionless line is purely real.

The overall transmission coefficient is

• Statements-1 and 2 are true.
• Statement-3 is not correct because in a lossless transmission line there is neither resistive loss (i.e. R = 0) nor any leakage current (i.e. G = 0).
• At radio frequency (RF), the inductive reactance is much larger than the resistance and so also the capacitie susceptance in comparison to the shunt conductance. Hence, at radio frequencies R and G both are neglected. Thus, statement-4 is correct.

The input impedance of a finite line terminated in its characteristic impedance (Z₀), is equivalent to an infinite line because the input impedance of an infinite line is the characteristic impedance of the line (Z₀). Hence, both assertion and reason are true and reason is the correct explanation of assertion.

• Propagation constant is dimensionless quantity because it is the ratio of voltages or currents.
  
  or,
  
  (where, P = Propagation constant) Hence, statement-1 is correct.
• For a lossless transmission line,
  
  ∴ V_P α f
  Thus, statement - 2 is also correct.
• Statement-3 is also true.
  Thus, all statements are true.

Group velocity is given by

Phase velocity or velocity of propagation is given by

The group velocity is usually less than the phase velocity. Hence, assertion is a false statement.

Assertion is correct because when a transmission line is open or short-circuited it behaves as resonant circuit. However, reason is false because this happens when length of the line is an integral multiple of λ/4.
We knnw that
Z_oc = -j cot βl
and Z_sc = jZ₀ tan βl
Thus, when βl = length of line = nλ/4 , then
cot βl = 0
and tan βl = ∞
∴ Z_oc = 0
and Z_sc = ∞
This means a quarter wave short-circuit line represents an infinite impedance at inpul terminals, just like a parallel resonant (LC) circuil and a λ/4 open circuit line present zero impedance at input terminals just like a series resonant LC circuit.

If the transmission line is to have neither frequency nor delay distortion, then α (attenuation constant) and velocity of propagation cannot be functions of frequency.
V = ω/β β 
must be a direct function of frequency to achieve this condition
LG = CR
L/C = R/G
z₀ = √((R + jωL)/(G + jωC))
For a lossless line,
z₀ = √(L/C)
α = √(RG) = 0 for R = 0, G = 0
β = ω√(LC)
A loss less line is always a distortion less line.

Input impedance of a short-circuited quarter wave line depends on the nature of load impedance Z_L.