# Microwave Engineering Questions and Answers – Terminated Lossless Transmission Lines – 2

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This set of Microwave Engineering Interview Questions and Answers focuses on “Terminated Lossless Transmission Lines – 2”.

1. Expression for input impedance of a transmission line in terms of load impedance and characteristic impedance is:
a) Z0 (ZL+j Z0tan βl)/ (Z0+j ZLtan βl)
b) (Z0+j ZLtan βl)/ (ZL+j Z0tan βl)
c) Z0 (ZL-j Z0tan βl)/ (Z0-j ZLtan βl)
d) (Z0-j ZLtan βl)/ (ZL-j Z0tan βl)

Explanation: Representing the input voltage as the ratio of voltage at current, representing voltage and currents in hyperbolic function form and simplifying, we get Z0 (ZL+j Z0tan βl)/ (Z0+j ZLtan βl).

2. Input impedance of a short circuited transmission line is :
a) -jZ0tanβl
b) jZ0tanβl
c) jZ0cotβl
d) – jZ0cotβl

Explanation: Since the load impedance of a short circuited transmission line is zero, substituting ZL=0 in the expression for input impedance of a transmission line Z0 (ZL+j Z0tan βl)/ (Z0+j ZLtan βl), input impedance of the transmission line comes out to be jZ0tanβl.

3. Input impedance of a transmission line can be represented in terms of this simple trigonometry function.
a) sine function
b) cosine function
c) cotangent function
d) tangent function

Explanation: The input impedance of a transmission line is expressed in the standard form as Z0 (ZL+j Z0tan βl)/ (Z0+j ZLtan βl) which is represented in terms of a tangent function.

4. If a ƛ/3 transmission line is short circuited that has a characteristic impedance of 50 Ω, then its input impedance is:
a) -j100Ω
b) 50Ω
c) 86.60Ω
d) –j86.60Ω

Explanation: For a short circuited transmission line, the input impedance is given by jZ0tanβl.substituting for characteristic impedance and ‘l’ in the above equation, input impedance is –j86.60Ω.

5. Expression for input impedance of an Open circuited transmission line is:
a) -jZ0tanβl
b) jZ0tanβl
c) jZ0cotβl
d) -jZ0cotβl

Explanation: Since the load impedance of a open circuited transmission line is infinity, substituting ZL=infinity (1/ ZL =0) in the expression for input impedance of a transmission line Z0 (ZL+j Z0tan βl)/ (Z0+j ZLtan βl), input impedance of the open circuited transmission line comes out to be- jZ0cotβl.

6. Input impedance of a open circuited transmission line is represented using this trigonometric function:
a) sine function
b) cosine function
c) cotangent function
d) tangent function

Explanation: The input impedance of a transmission line is expressed in the standard form as Z0 (ZL+j Z0tan βl)/ (Z0+j ZLtan βl). With ZL equal to infinity for open circuit termination, 1/ ZL equal to 0, substituting this, we get input impedance in terms of a cotangent function.

7. For a λ/2 transmission line, if the characteristic impedance of the line is 50 Ω and the terminated with a load of 100 Ω, then its input impedance is:
a) 100Ω
b) 50Ω
c) 88.86Ω
d) none of the mentioned

Explanation: Input impedance of a transmission line is given by Z0 (ZL+j Z0tan βl)/ (Z0+j ZLtan βl). Substituting β=2π/λ, and l=λ/2, we get input impedance of the transmission line equal to the load impedance or the terminated load.

8. If a λ/3 transmission line is open circuited and has characteristic impedance of 50 Ω then the input impedance is:
a) j28.86Ω
b) 50Ω
c) j50Ω
d) 28.86Ω

Explanation: Input impedance of an open circuited transmission line is given by – jZ0cotβl. Substituting l=λ/3 and β=2π/λ in the above equation, input impedance is j28.86Ω.

9. Expression for a transmission co-efficient of a transmission line is :
a) 2ZL/ ( ZL+Z0)
b) (ZL-Z0)/ (ZL+Z0)
c) 2Z0/( ZL+Z0)
d) (ZL+Z0)/ (ZL-Z0)

Explanation: T=┌+1, where T is the transmission co-efficient and ┌ is the reflection co-efficient substituting ┌= (ZL-Z0)/ (ZL+Z0) in the equation for transmission co-efficient, we get 2ZL/ ( ZL+Z0).

10. For a transmission line, if the reflection coefficient is 0.4, then the transmission coefficient is:
a) 0.4
b) 1.4
c) 0.8
d) 2.8

Explanation: T=┌+1, where T is the transmission co-efficient and ┌ is the reflection co-efficient substituting ┌=0.4 in the above equation, transmission co-efficient is equal to 1.4.

11. If the transmission coefficient of a transmission line is 1.6, then the reflection co efficient is:
a) 0.8
b) 0.6
c) 0.4
d) 0.3

Explanation: T=┌+1, where T is the transmission co-efficient and ┌ is the reflection co-efficient substituting T=1.6, we get ┌=0.6.

12. For a transmission line, if the transmission coefficient is 1.4, then the insertion loss in dB is:
a) -2.922dB
b) 29dB
c) 1.46dB
d) -29dB

Explanation: Insertion loss for a transmission line is given by the expression -20loglTl in dB. Substituting T=1.4 and taking logarithm to base 10, insertion loss is -2.922dB.

13. The relation between nepers and decibels is:
a) 1 Np= 8.686 dB
b) 1 dB=8.868 dB
c) Np≥dB
d) dB≥Np

Explanation: 1 Np=10log e2 dB. Substituting e=2.718 in the above equation , 1Np=8.686 dB.

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