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) Z_{0} (Z_{L}+j Z_{0}tan βl)/ (Z_{0}+j Z_{L}tan βl)

b) (Z_{0}+j Z_{L}tan βl)/ (Z_{L}+j Z_{0}tan βl)

c) Z_{0} (Z_{L}-j Z_{0}tan βl)/ (Z_{0}-j Z_{L}tan βl)

d) (Z_{0}-j Z_{L}tan βl)/ (Z_{L}-j Z_{0}tan βl)

View Answer

Explanation: Representing the input voltage as the ratio of voltage at current, representing voltage and currents in hyperbolic function form and simplifying, we get Z

_{0}(Z

_{L}+j Z

_{0}tan βl)/ (Z

_{0}+j Z

_{L}tan βl).

2. Input impedance of a short circuited transmission line is :

a) -jZ_{0}tanβl

b) jZ_{0}tanβl

c) jZ_{0}cotβl

d) – jZ_{0}cotβl

View Answer

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 Z

_{0}(Z

_{L}+j Z

_{0}tan βl)/ (Z

_{0}+j Z

_{L}tan βl), input impedance of the transmission line comes out to be jZ

_{0}tanβl.

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

View Answer

Explanation: The input impedance of a transmission line is expressed in the standard form as Z

_{0}(Z

_{L}+j Z

_{0}tan βl)/ (Z

_{0}+j Z

_{L}tan 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

View Answer

Explanation: For a short circuited transmission line, the input impedance is given by jZ

_{0}tanβl.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) -jZ_{0}tanβl

b) jZ_{0}tanβl

c) jZ_{0}cotβl

d) -jZ_{0}cotβl

View Answer

Explanation: Since the load impedance of a open circuited transmission line is infinity, substituting ZL=infinity (1/ Z

_{L}=0) in the expression for input impedance of a transmission line Z

_{0}(Z

_{L}+j Z

_{0}tan βl)/ (Z

_{0}+j Z

_{L}tan βl), input impedance of the open circuited transmission line comes out to be- jZ

_{0}cotβl.

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

View Answer

Explanation: The input impedance of a transmission line is expressed in the standard form as Z

_{0}(Z

_{L}+j Z

_{0}tan βl)/ (Z

_{0}+j Z

_{L}tan βl). With Z

_{L}equal to infinity for open circuit termination, 1/ Z

_{L}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

View Answer

Explanation: Input impedance of a transmission line is given by Z

_{0}(Z

_{L}+j Z

_{0}tan βl)/ (Z

_{0}+j Z

_{L}tan 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

View Answer

Explanation: Input impedance of an open circuited transmission line is given by – jZ

_{0}cotβl. 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) 2Z_{L}/ ( Z_{L}+Z_{0})

b) (Z_{L}-Z_{0})/ (Z_{L}+Z_{0})

c) 2Z_{0}/( Z_{L}+Z_{0})

d) (Z_{L}+Z_{0})/ (Z_{L}-Z_{0})

View Answer

Explanation: T=┌+1, where T is the transmission co-efficient and ┌ is the reflection co-efficient substituting ┌= (Z

_{L}-Z

_{0})/ (Z

_{L}+Z

_{0}) in the equation for transmission co-efficient, we get 2Z

_{L}/ ( Z

_{L}+Z

_{0}).

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

View Answer

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

View Answer

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

View Answer

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

View Answer

Explanation: 1 Np=10log e

^{2}dB. Substituting e=2.718 in the above equation , 1Np=8.686 dB.

**Sanfoundry Global Education & Learning Series – Microwave Engineering.**

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