Electromagnetic Theory Questions and Answers- Short and Open Circuit Lines and Answers

Electromagnetic Theory Multiple Choice Questions on “Short and Open Circuit Lines”.

1. The open circuit impedance of the transmission line is given by
A. Z_OC = j Zo tan βl
B. Z_OC = – j Zo tan βl
C. Z_OC = j Zo cot βl
D. Z_OC = -j Zo cot βl
Answer: D
Clarification: The open circuit in a transmission line refers to the load side open circuited. In this case, the load impedance will be infinite. Thus the transmission line equation will be Z_OC = -j Zo cot βl.

2. The short circuit impedance of the transmission line is given by
A. Z_SC = j Zo tan βl
B. Z_SC = -j Zo tan βl
C. Z_SC = j Zo cot βl
D. Z_SC = -j Zo cot βl
Answer: A
Clarification: The short circuit in a transmission line refers to the load side shorted. In this case, the load impedance will be zero. Thus the transmission line equation will be Z_SC = j Zo tan βl.

3. In a shorted line, the reflection coefficient will be
A. 0
B. 1
C. -1
D. ∞
Answer: C
Clarification: The shorted line will absorb more power than any other line. Thus the reflection coefficient is considered to be negative.

4. The open circuit line will have a reflection coefficient of
A. 0
B. 1
C. -1
D. ∞
Answer: B
Clarification: An open circuit line has infinite output impedance. Any wave incident at the output will be completely reflected. Thus the reflection coefficient is unity.

5. The standing wave ratio in short and open circuit transmission lines will be
A. 0
B. -1
C. 1
D. ∞
Answer: D
Clarification: The reflection coefficient is 1 and -1 in open and shorted lines respectively. This value of reflection coefficient will yield infinite standing wave ratio.

6. The characteristic impedance of a line having open and short impedances of 20 and 5 respectively is
A. 20
B. 100
C. 25
D. 10
Answer: D
Clarification: The characteristic impedance is the geometric mean of the short and open circuit impedance. It is given by Zo² = Zsc Zoc. On substituting Zoc = 20 and Zsc = 5, we get Zo² = 20 X 5 = 100. Thus Zo = 10 ohm.

7. The short circuit impedance is given by 18 ohm and the characteristic impedance is 50 ohm. Find the open circuit impedance.
A. 138.8
B. 188.3
C. 388.1
D. 838.1
Answer: A
Clarification: The relation between characteristic impedance, open and short impedance is given by Zo² = Zsc Zoc. For the given values Zo = 50 and Zsc = 18, we get Zoc = 50²/18 = 138.8 units.

8. For maximum power transfer theorem to be applied to the transmission line, the reflection coefficient has to be
A. 1
B. -1
C. 0
D. ∞
Answer: C
Clarification: Maximum power transfer between the load and source is possible, only when both are matched. This will lead to no reflections. Thus the reflection coefficient will be zero.

9. Find the transmission coefficient of a 75 ohm line with load impedance of 40 ohm.
A. 0.69
B. 0.96
C. 0.31
D. 0.13
Answer: A
Clarification: The transmission coefficient in terms of the load impedance is given by T = Z_L/Z₀. On substituting for Z_L = 40 and Zo = 75, we get T = 40/75 = 0.69.

10. The standing waves for open circuit voltage and short circuit current are the same. State true/false.
A. True
B. False
Answer: A
Clarification: The open circuit voltage and short circuit current will be same for a transmission line. The phase difference is λ/8.

11. The standing waves for open circuit current and short circuit voltage are the same. State true/false.
A. true
B. false
Answer: A
Clarification: The open circuit current and short circuit voltage will be same for a transmission line. The phase difference is λ/8.

12. The standing wave ratio for the maximum power transfer in a transmission line is
A. 1:2
B. 2:1
C. -1:1
D. 1:1
Answer: D
Clarification: The load and the source has to be matched for maximum power transfer. This is indicated by the ratio of 1:1.