# Electromagnetic Theory Questions and Answers – Power, Power Loss and Return Loss

This set of Electromagnetic Theory Multiple Choice Questions & Answers (MCQs) focuses on “Power, Power Loss and Return Loss”.

1. The power of the electromagnetic wave with electric and magnetic field intensities given by 12 and 15 respectively is
a) 180
b) 90
c) 45
d) 120

Explanation: The Poynting vector gives the power of an EM wave. Thus P = EH/2. On substituting for E = 12 and H = 15, we get P = 12 x 15/2 = 90 units.

2. The power of a wave of with voltage of 140V and a characteristic impedance of 50 ohm is
a) 1.96
b) 19.6
c) 196
d) 19600

Explanation: The power of a wave is given by P = V2/2Zo, where V is the generator voltage and Zo is the characteristic impedance. on substituting the given data, we get P = 1402/(2×50) = 196 units.

3. The power reflected by a wave with incident power of 16 units is(Given that the reflection coefficient is 0.5)
a) 2
b) 8
c) 6
d) 4

Explanation: The fraction of the reflected to the incident power is given by the reflection coefficient. Thus Pref = R2xPinc. On substituting the given data, we get Pref = 0.52 x 16 = 4 units.

4. The power transmitted by a wave with incident power of 16 units is(Given that the reflection coefficient is 0.5)
a) 12
b) 8
c) 16
d) 4

Explanation: The fraction of the transmitted to the incident power is given by the reflection coefficient. Thus Pref = (1-R2) Pinc. On substituting the given data, we get Pref = (1- 0.52) x 16 = 12 units. In other words, it is the remaining power after reflection.

5. The incident and the reflected voltage are given by 15 and 5 respectively. The transmission coefficient is
a) 1/3
b) 2/3
c) 1
d) 3

Explanation: The ratio of the reflected to the incident voltage is the reflection coefficient. It is given by R = 5/15 = 1/3. To get the transmission coefficient, T = 1 – R = 1 – 1/3 = 2/3.
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6. The current reflection coefficient is given by -0.75. Find the voltage reflection coefficient.
a) -0.75
b) 0.25
c) -0.25
d) 0.75

Explanation: The voltage reflection coefficient is the negative of the current reflection coefficient. For a current reflection coefficient of -0.75, the voltage reflection coefficient will be 0.75.

7. The attenuation is given by 20 units. Find the power loss in decibels.
a) 13.01
b) 26.02
c) 52.04
d) 104.08

Explanation: The attenuation refers to the power loss. Thus the power loss is given by 20 units. The power loss in dB will be 10 log 20 = 13.01 decibel.

8. The reflection coefficient is 0.5. Find the return loss.
a) 12.12
b) -12.12
c) 6.02
d) -6.02

Explanation: The return loss is given by RL = -20log R, where is the reflection coefficient. It is given as 0.5. Thus the return loss will be RL = -20 log 0.5 = 6.02 decibel.

9. The radiation resistance of an antenna having a power of 120 units and antenna current of 5A is
a) 4.8
b) 9.6
c) 3.6
d) 1.8

Explanation: The power of an antenna is given by Prad = Ia2 Rrad, where Ia is the antenna current and Rrad is the radiation resistance. On substituting the given data, we get Rrad = Prad/Ia2 = 120/52 = 4.8 ohm.

10. The transmission coefficient is given by 0.65. Find the return loss of the wave.
a) 9.11
b) 1.99
c) 1.19
d) 9.91

Explanation: The transmission coefficient is the reverse of the reflection coefficient, i.e, T + R = 1. When T = 0.65, we get R = 0.35. Thus the return loss RL = -20log R = -20log 0.35 = 9.11 decibel.

11. The return loss is given as 12 decibel. Calculate the reflection coefficient.
a) 0.35
b) 0.55
c) 0.25
d) 0.75

Explanation: The return loss is given by RL = -20log R. The reflection coefficient can be calculated as R = 10(-RL/20), by anti logarithm property. For the given return loss RL = 12, we get R = 10(-12/20) = 0.25.

12. Find the transmission coefficient of a wave, when the return loss is 6 decibel.
a) 0.498
b) 0.501
c) 0.35
d) 0.65

Explanation: The return loss is given by RL = -20log R. The reflection coefficient can be calculated as R = 10(-RL/20), by anti logarithm property. For the given return loss RL = 6, we get R = 10(-6/20) = 0.501. The transmission coefficient will be T = 1 – R = 1-0.501 = 0.498.

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