# Electromagnetic Theory Questions and Answers – Standing Waves and SWR

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This set of Electromagnetic Theory Questions and Answers for Campus interviews focuses on “Standing Waves and SWR”.

1. Standing waves occurs due to
a) Impedance match
b) Impedance mismatch
c) Reflection
d) Transmission

Explanation: Impedance mismatches result in standing waves along the transmission line. It shows the variation of the wave amplitudes due to mismatching.

2. Standing wave ratio is defined as the
a) Ratio of voltage maxima to voltage minima
b) Ratio of current maxima to current minima
c) Product of voltage maxima and voltage minima
d) Product of current maxima and current minima

Explanation: SWR is defined as the ratio of the partial standing wave’s amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line. It is given by S = VMAX/VMIN.

3. Given that the reflection coefficient is 0.6. Find the SWR.
a) 2
b) 4
c) 6
d) 8

Explanation: The relation between reflection coefficient and SWR is given by S = 1 + R/1 – R. On substituting for R = 0.6, we get S = 1 + 0.6/1 – 0.6 = 1.6/0.4 = 4.
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4. The maxima and minima voltage of the standing wave are 6 and 2 respectively. The standing wave ratio is
a) 2
b) 3
c) 1/2
d) 4

Explanation: The ratio of voltage maxima to voltage minima is given by the standing wave ratio SWR. Thus S = VMAX/VMIN. On substituting the given data, we get S = 6/2 = 3.

5. Find the standing wave ratio, when a load impedance of 250 ohm is connected to a 75 ohm line.
a) 0.3
b) 75
c) 250
d) 3.33

Explanation: The standing wave ratio is the ratio of the load impedance to the characteristic impedance. Thus S = ZL/Zo. On substituting for ZL = 250 and Zo = 75, we get S = 250/75 = 3.33.

6. Find the reflection coefficient of the wave with SWR of 3.5.
a) 0.55
b) 0.23
c) 0.48
d) 0.68

Explanation: The reflection coefficient in terms of the SWR is given by R = S – 1/S + 1. On substituting for S = 3.5, we get 3.5 – 1/3.5 + 1 = 0.55.

7. The range of the standing wave ratio is
a) 0 < S < 1
b) -1 < S < 1
c) 1 < S < ∞
d) 0 < S < ∞

Explanation: The standing wave ratio is given by S = 1 – R/1 + R. Thus the minimum value of S is 1. It can extend upto infinity for long lines. Thus the range is 1 < S < ∞.

8. For matched line, the standing wave ratio will be
a) 0
b) ∞
c) -1
d) 1

Explanation: In a matched line, maximum transmission occurs. The reflection will be zero. The standing wave ratio S = 1 – R/1 + R. For R = 0, the SWR is unity for matched line.

9. The maximum impedance of a 50 ohm transmission line with SWR of 3 is
a) 50/3
b) 3/50
c) 150
d) 450

Explanation: The maximum impedance is given by the product of the characteristic impedance and the SWR. Thus Zmax = S Zo. On substituting for S = 3 and Zo = 50, we get ZMAX = 3 X 50 = 150 units.

10. The minimum impedance of a 75 ohm transmission line with a SWR of 2.5 is
a) 100
b) 50
c) 25
d) 30

Explanation: The minimum impedance in terms of SWR is given by ZMIN = Zo/S. Substituting the given data for S = 2.5 and Zo = 75, we get Zmin = 75/2.5 = 30.

11. The standing wave ratio of short circuited and open circuited lines will be
a) 0
b) 1
c) -1
d) ∞

Explanation: The transmission line will reflect high power when it is short or circuited. This will lead to high reflection coefficient. Thus the standing wave ratio will be infinity for these extreme cases.

12. The current reflection coefficient of a line with voltage reflection coefficient of 0.65 is given by
a) 0
b) 0.65
c) -0.65
d) 0.35

Explanation: The current reflection coefficient at any point on the line is the negative of the voltage reflection coefficient at that point, i.e, -R. Given that the voltage reflection coefficient is 0.65, thus the current reflection coefficient is -0.65.

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