1. In the design of a single mode step index optical fiber close to upper cut off, the single mode operation is NOT preserved if
a) Radius as well as operating wavelength are halved
b) Radius as well as operating wavelength are doubled
c) Radius is halved and operating wavelength is doubled
d) Radius is doubled and operating wavelength is halved

Explanation: The cut off frequency is given by f = 2πa sin α/wavelength, where a is the radius of core. For a single mode step index fiber, f must lie between 0 and 2.405. Thus to get f below 2.405, the radius must be doubled and wavelength must be halved.

2. Which parameters cannot be computed from the Smith chart?
a) Impedance
c) Reflection coefficient and VSWR
d) Intrinsic impedance

Explanation: From the Smith chart, the parameters impedance, admittance, reflection coefficient and VSWR can be computed directly. The intrinsic impedance cannot be calculated.

3. Consider a transmission line of characteristic impedance 50 ohm. Let it be terminated at one end by +j50 ohm. The VSWR produced by it in the transmission line will be
a) 1
b) 0
c) Infinity
d) +j

Explanation: The reflection coefficient is given by R = j50 – 50/j50 + 50 = j – 1/j + 1. Thus the reflection coefficient is given by 1. The VSWR = 1 + R/1 – R = 2/0 = infinity.

4. A transmission line whose characteristic impedance is a pure resistance
a) Must be a lossless line
b) Must be a distortionless line
c) May not be a lossless line
d) May be a lossless and may not be a distortionless line

Explanation: When the characteristic line is a pure resistance, the line must be a distortionless line. But it may not be a lossless line.

5. A two wire transmission line of characteristic impedance Z is connected to a load impedance ZL(ZL and Zo are not equal). Impedance matching cannot be achieved with
a) A quarter wavelength transformer
b) A half wavelength transformer
c) An open circuited parallel stub
d) A short circuited parallel stub

Explanation: When the characteristic and load impedance are not same, a half wavelength transformer cannot be used for impedance matching because when l = wavelength/2, we get Zin = ZL.
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6. A transmission line of pure resistance characteristic impedance is terminated with an unknown load. The measured value of VSWR on the line is 2 and a voltage minimum point is found to be at the load. The load impedance is then
a) Complex
b) Purely capacitive
c) Purely resistive
d) Purely inductive

Explanation: Given, voltage minimum point is at load. If minimum voltage or maximum voltage occurs at the load for a lossless transmission line then the load impedance is purely resistive.

7. Which one of the following field patterns represents a TEM wave travelling in the positive x direction?
a) E = +8y, H = -4z
b) E = -2y, H = -3z
c) E = +2z, H = +2y
d) E = -3y, H = +4z

Explanation: To get wave travelling in the positive direction, the E component has to be in negative y direction and the H component has to be in negative z direction. Thus E = -2y and H = -3z is the right option.

8. The modes in a rectangular waveguide are denoted by TEmn / TMmn, where m and n are the Eigen values along the larger and smaller dimensions of the waveguide respectively. Which one of the following statements is true?
a) The TM10 mode does not exist
b) The TE10 mode does not exist
c) The TE10 and TM10 both exist and have same cut off frequencies
d) When m and n are increases, the cut off frequency decreases

Explanation: For TM mode, TM00, TM10, TM01 does not exist. For TE mode, TE00 does not exist but TE10 and TE01 exists.

9. In a microwave test bench, why is the microwave signal amplitude modulated at 1 kHz?
a) To increase the sensitivity of measurement
b) To transmit the signal to a far off place
c) To study amplitude modulation
d) Because crystal detector fails at microwave frequencies

Explanation: Since crystal detector fails at microwave frequencies, microwave signal amplitude is modulated at 1kHz.

10. Consider a lossless antenna with a directive gain of +6 decibel. If 1 milliwatt of power is fed to it, the total power radiated by the antenna will be (in milliwatt)
a) 4
b) 1
c) 7
d) 1/4

Explanation: For a lossless antenna, the input power and the radiated power will be same. The efficiency will be 100%. Thus the radiated power will also be 1 milliwatt.

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