# Microwave Engineering Questions and Answers – Parallel Plate Waveguide

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This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Parallel Plate Waveguide”.

1. The modes of wave propagation that a parallel plate waveguide can support are:
a) TEM, TE, TM modes
b) TM, TE modes
c) TEM, TM modes
d) TEM, TE modes

Explanation: Parallel plate waveguide is the simplest type of waveguide that can support TE and TM modes. It can also support a TEM mode since it is formed from two flat conducting plates.

2. The fringe effect can be neglected in a parallel plate waveguide is because of:
a) The dielectric material used
b) Width of the plates is greater than the distance between the plates
c) Material of the parallel plate waveguide used
d) None of the mentioned

Explanation: The strip width W of the parallel plate waveguide is assumed to be much greater than the separation d, hence the fringe effect or the fringing fields can be neglected.

3. The characteristic impedance of a parallel plate waveguide is given by:
a) η*D/W
b) η*W/D
c) D/ η*W
d) η*√(D/W)

Explanation: Characteristic impedance of a waveguide is the ratio of voltage and current. Defining voltage and current in the integral form of electric field and magnetic field respectively and solving the characteristic impedance is η*D/W. here η is the intrinsic impedance of the medium in the waveguide, D is the distance between the plates and W is the width of the rectangular plate.

4. If the width of a parallel plate waveguide is 30 mm and the distance between the parallel pates is 5 mm, with an intrinsic impedance of 377Ω, then the characteristic impedance of the wave is:
a) 50 Ω
b) 62.833 Ω
c) 100 Ω
d) None of the mentioned

Explanation: The expression for intrinsic impedance of a parallel plate waveguide is η*D/W. substituting the given values of intrinsic impedance and distance between plates and width of the plates, intrinsic impedance is 62.833Ω.

5. In TM mode, if the direction of wave propagation is in ‘z’ direction, then:
a) HZ=0
b) EZ=0
c) EY=0
d) HY=0

Explanation: In TM mode (transverse magnetic), when the wave propagation is along Z direction, magnetic field is absent in Z direction since the fields are transverse in nature. Hence HZ=0.

6. The wave impedance of a TM mode in a parallel plate waveguide is a:
a) Function of frequency
b) Independent of frequency
c) Proportional to square of frequency
d) Inversely proportional to square of frequency

Explanation: The wave impedance of a parallel plate waveguide in TM mode is β/k which is a function of frequency. The wave impedance is real for f>fC and purely imaginary for f<fC.

7. In a parallel plate waveguide, for a propagating mode, the value of β is:
a) Real
b) Complex
c) Imaginary
d) Cannot be defined

Explanation: The phase velocity and guide wavelength for a parallel plate waveguide are defined only for propagating modes. Propagating modes are those modes for which β are always positive. Hence β is always real for a parallel plate waveguide.

8. For TM2 mode, if the distance between two parallel plates of a waveguide are 40 mm, then the cut off wavelength for TM2 mode is:
a) 20 mm
b) 80 mm
c) 40 mm
d) 60 mm

Explanation: The cutoff wavelength of a TMn mode in a parallel plate waveguide is 2d/n, where d is the distance between the plates and n signifies the mode of operation. For the given condition, substituting the given values, cut off wavelength is 40 mm.

9. For a parallel waveguide, the dominant mode for TM propagation is:
a) TM0 mode
b) TM1 mode
c) TM2 mode
d) Dominant mode does not exist

Explanation: The mode of propagation for which the cutoff wavelength for wave propagation is maximum is called dominant mode. In TM mode of propagation, TM0 mode is similar to TEM mode of propagation. Hence, TM1 mode is the dominant mode.

10. Phase velocity of the plane waves in the two direction in a parallel plate waveguide is:
a) 1/√(μ∈)secant θ
b) 1/√(μ∈)cosecant θ
c) 1/√(μ∈)tangent θ
d) 1/√(μ∈)cosine θ

Explanation: The phase velocity of each plane wave along its direction of propagation (θ direction) is 1/√(μ∈), Which is the speed of light in the material filling the guide. But, the phase velocity of the plane waves in the z direction is 1/√(μ∈)secant θ.

11. For a parallel plate waveguide, which of the following is true?
a) No real power flow occurs in the ‘z’ direction
b) Power flow occurs in ‘z’ direction
c) No power flow occurs in any direction
d) Wave propagation in z direction is not possible in any mode

Explanation: The superposition of the two plane waves in Z direction is such that complete cancellation occurs at y=0 and y=d, to satisfy the boundary conditions that EZ=0 at these planes. As f decrease to fc, β approaches 0 so that θ approaches 90⁰. The two plane waves are then bouncing up and down, with no motion in +z direction, and no power flow occurs in the z direction.

12. TE mode is characterized by:
a) EZ=0
b) HZ=0
c) Ex=0
d) Ey=0

Explanation: In TE mode of wave propagation, the electric field is in transverse direction and hence electric field component in the direction of wave propagation is 0. Hence, EZ=0.

13. If in a parallel plate waveguide, PL=4 mW/m and Pₒ=10 mW/m, then what is the conduction loss?
a) 0.5
b) 0.4
c) 0.1
d) 0.2

Explanation: Conductor loss of a parallel plate waveguide is given by PL/2Pₒ. Substituting the given values in the above equation, conductor loss is 0.2.

14. If the distance between the two plates of a parallel plate waveguide is 20 mm and is operating in TE₂ mode, then the cut off frequency of TE₂ mode is:
a) 20 MHz
b) 15 GHz
c) 5 GHz
d) None of the mentioned

Explanation: The cutoff frequency for TEn mode is n/2d√(∈μ) for a parallel plate waveguide. Substituting the given values, the cutoff frequency is 15 GHz.

15. The wave impedance for a non-propagating mode in TE mode is:
a) K/β
b) Imaginary
c) Zero
d) Non-existing