(SEM VI) THEORY EXAMINATION 2024-25 MICROWAVE ENGINEERING

BEC064 – MICROWAVE ENGINEERING

Time: 3 Hours | Max Marks: 70

SECTION A – Short Answer Questions

(2 × 7 = 14 marks | Attempt ALL)
Write definition + 1–2 key points / formula

a. TEM Wave

A Transverse Electromagnetic (TEM) wave has both electric and magnetic fields perpendicular to the direction of propagation, with no longitudinal components.
Exists in two-conductor systems like coaxial cables.

b. Difference Between TE and TM Waves

c. Principle of Phase Shifter

A phase shifter works by changing the electrical length or propagation constant, resulting in a controlled phase change of the microwave signal.

d. Dominant Modes of Waveguides

Rectangular waveguide: TE₁₀                                Circular waveguide: TE₁₁

Dominant mode has lowest cutoff frequency.

e. Why Low-Frequency Parameters Are Not Used at Microwaves

At microwave frequencies:                                      Circuit dimensions ≈ wavelength

Distributed effects dominate                                   Lumped parameters become invalid

f. Unitary Properties of S-Matrix                              For a lossless network:

[S][S]∗=[I][S][S]^* = [I][S][S]∗=[I]

 Power is conserved, no energy loss.

g. Significance of VSWR Measurement                    VSWR indicates impedance matching quality.

VSWR = 1 → perfect match                                      High VSWR → reflections & power loss

SECTION B – Medium Answer Questions

(7 × 3 = 21 marks | Attempt ANY THREE)
Write working → explanation → neat diagram

a. Reflex Klystron – Operation                                    Uses velocity modulation

Electron beam passes through cavity gap                 Electrons bunch and transfer energy to RF field

Acts as a microwave oscillator                                Draw reflex klystron diagram.

b. Power Sensors for Microwave Power Measurement

Thermistor: High sensitivity                                     Bolometer: Accurate low-power measurement

Thermocouple: Wide frequency range

c. Backward Wave Crossed-Field Amplifier (CFA) – Key Points

Uses crossed electric & magnetic fields                Backward wave interaction

High gain and efficiency                                           Used in radar and satellite systems

d. E-Plane Tee, H-Plane Tee & Magic Tee

E-Plane Tee: Power divides with 180° phase shift     H-Plane Tee: Power divides in phase

Magic Tee: Combination of E & H plane tees           Draw waveguide tee diagrams.

e. GUNN Diode vs Tunnel Diode

SECTION C – Attempt ANY ONE

(7 marks)

a. TE Wave Equations + Numerical (Very Important)

For parallel plane waveguide:                              Plate separation = 0.03 m

Mode = TE₁₀                                                            Frequency = 6 GHz

Find:                                                                        Cutoff frequency

Cutoff wavelength                                                   Guide wavelength

Group velocity

Derivation + step-by-step numerical fetch full marks.

b. High-Field Domain Formation

When drift velocity decreases with increasing electric field, electrons form high-field domains, leading to microwave generation (basis of GUNN diode operation).