About this previous year paper
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
| TE Wave | TM Wave |
|---|---|
| Ez = 0 | Hz = 0 |
| Magnetic field has longitudinal component | Electric field has longitudinal component |
| Common in waveguides | Common in waveguides |
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
| GUNN Diode | Tunnel Diode |
|---|---|
| Transferred electron effect | Quantum tunneling |
| Used in microwave oscillators | Used in low-power circuits |
| No PN junction | Heavily doped PN junction |
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).