About this previous year paper
Microwave Engineering (REC-601)
Section-Wise Explanation & Answer Guidance (B.Tech Sem-VI)
Overview of the Paper
The REC-601 Microwave Engineering paper is designed to test both conceptual understanding and numerical/problem-solving ability. The syllabus mainly focuses on:
Waveguides and modes
Microwave tubes and solid-state devices
Microwave components (tees, couplers, isolators, circulators)
Microwave measurements
Numerical problems related to wave propagation
The paper is divided into three sections: A, B, and C, each with a specific purpose and difficulty level.
SECTION A (Very Short Answer – Concept Testing)
Pattern:
Attempt all questions
7 questions × 2 marks = 14 marks
Nature of Section A
Section A checks whether you understand basic definitions, differences, and applications. Answers should be short but conceptually correct. You don’t need derivations here—clarity matters more than length.
Explanation of Topics Asked
Dominant and Degenerate Modes:
Dominant mode is the mode with the lowest cut-off frequency, which propagates first in a waveguide (e.g., TE₁₀ in rectangular waveguide).
Degenerate modes are two or more modes having the same cut-off frequency but different field distributions.
E-Plane Tee Properties:
An E-plane tee splits power in such a way that outputs are out of phase and mainly affects the electric field plane.
Cut-off Wave Number (kc):
Cut-off wave number is the minimum wave number required for propagation. For lossless dielectric, it is related to angular frequency, permeability, and permittivity.
Insertion Loss:
It is the loss of signal power caused by inserting a device into a transmission line.
Reflex Klystron Applications:
Used as microwave oscillators in radar, microwave labs, and communication systems.
Microwave Solid-State vs Low-Frequency Devices:
Microwave devices operate at high frequencies with transit-time effects, unlike low-frequency devices.
Difference Between TWT and Klystron:
TWT provides wide bandwidth amplification, while klystron offers high power but narrow bandwidth.
SECTION B (Descriptive Theory – Concept + Explanation)
Pattern:
Attempt any three
3 questions × 7 marks = 21 marks
Nature of Section B
This section demands proper explanations, diagrams (if applicable), and working principles. You should explain how and why, not just what.
Important Topics Explained
1. IMPATT Diode Working Principle
IMPATT diode works on impact ionization and avalanche transit time. When a high reverse bias is applied, avalanche breakdown occurs, creating carriers that drift across the depletion region, producing microwave oscillations. Variation in electric field and carrier concentration is synchronized with AC signal, leading to negative resistance.
2. Gunn Diode and Electron Transfer Mechanism
Gunn diode operates based on the two-valley model. At high electric fields, electrons transfer from a low-energy valley to a high-energy valley, reducing drift velocity and causing negative resistance, which generates microwave oscillations.
3. Directional Coupler
A directional coupler is a passive microwave device that couples a specific amount of power in one direction. Types include single-hole, two-hole, and multi-hole couplers.
The two-hole directional coupler uses phase cancellation to ensure directivity. The S-matrix describes power flow between ports.
4. Microwave Isolator
An isolator allows power transmission in one direction and absorbs power in the reverse direction. It uses ferrite materials under magnetic bias and works on non-reciprocal behavior. Isolators protect microwave sources from reflected power.
5. E-Plane Tee vs H-Plane Tee
E-plane tee causes phase reversal at output ports, while H-plane tee divides power equally without phase change. Their propagation characteristics differ based on electric and magnetic field orientation.
SECTION C (Numericals + Long Theory)
Pattern:
Attempt one part from each question
5 questions × 7 marks = 35 marks
This is the most scoring section if concepts are clear.
Question 3: Waveguide / Magnetron
(a) Numerical on Circular Waveguide
You calculate: Cut-off frequency
Guide wavelength Wave impedance
Using standard microwave equations and given parameters.
(b) Magnetron Theory
Magnetron is called a cross-field device because electric and magnetic fields are perpendicular.
π-mode operation ensures maximum efficiency, and strapping separates π-mode from other modes.
Question 4: Gunn Effect / Cavity Resonator
Gunn Effect
Explained using two-valley energy band model, showing negative resistance region in current-density vs electric-field graph.
Rectangular Cavity Resonator
Cut-off wave number and phase constant are calculated using cavity dimensions and mode equations.
Question 5: Amplifiers / Circulator
Microwave Amplifiers & Oscillators
Classification includes: Klystron
TWT Gunn
IMPATT
Avalanche transit time devices generate oscillations due to delayed carrier movement.
Circulator
A circulator is a non-reciprocal microwave device that routes power sequentially between ports. It uses ferrite materials and magnetic biasing.
Question 6: Klystron / Frequency Measurement
Two-Cavity Klystron Amplifier
Working involves velocity modulation, bunching, and energy extraction.
Numericals include: Drift space length
Maximum efficiency Voltage gain
Frequency Measurement Methods
Includes:
Resonant cavity wavemeter Heterodyne method
Slotted line technique
Question 7: Waveguide Modes / Impedance Measurement
TM₀₁ and TM₁₀ Non-Existence
These modes violate boundary conditions in rectangular waveguides, hence cannot exist.
Impedance Measurement
Smith Chart is used to determine load impedance by plotting reflection coefficient and standing wave ratio.