(SEM VI) THEORY EXAMINATION 2018-19 MICROWAVE ENGINEERING

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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.