(SEM VI) THEORY EXAMINATION 2017-18 MICROWAVE ENGINEERING

Microwave Engineering (NEC-601)

Complete Section-Wise Explanation – B.Tech Semester VI

Introduction to the Subject

Microwave Engineering deals with the study and application of electromagnetic waves in the microwave frequency range, typically from 1 GHz to 300 GHz. At these high frequencies, conventional circuit theory fails, and special transmission lines, waveguides, and microwave devices are used.

This subject is extremely important for understanding:

Waveguides and propagation modes                                Microwave network analysis using S-parameters

Microwave sources and amplifiers (Klystron, TWT, BWO)  Microwave solid-state devices (IMPATT, TRAPATT, Tunnel diode)                                                                      Microwave measurements using test benches

Non-reciprocal devices like isolators and circulators

The question paper is divided into three sections: A, B, and C, and students must attempt all sections as per instructions.

SECTION A – Fundamental Concepts (Short Answers)

Pattern:
Attempt all questions
10 questions × 2 marks = 20 marks

Nature of Section A

Section A checks your basic conceptual clarity. Answers should be short, direct, and technically correct, usually written in two or three lines.

Explanation of Section A Questions

Why TEM mode cannot exist in rectangular waveguide
TEM mode requires both electric and magnetic fields to be entirely transverse with no longitudinal components. In a rectangular waveguide, boundary conditions do not allow simultaneous satisfaction of Maxwell’s equations for TEM mode, hence TEM mode cannot exist.

Dominant and Degenerate Modes
The dominant mode is the mode with the lowest cutoff frequency, which propagates first; in a rectangular waveguide, this is the TE₁₀ mode. Degenerate modes are different modes having the same cutoff frequency.

S-Matrix
The scattering matrix (S-matrix) relates incident and reflected waves at microwave ports and is widely used because voltage and current measurements are difficult at microwave frequencies.

Isolator vs Circulator
An isolator allows microwave power to pass in one direction only, protecting sources from reflections. A circulator routes power sequentially from one port to the next in a cyclic manner.

Drawbacks of Klystron Amplifier
Klystron amplifiers have narrow bandwidth, require high operating voltage, and are bulky compared to solid-state devices.

Backward Wave Oscillator (BWO)
A BWO is a microwave oscillator in which the electron beam interacts with a backward traveling wave. It is used in microwave signal generation and electronic warfare.

Conditions for IMPATT Oscillation
IMPATT diode oscillates when avalanche breakdown and transit-time delay occur simultaneously, producing negative resistance at microwave frequencies.

Transit Time Effect
Transit time effect refers to the delay experienced by charge carriers while crossing a device, which limits high-frequency performance.

VSWR
Voltage Standing Wave Ratio is the ratio of maximum to minimum voltage on a transmission line and indicates impedance matching quality.

Slotted Line
A slotted line is a waveguide with a longitudinal slot used to measure standing wave patterns, wavelength, VSWR, and impedance.

SECTION B – Descriptive Theory & Applications

Pattern:
Attempt any three questions
3 × 10 marks = 30 marks

Nature of Section B

Section B requires detailed explanations written in paragraph form, often supported by diagrams and equivalent circuits.

Explanation of Section B Questions

Microwave Cavity Resonator

A microwave cavity resonator is a closed metallic structure that stores electromagnetic energy at resonant frequencies. It works similarly to an LC resonant circuit at microwave frequencies. The equivalent circuit consists of inductance, capacitance, and resistance representing stored energy and losses. Cavity resonators are used in oscillators, filters, and frequency meters.

S-Parameters and Their Importance

S-parameters describe microwave networks in terms of incident and reflected waves. They are preferred at microwave frequencies because direct voltage and current measurements are impractical. The scattering matrix of a four-port circulator using magic tees demonstrates isolation and power flow control.

Two-Cavity Klystron Amplifier

A two-cavity klystron amplifier uses velocity modulation of an electron beam. The buncher cavity modulates velocity, and the catcher cavity extracts RF energy. This amplifier provides high power and efficiency but limited bandwidth.

Microwave Tunnel Diode

The tunnel diode operates based on quantum mechanical tunneling and exhibits negative resistance. It is used in microwave oscillators and amplifiers due to its fast response and low noise.

Microwave Frequency Measurement Methods

Frequency can be measured using cavity wavemeters, absorption wavemeters, heterodyne methods, and electronic frequency counters. Each method varies in accuracy and application range.

SECTION C – Analytical & Derivation-Based Questions

Pattern:
Attempt any one part from each question
5 questions × 10 marks = 50 marks

This section has the highest weightage and requires strong theoretical understanding, derivations, and numericals.

Question 3

Circular Waveguide
The advantages include higher power handling and lower attenuation, while disadvantages include mode degeneracy and complex excitation. Applications include radar and satellite communication.
The numerical problem calculates guide wavelength and cutoff wavelength using given diameter and operating frequency.

TE₁₀ Mode in Rectangular Waveguide
This derivation explains electric and magnetic field distribution in TE₁₀ mode. The field pattern shows maximum electric field at the center. TE₀₁ and TM₁₀ modes do not exist due to boundary condition violations.

Question 4

Directional Coupler
A directional coupler samples power flowing in a specific direction. The coupling factor and directivity expressions are derived. Single-hole couplers are simpler but less directional than double-hole couplers.

Circulator and Faraday Rotation
A circulator is a non-reciprocal device using ferrite materials. Faraday rotation rotates the plane of polarization under a magnetic field and is used to design isolators and circulators.

Question 5

Traveling Wave Tube (TWT) Amplifier
A TWT amplifier uses interaction between an electron beam and a slow-wave structure. It offers wide bandwidth and high gain. Different waves like fast wave, slow wave, and space-charge wave are explained.

Limitations of Conventional Devices
At microwave frequencies, conventional devices suffer from transit-time effects, parasitic capacitances, and excessive losses, making special microwave devices necessary.

Question 6

IMPATT and TRAPATT Diodes
IMPATT diodes offer high power but high noise, while TRAPATT diodes provide higher efficiency with lower frequency operation. Performance comparison is based on efficiency, noise, and frequency range.

Microwave BJT and Transferred Electron Devices
Microwave BJTs are designed for high-frequency operation, while transferred electron devices like Gunn diodes use negative differential resistance.

Question 7

Insertion Loss and Attenuation Measurement
Insertion loss measures power loss due to component insertion. Attenuation can be measured using slotted line and power meter methods on a microwave test bench.

Frequency Meter and Load Measurement
A frequency meter uses cavity resonance to determine frequency. Unknown load impedance is measured using VSWR and reflection coefficient on a microwave bench.