Microwave and Radar Communication (4351103) - Summer 2025 Solution

Table of Contents

Question 1(a) [3 marks]
#

List four microwave frequency bands with their frequency range and applications.

Answer:

Band	Frequency Range	Applications
L-band	1-2 GHz	GPS, Mobile communication
S-band	2-4 GHz	WiFi, Bluetooth, Radar
C-band	4-8 GHz	Satellite communication
X-band	8-12 GHz	Military radar, Weather radar

Mnemonic: “Little Satellites Communicate eXcellently”

Question 1(b) [4 marks]
#

Explain the impedance matching process using a single stub.

Answer:

Single stub matching removes reflections by adding a short-circuited stub at specific distance from load.

Process:

Stub length: Provides reactive impedance
Stub position: Calculated from load using Smith chart
Matching condition: Real part = Z₀, imaginary part = 0

graph LR
    A[Source] --> B[Transmission Line]
    B --> C[Stub Position]
    C --> D[Load]
    C --> E[Short Stub]

Mnemonic: “Stub Positioned for Perfect Matching”

Question 1(c) [7 marks]
#

State characteristics of lossless transmission line and obtain the general equation for a two-wire transmission line.

Answer:

Characteristics of Lossless Line:

No power loss: R = 0, G = 0
Constant amplitude: No attenuation
Phase delay only: Signal delayed but not weakened
Standing wave pattern: Due to reflections

General Equations:

For voltage: V(z) = V₊e^(-γz) + V₋e^(γz) For current: I(z) = (V₊/Z₀)e^(-γz) - (V₋/Z₀)e^(γz)

Where:

γ = α + jβ (propagation constant)
Z₀ = √(L/C) (characteristic impedance)
For lossless line: α = 0, γ = jβ

Mnemonic: “Lossless Lines Love Low Loss”

Question 1(c) OR [7 marks]
#

Define standing wave. Draw and explain the standing wave pattern for short circuit and open circuit line.

Answer:

Standing Wave: Fixed pattern formed by forward and reflected waves interfering constructively and destructively.

Short Circuit Line:

Current maximum at short circuit
Voltage minimum at short circuit
Distance between minima: λ/2

Open Circuit Line:

Voltage maximum at open circuit
Current minimum at open circuit
Distance between maxima: λ/2

Mnemonic: “Short Circuits Current, Open Circuits Voltage”

Question 2(a) [3 marks]
#

Draw and explain the working of Magic TEE.

Answer:

Magic TEE combines E-plane and H-plane tees with four ports providing isolation between opposite ports.

graph TD
    A[Port 1 - E-arm] --> C[Junction]
    B[Port 2 - H-arm] --> C
    C --> D[Port 3 - Collinear arm]
    C --> E[Port 4 - Collinear arm]

Working:

E-arm and H-arm: Isolated from each other
Sum port: Adds signals from collinear arms
Difference port: Subtracts signals

Mnemonic: “Magic Tee Mixes Modes”

Question 2(b) [4 marks]
#

Explain the working of Hybrid ring.

Answer:

Hybrid Ring is a circular waveguide with four ports spaced at specific intervals for power division and isolation.

Construction:

Ring circumference: 1.5λ
Port spacing: λ/4 between adjacent ports
Matched impedance: Each port matched to Z₀

Working:

Power splitting: Input splits equally between two output ports
Isolation: Opposite ports are isolated
Phase difference: 180° between output ports

Mnemonic: “Ring Runs Round for Power Sharing”

Question 2(c) [7 marks]
#

Explain the construction and working principle of “CIRCULATOR”. List its applications.

Answer:

Construction:

Three-port device with ferrite material
Permanent magnet creates magnetic field
Y-junction waveguide structure

graph LR
    A[Port 1] --> B[Ferrite Junction]
    B --> C[Port 2]
    C --> D[Port 3]
    D --> A
    style B fill:#ff9999

Working Principle:

Faraday rotation: Magnetic field rotates wave polarization
Unidirectional flow: Power flows in one direction only
Non-reciprocal: Different behavior for opposite directions

Applications:

Radar systems: Isolates transmitter from receiver
Communication: Antenna sharing for TX/RX
Microwave amplifiers: Prevents feedback

Mnemonic: “Circulator Circles Clockwise Continuously”

Question 2(a) OR [3 marks]
#

Compare rectangular waveguide and circular waveguide.

Answer:

Parameter	Rectangular	Circular
Cross-section	Rectangle	Circle
Dominant mode	TE₁₀	TE₁₁
Cutoff frequency	Easy calculation	Complex calculation
Manufacturing	Simple	Moderate
Power handling	Lower	Higher

Mnemonic: “Rectangles are Regular, Circles are Complex”

Question 2(b) OR [4 marks]
#

Draw and explain the working of a directional coupler.

Answer:

Directional Coupler samples forward power while providing isolation from reflected power.

graph LR
    A[Input] --> B[Main Line]
    B --> C[Output]
    B -.-> D[Coupled Port]
    B -.-> E[Isolated Port]
    style D fill:#99ff99
    style E fill:#ff9999

Working:

Coupling factor: Determines power extracted (10-20 dB typical)
Directivity: Isolates forward from reverse power
Insertion loss: Minimal loss in main line

Parameters:

C = 10 log(P₁/P₃) (Coupling factor)
D = 10 log(P₃/P₄) (Directivity)

Mnemonic: “Coupler Couples Carefully in Correct Direction”

Question 2(c) OR [7 marks]
#

Explain the construction and working principle of “Travelling Wave Tube”. List its applications.

Answer:

Construction:

Electron gun: Emits electron beam
Helix structure: Slows down RF wave
Collector: Collects spent electrons
Magnetic focusing: Keeps beam focused

graph LR
    A[Electron Gun] --> B[Helix]
    B --> C[Collector]
    D[RF Input] --> B
    B --> E[RF Output]
    F[Magnetic Field] -.-> B

Working Principle:

Velocity synchronization: Electron velocity ≈ RF wave velocity
Energy transfer: Electrons give energy to RF wave
Continuous interaction: Along entire helix length

Applications:

Satellite communication: High power amplification
Radar transmitters: High gain amplification
Electronic warfare: Jamming systems

Mnemonic: “TWT Transfers Tremendous power Through Travel”

Question 3(a) [3 marks]
#

Explain the Indirect method for higher VSWR measurement.

Answer:

Indirect Method measures high VSWR by using attenuator to reduce signal level for accurate measurement.

Procedure:

Insert calibrated attenuator (10-20 dB)
Measure reduced VSWR (VSWR₂)
Calculate actual VSWR: VSWR₁ = VSWR₂ × Attenuator ratio

Formula: VSWR_actual = VSWR_measured × 10^(Attenuation/20)

Mnemonic: “Indirect method uses Intermediate Attenuation”

Question 3(b) [4 marks]
#

Write and explain the frequency limitations of conventional tubes.

Answer:

Frequency Limitations:

Transit time effect: Electron transit time becomes significant
Interelectrode capacitance: Limits high frequency response
Lead inductance: Parasitic inductance reduces gain
Skin effect: Current flows on surface only

Effects:

Reduced gain: At frequencies above fα
Increased noise: Due to shot noise
Phase shift: Delays signal processing

Solutions:

Reduce electrode spacing
Use special tube designs
Employ cavity resonators

Mnemonic: “Transit Time Troubles Traditional Tubes”

Question 3(c) [7 marks]
#

Explain construction and working of Two cavity klystron with applegate diagram. List its advantages.

Answer:

Construction:

Electron gun: Produces electron beam
Input cavity: Velocity modulates beam
Drift region: Beam bunching occurs
Output cavity: Extracts RF energy
Collector: Collects electrons

Applegate Diagram:

Working:

Velocity modulation: Input cavity varies electron velocity
Density modulation: Electrons bunch in drift space
Energy extraction: Bunched beam transfers energy to output cavity

Advantages:

High power output: Several kilowatts
High efficiency: 40-60%
Low noise: Better than semiconductor devices
Stable operation: Excellent frequency stability

Mnemonic: “Klystron Kicks with Kinetic Bunching”

Question 3(a) OR [3 marks]
#

Explain construction and working of BWO.

Answer:

BWO (Backward Wave Oscillator) uses backward wave interaction for oscillation.

Construction:

Electron gun: Emits electron beam
Slow wave structure: Helix or coupled cavities
Collector: At input end
Output: From input end

Working:

Backward wave: Travels opposite to electron beam
Negative resistance: Beam provides energy to backward wave
Oscillation: When gain > losses

Mnemonic: “BWO goes Backward While Oscillating”

Question 3(b) OR [4 marks]
#

Explain hazards due to microwave radiation.

Answer:

Types of Hazards:

HERP: Hazards of Electromagnetic Radiation to Personnel
HERO: Hazards of Electromagnetic Radiation to Ordnance
HERF: Hazards of Electromagnetic Radiation to Fuel

Effects:

Thermal heating: Tissue heating at high power
Eye damage: Cataract formation
Reproductive effects: Potential fertility issues
Pacemaker interference: Electronic device malfunction

Protection:

Power density limits: < 10 mW/cm²
Safety distances: Far field calculations
Warning signs: Radiation hazard markers
Personal monitors: RF exposure meters

Mnemonic: “Microwaves Make Multiple Medical Maladies”

Question 3(c) OR [7 marks]
#

Explain construction and working of magnetron with neat sketch. List its applications.

Answer:

Construction:

Circular cathode: Central hot cathode
Cylindrical anode: With resonant cavities
Permanent magnet: Provides axial magnetic field
Output coupling: Loop or probe

graph TD
    A[Cathode] --> B[Interaction Space]
    B --> C[Anode Cavities]
    D[Magnetic Field] -.-> B
    C --> E[Output Coupling]
    style A fill:#ff9999
    style C fill:#99ff99

Working:

Electron cloud: Forms in interaction space
Cycloid motion: Due to E and B fields
Resonant cavities: Determine operating frequency
π-mode oscillation: Alternate cavities have opposite phase

Applications:

Microwave ovens: 2.45 GHz heating
Radar systems: High power pulses
Industrial heating: Material processing
Medical diathermy: Therapeutic heating

Mnemonic: “Magnetron Makes Microwaves Magnificently”

Question 4(a) [3 marks]
#

Explain working of P-i-N diode.

Answer:

P-i-N Diode has intrinsic layer between P and N regions, acting as voltage-controlled resistor.

Structure:

P region: Heavily doped
I region: Intrinsic (undoped)
N region: Heavily doped

Working:

Forward bias: Low resistance (1-10 Ω)
Reverse bias: High resistance (>10 kΩ)
RF switch: Controls microwave signals
Variable attenuator: Resistance varies with DC bias

Mnemonic: “PIN controls Power IN Networks”

Question 4(b) [4 marks]
#

Explain the working of Varactor diode with sketch.

Answer:

Varactor Diode acts as voltage-controlled capacitor using junction capacitance variation.

Working:

Reverse bias: Depletes junction, reduces capacitance
Bias voltage: Controls capacitance value
Capacitance ratio: Typically 3:1 to 10:1
Frequency tuning: Used in oscillators and filters

Applications:

VCO tuning: Voltage controlled oscillators
AFC circuits: Automatic frequency control
Parametric amplifiers: Low noise amplification

Mnemonic: “Varactor Varies Capacitance with Voltage”

Question 4(c) [7 marks]
#

Explain construction and working of Tunnel Diode and explain tunneling phenomenon in detail. List its applications.

Answer:

Construction:

Heavily doped P-N junction: Both sides degenerately doped
Thin junction: ~10 nm width
Quantum tunneling: Electrons tunnel through barrier

Tunneling Phenomenon:

Quantum effect: Electrons pass through energy barrier
Band overlap: Conduction band overlaps valence band
Probability function: Tunneling probability depends on barrier width
No thermal activation: Occurs at room temperature

Working:

Forward bias 0-Vp: Current increases (tunneling)
Vp to Vv: Negative resistance region
Beyond Vv: Normal diode operation

Applications:

High-speed switching: Picosecond switching
Oscillators: Microwave frequency generation
Amplifiers: Low noise amplification
Memory circuits: Bistable operation

Mnemonic: “Tunnel Diode Tunnels Through barriers Terrifically”

Question 4(a) OR [3 marks]
#

Describe the operation of IMPATT diode.

Answer:

IMPATT (Impact Avalanche Transit Time) diode uses avalanche multiplication and transit time delay for oscillation.

Operation:

Avalanche zone: Impact ionization creates carriers
Drift zone: Carriers drift with constant velocity
Transit time: Provides 180° phase shift
Negative resistance: Due to phase delay

Key parameters:

Breakdown voltage: Typically 20-100V
Efficiency: 10-20%
Frequency range: 1-300 GHz

Mnemonic: “IMPATT Impacts with Avalanche Transit Time”

Question 4(b) OR [4 marks]
#

Explain the frequency up and down conversion concepts for parametric amplifier.

Answer:

Parametric Amplifier uses time-varying reactance for amplification and frequency conversion.

Up-conversion:

Signal frequency: fs (input)
Pump frequency: fp (much higher)
Output frequency: fo = fp + fs
Energy transfer: From pump to signal

Down-conversion:

Signal frequency: fs (input)
Pump frequency: fp
Output frequency: fo = fp - fs
Mixer operation: Frequency translation

Advantages:

Low noise: Quantum-limited performance
High gain: 20-30 dB typical
Wide bandwidth: Several GHz

Mnemonic: “Parametric Pump Provides frequency conversion Plus gain”

Question 4(c) OR [7 marks]
#

Describe the construction and working principle of RUBY MASER. List its applications.

Answer:

Construction:

Ruby crystal: Cr³⁺ ions in Al₂O₃ lattice
Magnetic field: Strong DC magnetic field
Microwave cavity: Resonant at signal frequency
Pump source: High frequency klystron
Cryogenic cooling: Liquid helium temperature

graph TD
    A[Ruby Crystal] --> B[Microwave Cavity]
    C[Magnetic Field] -.-> A
    D[Pump Source] --> B
    E[Liquid Helium] -.-> A
    B --> F[Amplified Output]

Working Principle:

Energy levels: Cr³⁺ ions have three energy levels
Population inversion: Pump creates more atoms in upper level
Stimulated emission: Signal photons trigger emission
Coherent amplification: Phase-coherent amplification

Three-level system:

Ground state: E₁ (most populated)
Intermediate state: E₂ (signal frequency)
Upper state: E₃ (pump frequency)

Applications:

Radio astronomy: Ultra-low noise receivers
Satellite communication: Ground station amplifiers
Deep space communication: NASA tracking stations
Research: Quantum electronics experiments

Mnemonic: “RUBY MASER Makes Amazingly Sensitive Electromagnetic Receivers”

Question 5(a) [3 marks]
#

Draw and explain the functional block diagram of MTI RADAR.

Answer:

MTI RADAR detects moving targets by comparing successive echoes and canceling fixed targets.

graph LR
    A[Transmitter] --> B[Duplexer]
    B --> C[Antenna]
    C --> B
    B --> D[Receiver]
    D --> E[Phase Detector]
    F[STALO] --> E
    F --> G[COHO]
    G --> E
    E --> H[Canceller]
    H --> I[Display]

Components:

STALO: Stable Local Oscillator
COHO: Coherent Oscillator
Phase detector: Compares echo phases
Canceller: Removes fixed target echoes

Mnemonic: “MTI Makes Targets Intelligible by Motion”

Question 5(b) [4 marks]
#

Compare RADAR with SONAR.

Answer:

Parameter	RADAR	SONAR
Wave type	Electromagnetic	Acoustic
Medium	Air/vacuum	Water
Speed	3×10⁸ m/s	1500 m/s
Frequency	GHz	kHz
Range	100+ km	10-50 km
Applications	Air/space	Underwater

Common features:

Pulse-echo principle
Range measurement
Target detection

Mnemonic: “RADAR Radiates, SONAR Sounds”

Question 5(c) [7 marks]
#

Obtain the equation of maximum RADAR range. Explain the factors affecting the maximum radar range.

Answer:

RADAR Range Equation:

R_max = ⁴√[(P_t × G² × λ² × σ) / (64π³ × P_min × L)]

Where:

P_t: Transmitter power (W)
G: Antenna gain (dimensionless)
λ: Wavelength (m)
σ: Target cross-section (m²)
P_min: Minimum detectable power (W)
L: System losses (dimensionless)

Derivation steps:

Power density at target: P_t×G/(4πR²)
Power intercepted: σ × Power density
Power at receiver: Intercepted power × G/(4πR²)
Set equal to P_min and solve for R

Factors Affecting Range:

Increase Range:

Higher transmitter power: R ∝ P_t^(1/4)
Larger antenna gain: R ∝ G^(1/2)
Larger target RCS: R ∝ σ^(1/4)
Lower system losses: R ∝ L^(-1/4)

Decrease Range:

Higher frequency: R ∝ λ^(1/2)
Atmospheric losses: Absorption and scattering
Ground clutter: Interfering reflections

Mnemonic: “RADAR Range Requires Robust Power and Proper Parameters”

Question 5(a) OR [3 marks]
#

Describe the Doppler effect in CW Doppler RADAR.

Answer:

Doppler Effect causes frequency shift when target moves relative to RADAR.

Doppler Frequency: f_d = (2 × V_r × f_0) / c

Where:

V_r: Radial velocity (m/s)
f_0: Transmitted frequency (Hz)
c: Speed of light (3×10⁸ m/s)

Characteristics:

Approaching target: f_d positive
Receding target: f_d negative
Factor of 2: Due to two-way propagation

Mnemonic: “Doppler Detects Direction with Doubled frequency shift”

Question 5(b) OR [4 marks]
#

Explain PPI Display method for RADAR

Answer:

PPI (Plan Position Indicator) shows top view of RADAR coverage area with range and bearing information.

Display Features:

Circular screen: Center represents RADAR location
Rotating trace: Synchronized with antenna rotation
Range rings: Concentric circles for distance
Bearing scale: 0-360° around circumference

Operation:

Sweep rotation: Matches antenna rotation
Echo intensity: Controls brightness
Persistence: Afterglow maintains target visibility
Range scale: Selectable range settings

Applications:

Air traffic control: Aircraft positioning
Marine navigation: Ship and obstacle detection
Weather monitoring: Storm tracking

Mnemonic: “PPI Provides Position Information Perfectly”

Question 5(c) OR [7 marks]
#

Draw the block diagram of Pulse radar and explain the working principle.

Answer:

graph TD
    A[Master Oscillator] --> B[Modulator]
    B --> C[Power Amplifier]
    C --> D[Duplexer]
    D --> E[Antenna]
    E --> D
    D --> F[RF Amplifier]
    F --> G[Mixer]
    H[Local Oscillator] --> G
    G --> I[IF Amplifier]
    I --> J[Detector]
    J --> K[Video Amplifier]
    K --> L[Display]
    A --> M[Timer]
    M --> B
    M --> L

Working Principle:

Transmission: