Microwave and Radar Communication (4351103) - Summer 2025 Solution

Question 1(a) [3 marks]

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

Answer:

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

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

goat

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.

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

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:

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.

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

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:

goat

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

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.

goat

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

goat

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

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.

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:

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:

1. Power density at target: P_t×G/(4πR²)
2. Power intercepted: σ × Power density
3. Power at receiver: Intercepted power × G/(4πR²)
4. 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:

Working Principle:

Transmission:

• Master oscillator: Generates RF carrier
• Modulator: Creates short pulses
• Power amplifier: Amplifies pulse power
• Duplexer: Routes pulse to antenna

Reception:

• Echo reception: Antenna receives reflected signals
• RF amplification: Low noise amplification
• Mixing: Converts to intermediate frequency
• IF amplification: Further amplification
• Detection: Extracts video signal
• Display: Shows range vs amplitude

Key Parameters:

• Pulse width: Determines range resolution
• PRF: Pulse repetition frequency
• Peak power: Maximum range capability
• Duty cycle: Average power consideration

Advantages:

• High peak power: Long range capability
• Good range resolution: Narrow pulses
• Simple processing: Direct detection

Mnemonic: "Pulse RADAR Pulses Powerfully for Precise Position"