Microwave and Radar Communication (4351103) - Summer 2024 Solution

Question 1(a) [3 marks]

List different microwave bands with their frequency range.

Answer:

Table: Microwave Frequency Bands

Mnemonic: "Large Ships Can eXamine Kindly Using Knowledge Always"

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Question 1(b) [4 marks]

Draw the general equivalent circuit of the transmission line. Write the equation for characteristic impedance for a lossless line.

Answer:

Transmission Line Equivalent Circuit:

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Circuit Elements:

• R: Series resistance per unit length
• L: Series inductance per unit length
• C: Shunt capacitance per unit length
• G: Shunt conductance per unit length

For Lossless Line (R = 0, G = 0):

Characteristic Impedance: Z₀ = √(L/C)

Key Points:

• Lossless condition: No power loss during transmission
• Impedance matching: Z₀ determines reflection behavior

Mnemonic: "Lossless Lines Love Constant Impedance"

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Question 1(c) [7 marks]

Explain the impedance matching process using a single stub.

Answer:

Single Stub Matching Process:

Matching Steps:

Design Equations:

• Distance to stub: d = (λ/2π) × tan⁻¹(√(R_L/R₀))
• Stub length: l = (λ/2π) × tan⁻¹(B_stub/Y₀)

Applications:

• Antenna matching
• Amplifier input/output
• Filter design

Mnemonic: "Single Stubs Stop Standing Waves Successfully"

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Question 1(c) OR [7 marks]

Compare rectangular and circular waveguides.

Answer:

Comparison Table:

Key Advantages:

• Rectangular: Better mode control, easier fabrication
• Circular: Higher power capacity, rotating polarization

Mnemonic: "Rectangular is Regular, Circular Carries Current"

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Question 2(a) [3 marks]

Define group velocity and phase velocity in relation to them.

Answer:

Velocity Definitions:

Relationship: vₚ × vₘ = c²

Key Points:

• Phase velocity: Always > c (speed of light)
• Group velocity: Always < c
• Signal travels: At group velocity

Mnemonic: "Phase is Fast, Group Carries Message"

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Question 2(b) [4 marks]

Describe the principles and workings of the Directional coupler.

Answer:

Directional Coupler Principle:

Working Principle:

• Electromagnetic coupling between two transmission lines
• Power division based on coupling factor
• Directional sensitivity to wave direction

Key Parameters:

• Coupling Factor: C = 10 log(P₁/P₃) dB
• Directivity: D = 10 log(P₃/P₄) dB
• Insertion Loss: IL = 10 log(P₁/P₂) dB

Mnemonic: "Directional Couplers Divide Power Precisely"

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Question 2(c) [7 marks]

Explain Magic TEE with construction, operation and application.

Answer:

Magic TEE Construction:

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Operating Principles:

Scattering Properties:

• Isolation: Port 3 ↔ Port 4
• Power division: Equal split when matched
• Phase relationships: 0° and 180°

Applications:

• Mixers and modulators
• Power combiners
• Impedance bridges
• Antenna feeds

Mnemonic: "Magic TEE Creates Perfect Isolation"

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Question 2(a) OR [3 marks]

Draw TE₁₀, TE₂₀ modes for rectangular waveguide.

Answer:

TE₁₀ Mode (Dominant Mode):

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TE₂₀ Mode:

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Mode Characteristics:

• TE₁₀: One half-wave variation in x-direction
• TE₂₀: Two half-wave variations in x-direction
• Field patterns: Electric field perpendicular to propagation

Mnemonic: "TE modes have Electric Transverse"

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Question 2(b) OR [4 marks]

Describe the Hybrid Ring with a necessary sketch.

Answer:

Hybrid Ring Structure:

Operating Principle:

• Ring circumference: 3λ/2
• Port spacing: λ/4 apart
• Power division: Equal split between adjacent ports

Key Features:

• Isolation: Between opposite ports
• Phase relationships: 0° and 180°
• Impedance: Matched at all ports

Mnemonic: "Hybrid Rings Handle Half-wavelengths"

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Question 2(c) OR [7 marks]

Explain the Isolator with principles, construction and operation.

Answer:

Isolator Principle:

Construction Elements:

Operating Principle:

• Faraday rotation in magnetized ferrite
• Non-reciprocal phase shift
• Forward transmission: Low loss
• Reverse transmission: High attenuation

Applications:

• Amplifier protection
• Oscillator isolation
• Antenna systems

Specifications:

• Isolation: 20-30 dB typical
• Insertion Loss: < 0.5 dB

Mnemonic: "Isolators Ignore Reverse Reflections"

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Question 3(a) [3 marks]

Draw a Traveling wave tube amplifier.

Answer:

TWT Amplifier Structure:

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Key Components:

• Electron gun: Produces electron beam
• Helix: Slow-wave structure
• Couplers: Input/output RF connections
• Collector: Collects spent electrons

Mnemonic: "TWT Transfers Wave Through Helix"

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Question 3(b) [4 marks]

Describes various types of hazards due to microwave radiation.

Answer:

Microwave Radiation Hazards:

Biological Effects:

• Thermal effects: Tissue heating above 41°C
• Non-thermal effects: Cellular damage
• Sensitive organs: Eyes, reproductive organs

Protection Measures:

• Shielding: Conductive enclosures
• Distance: Power density ∝ 1/r²
• Time limits: Exposure duration control
• Warning systems: Radiation detectors

Mnemonic: "Heat Energy Requires Proper Protection"

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Question 3(c) [7 marks]

Explain two cavity klystrons construction and operation with an Applegate diagram.

Answer:

Two-Cavity Klystron Structure:

Applegate Diagram:

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Operation Principle:

Key Parameters:

• Transit time: Critical for bunching
• Drift space length: Optimized for maximum bunching
• Cavity tuning: Resonant frequency matching

Applications:

• Radar transmitters
• Satellite communications
• Linear accelerators

Mnemonic: "Klystrons Create Bunches Through Velocity Variation"

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Question 3(a) OR [3 marks]

Draw the block diagram of the attenuation measurement method for microwave frequency.

Answer:

Attenuation Measurement Setup:

Measurement Process:

• Reference measurement: Without DUT
• Insertion measurement: With DUT
• Attenuation calculation: A = P₁ - P₂ (dB)

Mnemonic: "Attenuation Appears After Accurate Assessment"

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Question 3(b) OR [4 marks]

Describe the limitation of vacuum tubes at microwave range.

Answer:

Vacuum Tube Limitations:

Frequency-Related Problems:

• Input impedance: Becomes reactive
• Gain-bandwidth: Product limitation
• Noise figure: Increases with frequency
• Power handling: Decreases

Solutions:

• Special tube designs: Lighthouse tubes
• Cavity resonators: Replace tuned circuits
• Short leads: Minimize inductance

Mnemonic: "Vacuum Tubes Fail Fast at High Frequencies"

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Question 3(c) OR [7 marks]

Explain the Principle, construction, effect of the electric and magnetic field and operation of the magnetron in detail.

Answer:

Magnetron Construction:

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Operating Principle:

Operation Stages:

1. Electron Emission: Heated cathode emits electrons
2. Cycloid Motion: E×B fields create spiral paths
3. Synchronization: Electrons synchronize with RF field
4. Energy Transfer: Kinetic energy → RF energy
5. Output Coupling: RF extracted through waveguide

Key Parameters:

• Magnetic flux density: B = 2πmf/e
• Hull cutoff voltage: VH = (eB²R²)/(8m)
• Frequency: f = eB/(2πm) × (anode modes)

Applications:

• Microwave ovens (2.45 GHz)
• Radar transmitters
• Industrial heating

Mnemonic: "Magnetrons Make Microwaves Through Magnetic Motion"

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Question 4(a) [3 marks]

Explain the working principle of a varactor diode using a graph.

Answer:

Varactor Diode Characteristics:

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Working Principle:

• Reverse bias operation: Diode operated in reverse
• Depletion layer: Acts as dielectric
• Variable capacitance: C ∝ 1/√VR
• Voltage tuning: Capacitance controlled by voltage

Applications:

• Voltage-controlled oscillators
• Frequency multipliers
• Parametric amplifiers

Mnemonic: "Varactors Vary Capacitance Via Voltage"

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Question 4(b) [4 marks]

Explain the Gunn Effect and negative resistance for Gunn diode.

Answer:

Gunn Effect Mechanism:

Transfer Characteristic:

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Negative Resistance:

• Threshold voltage: Electrons transfer to upper valley
• Current decrease: Due to reduced mobility
• Oscillation: Negative resistance enables
• Domain formation: High-field domains propagate

Key Points:

• Materials: GaAs, InP
• Frequency range: 1-100 GHz
• Efficiency: 5-20%

Mnemonic: "Gunn diodes Generate oscillations through Negative resistance"

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Question 4(c) [7 marks]

Explain frequency measurement method for microwave frequency.

Answer:

Direct Frequency Measurement:

Indirect Methods:

Cavity Wavemeter Setup:

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Measurement Procedure:

1. Coupling: Weakly couple to signal line
2. Tuning: Adjust cavity for resonance
3. Indication: Monitor output for minimum/maximum
4. Calibration: Read frequency from calibrated scale

Beat Frequency Method:

• Local oscillator: Known reference frequency
• Mixer: Generates beat frequency
• Measurement: fbeat = |fsignal - fLO|

Mnemonic: "Frequency Found through Careful Cavity Calibration"

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Question 4(a) OR [3 marks]

Explain the working of a PIN diode as a switch.

Answer:

PIN Diode Structure:

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Switching Operation:

Key Advantages:

• Fast switching: Nanosecond response
• Low insertion loss: When ON
• High isolation: When OFF
• Wide frequency range: DC to microwave

Applications:

• RF switches
• Modulators
• Attenuators
• Phase shifters

Mnemonic: "PIN diodes Perform Perfect switching"

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Question 4(b) OR [4 marks]

Explain stripeline and Microstrip circuits.

Answer:

Stripline Configuration:

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Microstrip Configuration:

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Comparison Table:

Applications:

• Stripline: High-performance systems
• Microstrip: PCB circuits, antennas

Design Equations:

• Characteristic impedance: Function of w/h ratio
• Effective permittivity: εeff = (εr + 1)/2

Mnemonic: "Striplines are Sandwiched, Microstrips are Mounted"

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Question 4(c) OR [7 marks]

Explain the principles and process of amplification for a Parametric amplifier.

Answer:

Parametric Amplifier Principle:

Frequency Relationships:

Amplification Process:

1. Nonlinear Element: Varactor diode provides time-varying capacitance
2. Pump Power: High-frequency pump supplies energy
3. Frequency Mixing: Three-frequency interaction
4. Energy Transfer: Pump energy → Signal energy
5. Impedance Matching: Optimize power transfer

Circuit Configuration:

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Key Advantages:

• Low noise figure: Near quantum limit
• High gain: 10-20 dB typical
• Wide bandwidth: Limited by pump circuit

Applications:

• Satellite receivers
• Radio astronomy
• Low-noise amplifiers

Design Considerations:

• Pump power: Sufficient for nonlinear operation
• Impedance matching: All three frequencies
• Stability: Prevent oscillation

Mnemonic: "Parametric amplifiers Pump Power into signal Perfectly"

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Question 5(a) [3 marks]

Compare RADAR and SONAR.

Answer:

RADAR vs SONAR Comparison:

Common Principles:

• Echo ranging: Measure time-of-flight
• Doppler effect: Detect moving targets
• Beam forming: Directional transmission

Key Differences:

• Propagation: EM waves vs sound waves
• Attenuation: Different loss mechanisms
• Resolution: Frequency dependent

Mnemonic: "RADAR sees Radio waves, SONAR hears Sound waves"

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Question 5(b) [4 marks]

Write the name of RADAR display method and explain anyone.

Answer:

RADAR Display Methods:

PPI Display Explanation:

PPI Features:

• Polar coordinates: Range and bearing
• Rotating sweep: Follows antenna rotation
• Persistence: Targets remain visible
• Scale selection: Adjustable range

Display Process:

1. Sweep generation: Synchronized with antenna
2. Target plotting: Distance and direction
3. Intensity modulation: Target strength
4. Map overlay: Geographic reference

Mnemonic: "PPI Provides Perfect Position Information"

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Question 5(c) [7 marks]

Explain the basic pulse radar system with a block diagram.

Answer:

Pulse Radar Block Diagram:

System Components:

Operating Sequence:

1. Transmission Phase:

   • Master oscillator generates RF
   • Modulator creates pulses
   • Power amplifier boosts signal
   • Duplexer routes to antenna

2. Reception Phase:

   • Antenna receives echoes
   • Duplexer routes to receiver
   • Signal processing extracts information
   • Display shows target data

Key Equations:

• Range: R = ct/2 (where t = round-trip time)
• Maximum range: Rmax = cPRT/2
• Range resolution: ΔR = cτ/2

Performance Parameters:

• PRF: Pulse Repetition Frequency
• Duty cycle: τ × PRF
• Average power: Peak power × duty cycle

Mnemonic: "Pulse Radar Properly Processes Reflected signals"

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Question 5(a) OR [3 marks]

List the application of microwave frequency.

Answer:

Microwave Applications:

Key Points:

• ISM bands (Industrial, Scientific, Medical): License-free
• Penetration ability: Depends on frequency and material
• Atmospheric absorption: Increases with frequency

Mnemonic: "Microwaves Serve Many Applications Perfectly"

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Question 5(b) OR [4 marks]

Compare PULSED RADAR and CW RADAR.

Answer:

PULSED vs CW RADAR Comparison:

Operational Differences:

Pulsed RADAR:

• Range equation: R = ct/2
• Maximum range: Limited by PRF
• Blind ranges: Multiple of cPRT/2
• Applications: Long-range detection

CW RADAR:

• Doppler equation: fd = 2vr/λ
• Range measurement: Requires FM modulation
• No blind ranges: Continuous operation
• Applications: Speed measurement, proximity

Key Advantages:

• Pulsed: Better range capability, target separation
• CW: Better velocity accuracy, simpler design

Mnemonic: "Pulsed measures Range, CW measures Velocity"

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Question 5(c) OR [7 marks]

Explain MTI Radar with the block diagram.

Answer:

MTI RADAR Block Diagram:

MTI System Components:

MTI Operating Principle:

Pulse-to-Pulse Comparison:

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MTI Process:

1. Coherent transmission: Maintain phase relationships
2. Echo reception: Preserve phase information
3. Phase comparison: Compare successive pulses
4. Clutter cancellation: Subtract stationary returns
5. Moving target detection: Enhance moving targets

Key Equations:

• Doppler frequency: fd = 2vr cos(θ)/λ
• Phase change: Δφ = 4πvr/λ × PRT
• Blind speeds: vb = nλ/(2PRT)

MTI Improvement Factor:

• Definition: Ratio of clutter power before/after MTI
• Typical values: 20-40 dB
• Factors affecting: System stability, clutter characteristics

Limitations:

• Blind speeds: Targets invisible at certain velocities
• Tangential targets: Radial velocity component needed
• Weather effects: Atmospheric fluctuations

Applications:

• Air traffic control: Separate aircraft from ground clutter
• Weather radar: Distinguish precipitation from terrain
• Military radar: Detect moving vehicles/aircraft

Mnemonic: "MTI Makes Targets Identifiable by Movement"