Microwave and Radar Communication (4351103) - Winter 2024 Solution

Question 1(a) [3 marks]

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Give comparison between transmission line and waveguide.

Answer:

Mnemonic: “WAVES Travel Better” (Waveguides - Advanced Versions Enabling Superior Transmission)

Question 1(b) [4 marks]

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Define the following terms: (1) Lossless Line (2) VSWR (3) STUB (4) Reflection coefficient

Answer:

• Lossless Line: A transmission line with zero resistance and conductance, having no power loss during signal transmission.

• VSWR (Voltage Standing Wave Ratio): Ratio of maximum voltage to minimum voltage on a transmission line, indicating impedance mismatch.

• STUB: Short length of transmission line connected to main line for impedance matching purposes.

• Reflection Coefficient: Ratio of reflected wave amplitude to incident wave amplitude at any point on transmission line.

Mnemonic: “Light Volumes Stay Reflected” (Lossless-VSWR-Stub-Reflection)

Question 1(c) [7 marks]

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Explain isolator and circulator with the help of sketch.

Answer:

graph LR
    A[Port 1] --> B[Isolator] --> C[Port 2]
    B -.X.-> A
    
    D[Port 1] --> E((Circulator)) --> F[Port 2]
    F --> G[Port 3] --> D

Isolator:

• Function: Allows signal flow in one direction only
• Construction: Uses ferrite material with magnetic bias
• Applications: Protects sources from reflections

Circulator:

• Function: Routes signals in circular pattern between three or four ports
• Construction: Y-junction with ferrite material
• Applications: Duplexers in radar systems

Mnemonic: “Isolated Circuits Flow Forward” (Isolator-Circulator-Forward-Flow)

Question 1(c OR) [7 marks]

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What is dominant mode in a waveguide? What will be the cutoff wavelength for dominant mode, in a rectangular waveguide whose breadth is 10 cm? For a 2.5 GHz signal propagated through it calculate guide wavelength, group velocity and phase velocity and Z₀.

Answer:

Dominant Mode: Lowest order mode that can propagate in a waveguide. For rectangular waveguide, it’s TE₁₀ mode.

Given Data:

• Breadth (a) = 10 cm = 0.1 m
• Frequency (f) = 2.5 GHz = 2.5 × 10⁹ Hz
• c = 3 × 10⁸ m/s

Calculations:

Mnemonic: “Dominant Modes Calculate Guide Parameters”

Question 2(a) [3 marks]

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What is single stub impedance matching, and how does it work?

Answer:

Single Stub Matching: Technique using one short-circuited or open-circuited stub connected in parallel to transmission line for impedance matching.

Working Principle:

• Stub acts as reactive element (inductive or capacitive)
• Cancels reactive component of load impedance
• Transforms impedance to characteristic impedance

Mnemonic: “Single Stubs Transform Reactance” (Single-Stub-Transform-Reactive)

Question 2(b) [4 marks]

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Differentiate between rectangular and circular waveguide any three points.

Answer:

Mnemonic: “Rectangles Dominate Ten” vs “Circles Dominate Eleven”

Question 2(c) [7 marks]

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Explain the construction and working of Hybrid Ring with diagram.

Answer:

graph TD
    A[Port 1] --- B[Hybrid Ring]
    C[Port 2] --- B
    D[Port 3] --- B
    E[Port 4] --- B
    B -.-> F[λ/4 sections]

Construction:

• Ring structure with four ports
• Circumference = 1.5λ (one and half wavelengths)
• Adjacent ports separated by λ/4
• Opposite ports separated by 3λ/4

Working:

• Power division: Input at one port divides equally between two adjacent ports
• Isolation: Opposite port receives no power
• Phase relationship: 180° phase difference between output ports

Applications:

• Balanced mixers
• Power combiners/dividers
• Antenna feeds

Mnemonic: “Hybrid Rings Divide Power Equally”

Question 2(a OR) [3 marks]

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What is Microwave? List out any four applications of microwave.

Answer:

Microwave: Electromagnetic waves with frequency range from 1 GHz to 300 GHz.

Applications:

• Radar systems for detection and ranging
• Satellite communication for long-distance transmission
• Microwave ovens for heating food
• Mobile communication (cellular networks)

Mnemonic: “Microwaves Reach Space Mobile” (Microwave-Radar-Satellite-Mobile)

Question 2(b OR) [4 marks]

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Write short note on cavity resonator.

Answer:

Cavity Resonator: Closed metallic structure that confines electromagnetic energy at specific resonant frequencies.

Construction:

• Metallic enclosure of specific dimensions
• High Q factor (low losses)
• Resonant frequency depends on cavity dimensions

Types:

• Rectangular cavity
• Cylindrical cavity
• Spherical cavity

Applications:

• Frequency meters
• Oscillator circuits
• Filter circuits

Mnemonic: “Cavities Resonate High Quality” (Cavity-Resonant-High-Q)

Question 2(c OR) [7 marks]

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Explain MAGIC TEE with the help of sketch and how it works as an isolator?

Answer:

graph TD
    A[E-arm] --- B[Magic Tee Junction]
    C[H-arm] --- B
    D[Arm 1] --- B
    E[Arm 2] --- B

Magic Tee Construction:

• E-plane Tee and H-plane Tee combined
• Four ports: E-arm, H-arm, and two side arms
• E-arm perpendicular to H-arm

Working as Isolator:

• Signal at E-arm divides equally between side arms (in-phase)
• Signal at H-arm divides equally between side arms (out-of-phase)
• Isolation between E-arm and H-arm
• No coupling between perpendicular arms

Properties:

• Matched at all ports
• Reciprocal device
• Power division and isolation

Mnemonic: “Magic Isolates Perpendicular Arms”

Question 3(a) [3 marks]

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Describe the working principle of MASER.

Answer:

MASER (Microwave Amplification by Stimulated Emission of Radiation):

• Population inversion created in active medium
• Stimulated emission produces coherent microwaves
• Amplification occurs through energy level transitions

Working Principle:

• Atoms excited to higher energy levels
• Stimulated photons trigger emission
• Coherent amplification of microwave signals

Mnemonic: “Microwaves Amplify Stimulated Emission Radiation”

Question 3(b) [4 marks]

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List four microwave diodes and explain any one.

Answer:

Four Microwave Diodes:

1. GUNN Diode
2. IMPATT Diode
3. TRAPATT Diode
4. PIN Diode

GUNN Diode Explanation:

• Principle: Transferred electron effect in GaAs
• Construction: N-type GaAs with ohmic contacts
• Operation: Negative resistance at microwave frequencies
• Applications: Oscillators, amplifiers

VI Characteristic:

Mnemonic: “GUNN Generates Negative Resistance”

Question 3(c) [7 marks]

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Write a detailed explanation of the Magnetron Oscillator, covering its construction, working principle, and applications?

Answer:

graph LR
    A[Cathode] --> B[Interaction Space]
    B --> C[Anode with Cavities]
    C --> D[Output Coupling]
    E[Magnetic Field] -.-> B

Construction:

• Cylindrical cathode at center
• Anode with resonant cavities surrounding cathode
• Strong magnetic field perpendicular to electric field
• Output coupling through waveguide

Working Principle:

• Electrons emitted from heated cathode
• Cycloid motion due to crossed E and B fields
• Bunching effect creates electron clouds
• Energy transfer from electrons to RF field
• Oscillation at cavity resonant frequency

Applications:

• Radar transmitters
• Microwave ovens
• Industrial heating
• Medical diathermy

Mnemonic: “Magnetrons Make Microwave Oscillations”

Question 3(a OR) [3 marks]

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Describe the working of RUBY MASER.

Answer:

Ruby MASER Working:

• Ruby crystal (Al₂O₃ with Cr³⁺ ions) as active medium
• Three energy levels in chromium ions
• Pump frequency creates population inversion
• Signal amplification at 2.9 GHz

Process:

• Optical pumping excites electrons to higher level
• Stimulated emission produces coherent microwaves
• Low noise amplification achieved

Mnemonic: “Ruby Radiates Amplified Microwaves”

Question 3(b OR) [4 marks]

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Draw and explain the VI characteristic of Gun diode

Answer:

VI Characteristic Explanation:

• Region OA: Ohmic region (positive resistance)
• Region AB: Negative resistance region
• Region BC: Valley current region
• Region CD: Saturation region

Key Points:

• Peak voltage: Maximum voltage before negative resistance
• Valley current: Minimum current in negative resistance region
• Negative resistance: Current decreases with increasing voltage

Mnemonic: “Valley Peak Negative Resistance”

Question 3(c OR) [7 marks]

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Explain “frequency measurement method” as well as “attenuation measurement method” at microwave frequency.

Answer:

Frequency Measurement Methods:

Attenuation Measurement Methods:

Setup for Measurement:

• Signal generator provides test signal
• Calibrated attenuator for reference
• Power meter measures signal levels
• VSWR meter monitors impedance matching

Mnemonic: “Frequency Attenuation Measured Precisely”

Question 4(a) [3 marks]

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Explain working of P-i-N diode.

Answer:

P-i-N Diode Structure:

• P-type region (heavily doped)
• Intrinsic region (undoped, high resistance)
• N-type region (heavily doped)

Working:

• Forward bias: Low resistance, acts as conductor
• Reverse bias: High resistance, acts as insulator
• RF switching: Fast switching due to charge storage

Applications:

• RF switches
• Attenuators
• Phase shifters

Mnemonic: “PIN Provides Instant Switching”

Question 4(b) [4 marks]

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Explain π mode oscillations for magnetron.

Answer:

π Mode Oscillation:

• Adjacent cavities oscillate 180° out of phase
• Electron bunching synchronized with RF field
• Maximum power transfer from electrons to RF
• Stable oscillation at designed frequency

Characteristics:

• Phase difference: π radians between adjacent cavities
• Frequency: Determined by cavity dimensions
• Efficiency: Highest among all modes
• Stability: Most stable oscillation mode

Mode Chart:

Mnemonic: “Pi Mode Produces Maximum Power”

Question 4(c) [7 marks]

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Explain the construction and working of two cavity klystron amplifiers with necessary diagram.

Answer:

graph LR
    A[Electron Gun] --> B[Input Cavity]
    B --> C[Drift Space]
    C --> D[Output Cavity]
    D --> E[Collector]
    F[Input Signal] --> B
    D --> G[Output Signal]

Construction:

• Electron gun produces electron beam
• Input cavity (buncher) modulates electron beam
• Drift space allows velocity modulation
• Output cavity (catcher) extracts RF energy
• Collector collects spent electrons

Working Principle:

• Velocity modulation in input cavity
• Electron bunching in drift space
• Density modulation creates current variation
• Energy extraction in output cavity
• Amplification achieved through beam-field interaction

Key Parameters:

• Beam voltage: Determines electron velocity
• Cavity tuning: Sets operating frequency
• Drift space length: Controls bunching effectiveness

Applications:

• Radar transmitters
• Satellite communication
• Linear accelerators

Mnemonic: “Klystrons Amplify Through Bunching”

Question 4(a OR) [3 marks]

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Explain parametric amplifier.

Answer:

Parametric Amplifier:

• Variable reactance device using varactor diode
• Pump frequency modulates diode capacitance
• Energy transfer from pump to signal
• Low noise amplification achieved

Working:

• Pump power varies diode reactance
• Signal mixing produces sum and difference frequencies
• Idler frequency fp = fs + fi
• Power gain through nonlinear mixing

Advantages:

• Very low noise figure
• High gain possible
• Wide bandwidth

Mnemonic: “Parametric Amplifiers Pump Low Noise”

Question 4(b OR) [4 marks]

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Draw and explain schematic diagram of travelling wave tube with necessary notation

Answer:

graph LR
    A[Electron Gun] --> B[Input]
    B --> C[Helix]
    C --> D[Output]
    D --> E[Collector]
    F[Attenuator] -.-> C
    G[Focusing System] -.-> C

Components:

• Electron gun: Produces electron beam
• Helix: Slow-wave structure
• Attenuator: Prevents oscillation
• Collector: Collects electrons
• Focusing system: Maintains beam alignment

Working:

• Electron beam travels through helix center
• RF signal propagates along helix
• Synchronism between beam and RF wave
• Energy transfer from beam to RF
• Continuous amplification along helix length

Mnemonic: “TWT Travels With Waves”

Question 4(c OR) [7 marks]

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Explain the working principle of a reflex klystron in detail with suitable diagram.

Answer:

graph LR
    A[Cathode] --> B[Resonant Cavity]
    B --> C[Drift Space]
    C --> D[Repeller]
    D -.-> C
    C -.-> B
    B --> E[Output]

Construction:

• Single resonant cavity acts as buncher and catcher
• Repeller electrode reflects electron beam
• Drift space allows velocity modulation
• Output coupling extracts RF power

Working Principle:

Applegate Diagram:

Process:

1. Electrons enter cavity and get velocity modulated
2. Electrons drift toward repeller
3. Repeller reflects electrons back to cavity
4. Transit time determines bunching phase
5. Bunched electrons deliver energy to cavity
6. Oscillation maintained through feedback

Frequency Tuning:

• Repeller voltage controls transit time
• Cavity tuning sets center frequency
• Electronic tuning possible

Applications:

• Local oscillators
• Frequency meters
• Microwave sources

Mnemonic: “Reflex Returns Electron Bunches”

Question 5(a) [3 marks]

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“PIN diode acts as a switch and VARACTOR diode acts as a variable capacitor” explain.

Answer:

PIN Diode as Switch:

• Forward bias: Low resistance (~1Ω), switch ON
• Reverse bias: High resistance (~10kΩ), switch OFF
• Fast switching due to charge storage in I-region
• RF isolation in OFF state

VARACTOR Diode as Variable Capacitor:

• Reverse bias voltage controls junction capacitance
• Capacitance decreases with increasing reverse voltage
• Voltage-controlled reactance for tuning circuits
• Electronic tuning without mechanical adjustment

Mnemonic: “PIN Switches, VARACTOR Varies”

Question 5(b) [4 marks]

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List the display methods used in RADAR and explain any one.

Answer:

RADAR Display Methods:

1. A-Scope Display
2. PPI (Plan Position Indicator)
3. B-Scope Display
4. RHI (Range Height Indicator)

PPI Display Explanation:

• Circular display showing target positions
• Center represents radar location
• Radial distance indicates target range
• Angular position shows target bearing
• Rotating sweep synchronized with antenna rotation

Features:

• Real-time display of target positions
• Range and bearing information
• Multiple target tracking
• Clutter suppression

Mnemonic: “PPI Pictures Position Indicators”

Question 5(c) [7 marks]

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What is radar? List out the different types of radar systems? Explain any One of radar in detail?

Answer:

RADAR (Radio Detection And Ranging): System using radio waves to detect objects and determine their range, velocity, and characteristics.

Types of RADAR Systems:

Pulse Radar Detailed Explanation:

graph LR
    A[Transmitter] --> B[Duplexer]
    B --> C[Antenna]
    C --> D[Target]
    D --> C
    C --> B
    B --> E[Receiver]
    E --> F[Display]
    G[Timer] --> A
    G --> F

Working:

• Transmits short pulses of RF energy
• Receives echoes from targets
• Measures time delay for range calculation
• Processes signals for display

Range Equation: R = (c × t)/2

Where:

• R = Range to target
• c = Speed of light
• t = Time delay

Applications:

• Air traffic control
• Weather monitoring
• Military surveillance
• Navigation aids

Mnemonic: “Radar Ranges Radio Waves”

Question 5(a OR) [3 marks]

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Describe the operation of TRAPATT diode with diagram.

Answer:

TRAPATT Operation:

• TRApped Plasma Avalanche Triggered Transit diode
• High field region creates avalanche breakdown
• Plasma formation traps charge carriers
• Transit time effects create negative resistance
• Oscillation frequency determined by transit time

Applications:

• High power oscillators
• Radar transmitters
• Communication systems

Mnemonic: “TRAPATT Traps Plasma Avalanche”

Question 5(b OR) [4 marks]

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Compare RADAR with SONAR.

Answer:

Similarities:

• Echo principle for detection
• Range measurement using time delay
• Doppler effect for velocity measurement

Mnemonic: “RADAR Radiates, SONAR Sounds”

Question 5(c OR) [7 marks]

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Obtain the equation for maximum radar range.

Answer:

RADAR Range Equation Derivation:

Power Transmitted: Pt

Power Density at Target: Pd = Pt/(4πR²)

Power Intercepted by Target: Pi = Pd × σ = (Pt × σ)/(4πR²)

Power Returned to Radar: Pr = Pi/(4πR²) = (Pt × σ)/(4πR²)²

Power Received: Pr = (Pt × G² × λ² × σ)/((4π)³ × R⁴)

Maximum Range Equation:

Rmax = ⁴√[(Pt × G² × λ² × σ)/((4π)³ × Prmin)]

Where:

• Pt = Transmitted power
• G = Antenna gain
• λ = Wavelength
• σ = Radar cross section
• Prmin = Minimum detectable signal
• R = Range

Factors Affecting Range:

• Transmitted power (increases range)
• Antenna gain (increases range)
• Target cross-section (increases range)
• Frequency (affects propagation)
• Receiver sensitivity (affects minimum signal)

Practical Considerations:

• Atmospheric losses
• Ground reflections
• Noise limitations
• Clutter effects

Mnemonic: “Power Gain Lambda Sigma Range”