Question 1(a) [3 marks]

Sketch the standing wave pattern for voltage and current along the transmission line when it is terminated with (i) Short Circuit, (ii) Open circuit, and (iii) Matched Load.

Answer:

Diagram:

Short Circuit (Z_L = 0):
     V      
     |      
     |      
     |______|______|______|______ 0V
       λ/4   λ/2   3λ/4    λ
     
     I      
   I_max ---|---|---|---
     |                  
     |      
     0 __________________ 0A

Open Circuit (Z_L = ∞):
   V_max ---|---|---|---
     |                  
     V      
     0 __________________ 0V
       λ/4   λ/2   3λ/4    λ
     
     I      
     |      
     |      
     |______|______|______|______ 0A
       λ/4   λ/2   3λ/4    λ

Matched Load (Z_L = Z_0):
     V      
   ---|---|---|---|--- Constant
     
     I      
   ---|---|---|---|--- Constant

Short Circuit: Voltage minimum at load, current maximum at load
Open Circuit: Voltage maximum at load, current minimum at load
Matched Load: Constant voltage and current, no reflections

Mnemonic: "SOC - Short Opens Current, Open Shorts Current"

Question 1(b) [4 marks]

Draw and Explain equivalent circuit of two parallel wire transmission line at microwave frequency.

Answer:

Diagram:

       R      L      R      L
    ---|^^^|--|||||---|^^^|--|||||---
    |      |       |      |       |
    |      G       C      G       C
    |     |||     ---    |||     ---
    |     |||     ---    |||     ---
    ---|^^^|--|||||---|^^^|--|||||---
       R      L      R      L
       
       ←-- Δz --→

R: Series resistance per unit length (conductor losses)
L: Series inductance per unit length (magnetic field storage)
G: Shunt conductance per unit length (dielectric losses)
C: Shunt capacitance per unit length (electric field storage)

Primary Constants Table:

Parameter	Symbol	Unit	Effect
Resistance	R	Ω/m	Power loss
Inductance	L	H/m	Magnetic energy
Conductance	G	S/m	Leakage current
Capacitance	C	F/m	Electric energy

Mnemonic: "RLGC - Really Largeガイド Cables"

Question 1(c) [7 marks]

Explain Principle, construction and working of Isolator with necessary sketch.

Answer:

Principle: Isolator allows microwave signal to pass in forward direction only using ferrite material and Faraday rotation effect.

Construction Diagram:

graph LR
    A[Input Port] --> B[Ferrite Rod]
    B --> C[Permanent Magnet]
    C --> D[Output Port]
    E[Resistive Load] --> B
    F[Waveguide] --> B

Working:

Forward direction: Signal passes through ferrite with minimal loss
Reverse direction: Signal is rotated 45° and absorbed by resistive load
Magnetic field biases ferrite material
Isolation: 20-30 dB typical

Applications:

Protects transmitter from reflected power
Prevents oscillations in amplifier circuits
Maintains source impedance matching

Specifications Table:

Parameter	Value	Unit
Isolation	20-30	dB
Insertion Loss	0.5-1	dB
VSWR	<1.5	-

Mnemonic: "Isolate Forward, Absorb Reverse"

Question 1(c OR) [7 marks]

Compare Transmission Line and Waveguide.

Answer:

Comparison Table:

Parameter	Transmission Line	Waveguide
Frequency Range	DC to microwave	Above cutoff frequency
Power Handling	Limited	High power capability
Losses	Higher (I²R losses)	Lower (no center conductor)
Size	Compact	Bulky at low frequencies
Modes	TEM mode	TE and TM modes
Installation	Easy	Complex mounting
Cost	Lower	Higher
Bandwidth	Wide	Limited by modes

Key Differences:

Transmission line: Uses two conductors, supports TEM mode
Waveguide: Single hollow conductor, supports TE/TM modes
Cutoff frequency: Waveguide has minimum operating frequency
Field pattern: Different electromagnetic field distributions

Applications:

Transmission lines: Low power, broadband applications
Waveguides: High power radar, satellite communication

Mnemonic: "Transmission Travels Two-wire, Waveguide Walks Wide"

Question 2(a) [3 marks]

Define: (i) VSWR, (ii) Reflection Coefficient, and (iii) Skin effect

Answer:

Definitions:

VSWR (Voltage Standing Wave Ratio): Ratio of maximum to minimum voltage amplitudes on transmission line
- Formula: VSWR = V_max/V_min = (1+|Γ|)/(1-|Γ|)
Reflection Coefficient (Γ): Ratio of reflected to incident voltage amplitude
- Formula: Γ = (Z_L - Z_0)/(Z_L + Z_0)
Skin Effect: Current flows mainly on conductor surface at high frequencies
- Skin depth: δ = √(2/ωμσ)

Parameters Table:

Parameter	Range	Ideal Value
VSWR	1 to ∞	1 (matched)
	Γ
Skin Depth	μm to mm	Frequency dependent

Mnemonic: "VSWR Varies, Gamma Guides, Skin Shrinks"

Question 2(b) [4 marks]

Explain working of Two-hole Directional Coupler with Proper sketch.

Answer:

Construction Diagram:

Main Waveguide:
|===============================|
|    →  P1      →  P2           |
|===============================|
         ○    ○  ← Two holes
|===============================|
|    ←  P4      ←  P3           |
|===============================|
Auxiliary Waveguide

Working Principle:

Two holes spaced λ/4 apart couple energy between waveguides
Forward wave: Coupled signals add at P3, cancel at P4
Reverse wave: Coupled signals add at P4, cancel at P3
Directivity: Achieved by proper hole spacing and size

Coupling Mechanism:

Electric field coupling through holes
Phase difference creates directional coupling
Coupling factor: C = 10 log(P1/P3) dB

Performance Parameters:

Parameter	Typical Value
Coupling	10-30 dB
Directivity	25-40 dB
VSWR	<1.3

Mnemonic: "Two Holes, Two Directions, Total Control"

Question 2(c) [7 marks]

Describe Propagation of microwaves through waveguide and get the equation of cut off wavelength.

Answer:

Propagation Theory: Electromagnetic waves propagate through waveguide in TE and TM modes with specific field patterns.

Wave Equation: For rectangular waveguide, the wave equation is: ∇²E + γ²E = 0

Where γ² = β² - k²

Cutoff Wavelength Derivation:

For TE_mn mode in rectangular waveguide:

Cutoff frequency: f_c = (c/2)√[(m/a)² + (n/b)²]
Cutoff wavelength: λ_c = 2/√[(m/a)² + (n/b)²]

For dominant TE₁₀ mode:

λ_c = 2a (where a is broad dimension)

Propagation Conditions:

Below cutoff (f < f_c): Evanescent wave, exponential decay
Above cutoff (f > f_c): Propagating wave
Phase velocity: v_p = c/√[1 - (f_c/f)²]
Group velocity: v_g = c√[1 - (f_c/f)²]

Mode Chart:

graph LR
    A[TE₁₀] --> B[TE₂₀]
    A --> C[TE₀₁]
    B --> D[TE₁₁]
    C --> D

Key Relations:

v_p × v_g = c²
λ_g = λ₀/√[1 - (λ₀/λ_c)²]

Mnemonic: "Cut-off Comes, Propagation Proceeds"

Question 2(a OR) [3 marks]

Explain Impedance Matching using Single stub.

Answer:

Principle: Single stub matching uses a short-circuited or open-circuited stub to cancel reactive component of load impedance.

Stub Diagram:

Source ---|---●---|--- Load
  Z₀      |       |    Z_L
          |       |
         ---      |
        Stub      |
         l_s      d

Design Steps:

Step 1: Find distance 'd' where normalized conductance = 1
Step 2: Calculate required stub susceptance: B_s = -B_load
Step 3: Determine stub length: l_s from B_s

Smith Chart Method:

Plot normalized load impedance
Move toward generator to find matching point
Add stub susceptance to achieve center point

Mnemonic: "Single Stub Solves Susceptance"

Question 2(b OR) [4 marks]

Explain Hybrid ring with necessary sketch.

Answer:

Construction Diagram:

graph TB
    A[Port 1] --> B[Ring Junction]
    C[Port 2] --> B
    D[Port 3] --> B
    E[Port 4] --> B
    B --> F[3λ/2 Ring Path]

Working Principle:

Ring circumference: 3λ/2 (1.5 wavelengths)
Equal path lengths from each port to opposite port
180° phase difference between adjacent ports

S-Matrix Properties:

Isolation: Ports 1-3 and ports 2-4 are isolated
Power division: Equal split with 180° phase difference
Impedance: All ports matched to Z₀

Applications:

Balanced mixers
Push-pull amplifiers
Phase comparison circuits

Performance Table:

Parameter	Value
Isolation	>25 dB
Return Loss	>20 dB
Phase Balance	±5°

Mnemonic: "Ring Rotates, Ports Pair-up"

Question 2(c OR) [7 marks]

Explain construction, working and any one application of Magic Tee with necessary diagram.

Answer:

Construction: Magic Tee is formed by joining E-plane and H-plane tees at their junction.

Structure Diagram:

       H-arm (Sum port)
           |
           |
    -------●------- 
   |               |
E-arm     Junction  Collinear
(Diff)              arms
   |               |
    -------●-------
           |
           |
       Matched load

Working Principle:

Ports 1,2: Collinear arms (input/output ports)
Port 3: H-arm (sum/Σ port)
Port 4: E-arm (difference/Δ port)
Isolation: Between sum and difference ports

S-Matrix Properties:

graph LR
    A[Port 1] -.->|In phase| B[H-arm]
    C[Port 2] -.->|In phase| B
    A -->|Out of phase| D[E-arm]
    C -->|180° phase| D

Application - Radar Duplexer:

Transmit: Power fed to H-arm, splits equally to ports 1,2
Receive: Received signals combine at E-arm for receiver
Isolation: Protects receiver during transmission
Advantage: Single antenna for transmit/receive

Performance Specifications:

Parameter	Value
Isolation	>30 dB
VSWR	<1.3
Power Split	3 dB
Phase Balance	±5°

Key Features:

Symmetric structure ensures equal power division
Orthogonal fields provide port isolation
Broadband operation over octave bandwidth

Mnemonic: "Magic Makes Isolation, Tee Transmits Together"

Question 3(a) [3 marks]

Explain Attenuation measurement with the help of block diagram.

Answer:

Block Diagram:

graph LR
    A[Signal Generator] --> B[Attenuator Under Test]
    B --> C[Power Meter]
    D[Reference Path] --> C
    E[Switch] --> B
    E --> D

Measurement Procedure:

Step 1: Measure power without attenuator (P₁)
Step 2: Insert attenuator, measure power (P₂)
Step 3: Calculate attenuation = 10 log(P₁/P₂) dB

Methods:

Substitution method: Compare with known attenuator
Direct method: Measure input and output power
IF substitution: Use intermediate frequency

Mnemonic: "Attenuation = Power₁/Power₂"

Question 3(b) [4 marks]

Explain velocity modulation in two cavity klystron with the help of Applegate diagram.

Answer:

Two Cavity Klystron Diagram:

Electron -----> ●======● -----> ●======● -----> Collector
Gun            Input    Drift   Output
               Cavity   Space   Cavity
                 |               |
              RF Input         RF Output

Applegate Diagram:

Distance →
   |     
   |  ╱  ╲     ╱  ╲     ╱  ╲
   | ╱    ╲   ╱    ╲   ╱    ╲
   |╱      ╲ ╱      ╲ ╱      ╲
   |        X        X        X  ← Bunching
Tim|       ╱ ╲      ╱ ╲      ╱ ╲  
   ↓      ╱   ╲    ╱   ╲    ╱   ╲
         ╱     ╲  ╱     ╲  ╱     ╲
        ╱       ╲╱       ╲╱       ╲
Fast electrons ←→ Slow electrons

Velocity Modulation Process:

Input cavity: Electrons gain/lose energy from RF field
Drift space: Fast electrons catch up to slow electrons
Bunching: Electron density varies periodically
Output cavity: Bunched electrons induce RF current

Key Parameters:

Transit time: τ = L/v₀ (where L = drift space length)
Bunching parameter: X = βn/2
Optimum bunching: X = 1.84

Mnemonic: "Velocity Varies, Bunching Builds"

Question 3(c) [7 marks]

Explain the principle, construction and effect of electric and magnetic field in Magnetron.

Answer:

Principle: Magnetron uses crossed electric and magnetic fields to generate high-power microwave oscillations through cyclotron motion of electrons.

Construction Diagram:

    Permanent Magnet (N)
         ↓  ↓  ↓  ↓
    ┌─────────────────┐
    │  ○  ○  ○  ○  ○  │ ← Resonant Cavities
    │○               ○│
    │  ●─ Cathode ─●  │ ← Central cathode
    │○               ○│
    │  ○  ○  ○  ○  ○  │
    └─────────────────┘
         ↑  ↑  ↑  ↑
    Permanent Magnet (S)

Field Effects:

Electric Field (E): Radial, from cathode to anode
Magnetic Field (B): Axial, perpendicular to E-field
Crossed fields: Create cycloidal electron motion

Electron Motion Analysis:

graph LR
    A[Electron Emission] --> B[Cyclotron Motion]
    B --> C[Spiral Path]
    C --> D[Energy Transfer]
    D --> E[RF Oscillation]

Operating Conditions:

Cutoff condition: E/B = v_drift
Synchronism: Electron drift velocity matches phase velocity
Hull cutoff: Minimum magnetic field for operation

Resonant Cavities:

π-mode operation: Alternate cavities have opposite phases
Frequency: f = c/(2√LC) for cavity resonance
Mode separation: Prevents mode competition

Performance Characteristics:

Parameter	Typical Value
Efficiency	60-80%
Power Output	10 kW - 10 MW
Frequency	1-100 GHz
Pulse/CW	Both modes

Advantages:

High efficiency compared to other tubes
High power capability
Compact structure
Good frequency stability

Applications:

Radar transmitters
Microwave ovens
Industrial heating
Electronic warfare

Mnemonic: "Magnetron Makes Microwaves via Magnetic Motion"

Question 3(a OR) [3 marks]

Explain working of TWT (Travelling Wave Tube) as an Amplifier.

Answer:

TWT Structure:

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

Amplification Process:

Electron beam travels along helix axis
RF signal propagates along helix (slow wave structure)
Velocity synchronism: v_electron ≈ v_RF
Energy transfer from DC beam to RF wave

Gain Mechanism:

Bunching: RF field modulates electron velocity
Induced current: Bunched electrons induce RF current in helix
Progressive amplification along helix length

Mnemonic: "Travelling Wave Transfers Energy"

Question 3(b OR) [4 marks]

Explain Bolometer method for low power measurement at microwave frequency.

Answer:

Principle: Bolometer measures microwave power by detecting temperature rise in resistive element.

Bolometer Types:

Thermistor: Negative temperature coefficient
Barretter: Positive temperature coefficient

Circuit Diagram:

    RF Power -----> [Bolometer] -----> Temperature
         |              |               Change
         |              |                 |
    DC Bridge ←---------●---------→ DC Voltmeter

Measurement Process:

Step 1: Balance bridge with DC power only
Step 2: Apply RF power, note bridge unbalance
Step 3: Reduce DC power to rebalance bridge
Step 4: RF power = Reduction in DC power

Advantages:

High sensitivity (µW to mW range)
Square law response
Broadband operation

Mnemonic: "Bolometer Burns, Bridge Balances"

Question 3(c OR) [7 marks]

Explain frequency and wavelength measurement method with the help of block diagram.

Answer:

Frequency Measurement - Direct Method:

graph LR
    A[Microwave Source] --> B[Frequency Counter]
    B --> C[Digital Display]
    D[Reference Oscillator] --> B

Frequency Measurement - Heterodyne Method:

graph LR
    A[Unknown Frequency] --> B[Mixer]
    C[Local Oscillator] --> B
    B --> D[IF Amplifier]
    D --> E[Frequency Counter]

Wavelength Measurement - Slotted Line Method:

Setup Diagram:

Microwave --|-----|====|-----|-- Slotted Line --|-- Load
Source      |  Isolator  |                     |
            |            |                     |
         Attenuator   Detector              Movable
                                           Probe

Measurement Procedure:

Free Space Wavelength (λ₀):

Step 1: Connect matched load, measure frequency
Step 2: Calculate λ₀ = c/f

Guided Wavelength (λ_g):

Step 1: Connect short circuit, find two consecutive minima
Step 2: λ_g = 2 × distance between minima
Step 3: Verify: λ_g = λ₀/√[1-(λ₀/λ_c)²]

Cut-off Wavelength (λ_c):

Method 1: From waveguide dimensions: λ_c = 2a (for TE₁₀)
Method 2: From λ₀ and λ_g: λ_c = λ₀/√[1-(λ₀/λ_g)²]

Measurement Table:

Parameter	Method	Accuracy
Frequency	Direct counting	±0.01%
λ₀	Calculate from f	±0.01%
λ_g	Slotted line	±1%
λ_c	Calculation/measurement	±2%

Advantages of Each Method:

Direct method: High accuracy, simple
Heterodyne method: Extended frequency range
Slotted line: Measures guided parameters directly

Error Sources:

Probe coupling variations
Temperature effects on dimensions
Detector nonlinearity
Standing wave disturbances

Applications:

Waveguide characterization
Material property measurement
Antenna testing
Component verification

Mnemonic: "Frequency First, Wavelength With-measurement"

Question 4(a) [3 marks]

State Frequency limitations of vacuum tubes at microwave frequency.

Answer:

Frequency Limitations:

Transit time effects: Electron transit time becomes comparable to RF period
Inter-electrode capacitance: Reduces gain at high frequencies
Lead inductance: Parasitic inductances limit performance
Skin effect: Current concentration reduces effective conductance

Limiting Factors Table:

Factor	Effect	Frequency Impact
Transit Time	Phase delay	f < 1/(2πτ)
Capacitance	Reactance loading	Gain ∝ 1/f
Inductance	Resonance effects	Stability issues
Skin Effect	Increased resistance	Efficiency ↓

Solutions:

Reduce electrode spacing
Use special geometries
Employ microwave tubes (Klystron, Magnetron)

Mnemonic: "Transit Time Troubles Traditional Tubes"

Question 4(b) [4 marks]

Explain Negative resistance effect in IMPATT Diode.

Answer:

IMPATT Structure:

P+ |--| I |--| P |--| N+ |
   ←--|------|------|--→
   Avalanche  Drift
   Region     Region

Negative Resistance Mechanism:

Two-step Process:

Impact Ionization: High field creates electron-hole pairs
Transit Time Delay: Carriers drift across depletion region

Phase Relationships:

Current: Lags voltage by 90° (avalanche delay) + 90° (transit delay) = 180°
Result: I = -G*V (negative conductance)

Operating Cycle:

graph LR
    A[High Field] --> B[Avalanche]
    B --> C[Carrier Generation]
    C --> D[Transit Delay]
    D --> E[Current Peak]
    E --> A

Key Parameters:

Avalanche frequency: f_a = v_s/(2W_a)
Transit frequency: f_t = v_d/(2W_d)
Optimum frequency: f_0 = 1/(2π√L*|C_negative|)

Mnemonic: "Impact Ionization, Transit Time = Negative Resistance"

Question 4(c) [7 marks]

Explain Principle, tunneling phenomenon and any one application of Tunnel Diode.

Answer:

Principle: Tunnel diode operates on quantum mechanical tunneling effect through thin potential barrier in heavily doped p-n junction.

Energy Band Diagram:

Forward Bias States:

State 1 (Low bias):    State 2 (Peak):      State 3 (Valley):
   P side | N side        P side | N side        P side | N side
    ___   |  ___           ___   |  ___           ___   |  ___
   |   |  | |   |         |   |  | |   |         |   |  | |   |
   |___|  | |___|         |___| /| |___|         |___| /| |___|
          |               Tunneling              No tunnel
       Tunneling

I-V Characteristics:

Current ↑
        |    
     Ip |●     
        |  ●    
        |    ●   Forward region
        |      ●  
        |        ●
     Iv |         ●____
        |                ●
        |                  ●
        |________________________→ Voltage
        0   Vp    Vv    Vf
        
    Peak    Valley  Forward
    point   point   region

Tunneling Phenomenon:

Quantum Mechanics: Electrons can penetrate potential barrier even if their energy is less than barrier height.

Tunneling Probability: T = exp(-2√(2mφd²)/ħ) Where:

m = electron mass
φ = barrier height
d = barrier width
ħ = reduced Planck constant

Operating Regions:

Tunneling region (0 to Vp): Current increases with voltage
Negative resistance (Vp to Vv): Current decreases with increasing voltage
Forward bias (>Vv): Normal diode behavior

Key Parameters Table:

Parameter	Symbol	Typical Value
Peak Current	Ip	1-100 mA
Peak Voltage	Vp	50-100 mV
Valley Current	Iv	0.1*Ip
Valley Voltage	Vv	300-500 mV

Application - High Frequency Oscillator:

Circuit Diagram:

    +Vcc
      |
      R ← Bias resistor
      |
      ●---L---●---Output
      |       |
   Tunnel     C
   Diode      |
      |       |
    ──┴──   ──┴──
     GND     GND

Oscillator Operation:

Bias point: Set in negative resistance region
Tank circuit: LC determines oscillation frequency
Condition: |R_negative| > R_positive for oscillation
Frequency: f = 1/(2π√LC)

Advantages:

Ultra-high frequency operation (up to 100 GHz)
Low noise figure
Fast switching (picosecond range)
Low power consumption
Temperature stable

Applications:

Microwave oscillators
High-speed switches
Microwave amplifiers
Frequency converters
Computer memory circuits

Limitations:

Low power handling
Critical bias requirements
Limited temperature range
Expensive manufacturing

Design Considerations:

Doping concentration: >10¹⁹ cm⁻³ for both sides
Junction area: Small for high frequency operation
Parasitic elements: Minimize package inductance/capacitance
Bias stability: Temperature compensation required

Mnemonic: "Tunnel Through, Negative Grows, Oscillator Flows"

Question 4(a OR) [3 marks]

Explain Hazards due to microwave radiation.

Answer:

Types of Hazards:

HERP (Hazards of Electromagnetic Radiation to Personnel):

Thermal effects: Tissue heating above 41°C
Non-thermal effects: Cellular damage at low power levels
Cumulative effects: Long-term exposure risks

HERO (Hazards of Electromagnetic Radiation to Ordnance):

Premature ignition: RF energy can trigger explosive devices
Fuel ignition: Microwave heating of fuel vapors
Electronic interference: Disruption of control systems

HERF (Hazards of Electromagnetic Radiation to Fuels):

Fuel heating: Dielectric heating of hydrocarbon fuels
Static discharge: RF-induced sparking in fuel systems
Vapor ignition: Heating of fuel-air mixtures

Safety Guidelines Table:

Exposure Level	Power Density	Duration	Effect
Safe	<10 mW/cm²	8 hours	No effect
Caution	10-100 mW/cm²	Limited	Possible heating
Danger	>100 mW/cm²	Avoid	Tissue damage

Mnemonic: "HERP-HERO-HERF = Health-Explosive-Fuel Risks"

Question 4(b OR) [4 marks]

Explain Degenerate and non-degenerate mode in Parametric Amplifier.

Answer:

Parametric Amplifier Principle: Uses time-varying reactance to transfer energy from pump to signal.

Mode Classifications:

Non-degenerate Mode:

Three frequencies: f_s (signal), f_i (idler), f_p (pump)
Frequency relation: f_p = f_s + f_i
Two separate circuits for signal and idler
Higher gain but more complex

Degenerate Mode:

Two frequencies: f_s (signal), f_p (pump)
Frequency relation: f_p = 2f_s
Single resonant circuit
Simpler design but lower gain

Comparison Table:

Parameter	Non-degenerate	Degenerate
Frequencies	3 (fs, fi, fp)	2 (fs, fp)
Circuits	Separate	Combined
Gain	Higher	Lower
Complexity	More	Less
Bandwidth	Narrower	Wider

Energy Transfer:

graph LR
    A[Pump Power] --> B[Variable Reactance]
    B --> C[Signal Amplification]
    D[Idler] -.-> B

Mnemonic: "Non-degenerate = Not-single, Degenerate = Doubled-frequency"

Question 4(c OR) [7 marks]

Explain principle and Gunn effect in Gunn Diode. Also Explain Gunn Diode as an Oscillator.

Answer:

Gunn Effect Principle: Based on transferred electron effect in compound semiconductors (GaAs, InP).

Energy Band Structure:

Energy ↑
       |     Upper valley
       |    /
       |   /  ΔE = 0.36 eV
       |  /
       |_/_______ Lower valley
              |
              |→ k (momentum)
        Γ valley   L valley

Gunn Effect Mechanism:

Differential Mobility:

Low field (<3 kV/cm): Electrons in Γ valley (high mobility)
High field (>3 kV/cm): Electrons transfer to L valley (low mobility)
Result: Negative differential mobility (NDM)

Domain Formation:

graph LR
    A[Uniform Field] --> B[Instability]
    B --> C[Domain Nucleation]
    C --> D[Domain Growth]
    D --> E[Domain Transit]
    E --> F[Domain Collection]
    F --> A

Current-Voltage Characteristics:

Current ↑
        |
    I_p |●
        | ●
        |  ●
        |   ●_____ NDM region
        |        ●
        |         ●
        |__________●_________→ Voltage
        0    V_th    V_s
        
    Threshold  Sustaining
    voltage    voltage

Gunn Diode Oscillator:

Basic Configuration:

    +V_bias
      |
      R ← Bias resistor  
      |
    ──●──── RF Output
      |
   [Gunn]  ← Gunn diode
   Diode   
      |
    ──┴──── Ground
     GND

Oscillator Modes:

Transit Time Mode:

Domain formation at cathode
Domain transit across active region
Current pulse when domain reaches anode
Frequency: f = v_d/L (where v_d = drift velocity, L = length)

Quenched Domain Mode:

Resonant circuit quenches domain before transit
Higher frequency operation possible
Efficiency: 5-20%

LSA (Limited Space-charge Accumulation) Mode:

High frequency prevents domain formation
Uniform field maintained
Higher efficiency: 10-25%

Performance Parameters:

Parameter	Value	Unit
Frequency Range	1-100	GHz
Power Output	1 mW-10 W	-
Efficiency	5-25	%
Noise Figure	35-50	dB

Advantages:

Simple structure - no external resonator needed
Broadband tuning capability
Low noise at microwave frequencies
Reliable operation

Applications:

Local oscillators in receivers
CW radar transmitters
Microwave communication systems
Test equipment signal sources

Design Considerations:

Doping profile: Uniform n-type doping
Length optimization: L = v_d/f for transit time mode
Thermal management: Heat dissipation critical
Circuit design: Impedance matching important

Comparison with Other Oscillators:

Oscillator	Frequency	Power	Efficiency
Gunn Diode	1-100 GHz	mW-10W	5-25%
IMPATT	1-300 GHz	1W-100W	10-20%
Klystron	1-20 GHz	1kW-1MW	30-60%

Mnemonic: "Gunn Gets Going via Gallium-Arsenide"

Question 5(a) [3 marks]

Explain working principle of Basic RADAR system with the help of block diagram.

Answer:

RADAR Principle: Radio Detection And Ranging - transmits RF pulses and detects reflected signals from targets.

Basic RADAR Block Diagram:

graph LR
    A[Master Oscillator] --> B[Modulator]
    B --> C[Power Amplifier]
    C --> D[Duplexer]
    D --> E[Antenna]
    E --> F[Target]
    F --> E
    E --> D
    D --> G[Receiver]
    G --> H[Signal Processor]
    H --> I[Display]
    J[Timing Control] --> B

Working Principle:

Transmission: High power RF pulse transmitted toward target
Propagation: EM wave travels at speed of light (c)
Reflection: Target reflects portion of energy back to radar
Reception: Reflected signal received and processed
Range calculation: R = (c × t)/2

Key Parameters:

Pulse width: τ = 0.1 to 10 μs
Pulse repetition frequency: PRF = 100 Hz to 10 kHz
Peak power: 1 kW to 10 MW

Mnemonic: "RADAR Ranges by Round-trip Reflection"

Question 5(b) [4 marks]

Explain A-scope display method with the help of proper figure.

Answer:

A-Scope Display: Shows amplitude vs time relationship of received echoes.

A-Scope Presentation:

Amplitude ↑
          |
          |    ●  Target echo
          |   /|\  
     Main |  / | \  
    pulse | /  |  \  
          |/   |   \
          |    |    \___
          |____|_________\______→ Time
          0    |         
               |
           2R/c (Range)
           
    ←Ground→←Sea→←Target→←Noise→
     clutter clutter

Display Components:

Main pulse: Initial transmitted pulse (reference)
Ground clutter: Reflections from nearby terrain
Sea clutter: Reflections from sea surface
Target echo: Reflection from actual target
Noise: Random background signals

Range Measurement:

Horizontal axis: Time (proportional to range)
Vertical axis: Signal amplitude
Range formula: R = (c × t)/2

Applications:

Air traffic control
Height finding radars
Range measurement
Signal analysis

Mnemonic: "A-scope shows Amplitude Along time Axis"

Question 5(c) [7 marks]

Explain Doppler effect and working of MTI (Moving Target Indicator) RADAR system with the help of block diagram.

Answer:

Doppler Effect: Frequency shift occurs when there is relative motion between radar and target.

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

Where:

f_d = Doppler frequency shift
v_r = radial velocity of target
f_0 = transmitted frequency
c = speed of light

Doppler Shift Cases:

Approaching target: f_d > 0 (positive shift)
Receding target: f_d < 0 (negative shift)
Stationary target: f_d = 0 (no shift)

MTI RADAR Block Diagram:

graph LR
    A[Transmitter] --> B[Duplexer]
    B --> C[Antenna]
    C --> D[Target]
    D --> C
    C --> B
    B --> E[Receiver]
    F[STALO] --> G[Mixer 1]
    H[COHO] --> I[Phase Detector]
    E --> G
    G --> J[IF Amplifier]
    J --> K[Mixer 2]
    H --> K
    K --> L[Video Amplifier]
    L --> M[Delay Line]
    M --> N[Subtractor]
    L --> N
    N --> O[Display]
    P[Sync] --> A
    P --> H

MTI System Components:

STALO (Stable Local Oscillator):

Frequency: Close to transmitted frequency
Stability: High frequency stability required
Function: First mixer LO

COHO (Coherent Oscillator):

Phase reference: Maintains phase coherence
Synchronization: Locked to transmitter phase
Function: Second mixer LO and phase reference

MTI Processing:

Delay line: Stores previous pulse echo
Subtractor: Subtracts current from previous pulse
Result: Stationary targets cancelled, moving targets enhanced

MTI Transfer Function:

|H(f)| ↑
       |     
    1.0|     ●●●     ●●●     ●●●
       |    ●   ●   ●   ●   ●   ●
    0.5|   ●     ● ●     ● ●     ●
       |  ●       ●       ●       ●
     0 |_●_______●_______●_______●___→ fd
       0  PRF/4  PRF/2  3PRF/4  PRF
       
       ← Blind speeds →

Blind Speeds: Targets with certain velocities appear stationary: v_blind = (n × λ × PRF)/2 (where n = 1,2,3...)

Performance Improvements:

Multi-pulse MTI:

Multiple delay lines for better clutter rejection
Staggered PRF to reduce blind speeds
Weighted coefficients for optimum response

Clutter Map:

Digital memory stores clutter pattern
Adaptive threshold adjusts to local clutter level
Automatic updates track slow clutter changes

MTI Performance Metrics:

Parameter	Typical Value
Clutter Attenuation	30-60 dB
Minimum Detectable Velocity	1-10 m/s
Blind Speed	λ×PRF/2
Improvement Factor	20-40 dB

Advantages:

Clutter suppression: Eliminates stationary clutter
Moving target emphasis: Enhances moving targets
Automatic operation: Reduces operator workload

Limitations:

Blind speeds: Some velocities undetectable
Tangential targets: No radial component
Weather effects: Rain/snow can mask targets

Applications:

Air traffic control: Separates aircraft from ground clutter
Weather radar: Detects precipitation movement
Military surveillance: Detects moving vehicles
Marine radar: Reduces sea clutter

Mnemonic: "MTI Makes Targets Identifiable via Doppler Difference"

Question 5(a OR) [3 marks]

Define: a) Blind speed, and b) MUR

Answer:

Blind Speed:

Definition: Target radial velocities that produce zero Doppler shift in MTI radar
Formula: v_blind = (n × λ × PRF)/2
Cause: Target motion synchronized with pulse repetition
Result: Moving target appears stationary

MUR (Maximum Unambiguous Range):

Definition: Maximum range at which targets can be detected without range ambiguity
Formula: R_max = (c × PRT)/2 = c/(2 × PRF)
Limitation: Next pulse transmitted before echo returns
Ambiguity: Targets beyond MUR appear at incorrect range

Relationship Table:

Parameter	Formula	Unit
Blind Speed	nλPRF/2	m/s
MUR	c/(2×PRF)	meters
PRT	1/PRF	seconds

Mnemonic: "Blind speed Blocks, MUR Measures maximum"

Question 5(b OR) [4 marks]

Explain the factors affecting Maximum RADAR range.

Answer:

RADAR Range Equation: R_max = [(P_t × G² × λ² × σ)/(64π³ × P_min × L)]^(1/4)

Factors Affecting Maximum Range:

Transmitted Power (P_t):

Higher power = greater range
Relationship: R ∝ P_t^(1/4)
Limitation: Peak power limited by transmitter

Antenna Gain (G):

Directional antenna concentrates energy
Relationship: R ∝ G^(1/2)
Trade-off: Higher gain = narrower beamwidth

Wavelength (λ):

Lower frequency = better propagation
Relationship: R ∝ λ^(1/2)
Consideration: Atmospheric absorption increases with frequency

Target Cross Section (σ):

Larger targets reflect more energy
Relationship: R ∝ σ^(1/4)
Variation: Depends on target shape, material, aspect angle

Factors Table:

Factor	Effect on Range	Typical Values
Peak Power	R ∝ Pt^0.25	1 kW - 10 MW
Antenna Gain	R ∝ G^0.5	20 - 50 dB
Frequency	R ∝ λ^0.5	1 - 100 GHz
Target RCS	R ∝ σ^0.25	0.1 - 1000 m²

Mnemonic: "Power-Gain-Lambda-Sigma determine Range"

Question 5(c OR) [7 marks]

Compare Pulsed RADAR and CW Doppler RADAR.

Answer:

Comprehensive Comparison:

Basic Principle:

Pulsed RADAR: Transmits high-power pulses, measures round-trip time
CW Doppler: Transmits continuous wave, measures Doppler frequency shift

System Block Diagrams:

Pulsed RADAR:

graph LR
    A[Pulse Generator] --> B[Transmitter]
    B --> C[Duplexer]
    C --> D[Antenna]
    C --> E[Receiver]
    E --> F[Display]

CW Doppler RADAR:

graph LR
    A[CW Oscillator] --> B[Directional Coupler]
    B --> C[Transmit Antenna]
    D[Receive Antenna] --> E[Mixer]
    B --> E
    E --> F[Audio Amplifier]
    F --> G[Display]

Detailed Comparison Table:

Parameter	Pulsed RADAR	CW Doppler RADAR
Transmission	High power pulses	Continuous low power
Information	Range + velocity	Velocity only
Antenna	Single (duplexer)	Separate Tx/Rx
Range Capability	Excellent	None (unless FM-CW)
Velocity Resolution	Limited	Excellent
Peak Power	Very high (MW)	Low (mW to W)
Average Power	Low	Moderate
Complexity	High	Simple
Cost	Expensive	Economical
Size	Large	Compact

Performance Characteristics:

Aspect	Pulsed RADAR	CW Doppler RADAR
Range Accuracy	±10-100 m	Not applicable
Velocity Accuracy	±1-10 m/s	±0.1-1 m/s
Minimum Range	Limited by pulse width	Zero
Maximum Range	10-1000 km	1-50 km
Clutter Rejection	Moderate	Excellent
Weather Effects	Significant	Minimal

Advantages & Disadvantages:

Pulsed RADAR Advantages:

Range measurement capability
High peak power for long range
Single antenna system
Well-established technology

Pulsed RADAR Disadvantages:

Complex circuitry (duplexer, timing)
High cost and maintenance
Power supply requirements
Blind ranges due to pulse width

CW Doppler Advantages:

Simple design and low cost
Excellent velocity resolution
Continuous monitoring
Low power consumption
Compact size

CW Doppler Disadvantages:

No range information
Separate antennas required
Limited range capability
Vulnerable to interference

Applications:

Pulsed RADAR Applications:

Air traffic control
Weather monitoring
Military surveillance
Maritime navigation
Satellite tracking

CW Doppler Applications:

Traffic speed monitoring
Sports radar guns
Burglar alarms
Automatic door openers
Heart rate monitoring

Hybrid Systems:

Pulse Doppler RADAR:

Combines advantages of both systems
Range and velocity measurement
Higher complexity but better performance

FM-CW RADAR:

Frequency modulated continuous wave
Range capability added to CW system
Used in automotive radar applications

Selection Criteria:

Requirement	Choose Pulsed	Choose CW Doppler
Range measurement needed	✓	✗
High velocity accuracy	✗	✓
Long range operation	✓	✗
Low cost requirement	✗	✓
Portable application	✗	✓
Weather radar	✓	✗

Future Trends:

Digital signal processing improving both types
Software-defined radars offering flexibility
MIMO techniques enhancing performance
Integration with other sensors

Mnemonic: "Pulsed gives Position, CW gives Continuous-Velocity"