Antenna & Wave Propagation (4341106) - Winter 2023 Solution

Question 1(a) [3 marks]

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Define: (1) Directivity, (2) Gain, and (3) HPBW

Answer:

Table: Key Antenna Parameters

Mnemonic: “DGH: Direction Gives Half-power”

Question 1(b) [4 marks]

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List the properties of electromagnetic waves

Answer:

Table: Properties of Electromagnetic Waves

Mnemonic: “TVNPER: Transverse Velocity No-medium Polarized Energy Reflection”

Question 1(c) [7 marks]

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Explain physical concept of generation of Electromagnetic wave

Answer:

Diagram: Generation of Electromagnetic Wave

graph TD
    A[Accelerating Charge] -->|Produces| B[Time-varying Electric Field]
    B -->|Produces| C[Time-varying Magnetic Field]
    C -->|Produces| D[Time-varying Electric Field]
    D --> C
    C --> E[Self-sustaining EM Wave]

• Charge Acceleration: When electric charges accelerate, they generate changing electric fields
• Field Coupling: A changing electric field produces a changing magnetic field and vice versa
• Self-Propagation: This cyclic generation of fields allows waves to travel without a medium
• Field Orientation: Electric and magnetic fields are perpendicular to each other and the direction of propagation
• Energy Transport: Energy alternates between electric and magnetic fields as wave propagates

Mnemonic: “CASES: Charge Acceleration Self-propagates Electro-magnetic Signals”

Question 1(c) OR [7 marks]

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Explain how electromagnetic field radiated from a center fed dipole

Answer:

Diagram: Field Radiation from Center-Fed Dipole

graph TD
    A[Alternating Current Input] -->|Creates| B[Oscillating Charges]
    B -->|Generates| C[Time-varying Electric Field]
    C -->|Generates| D[Time-varying Magnetic Field]
    C --> E[EM Wave Radiation]
    D --> E

• Center Feeding: AC signal applied at center of dipole creates oscillating current
• Charge Distribution: Current creates opposite charges at dipole ends that change with AC frequency
• Field Generation: Oscillating charges create time-varying electric field
• Magnetic Coupling: Time-varying electric field generates perpendicular magnetic field
• Near/Far Fields: Near dipole, fields are complex; far from dipole, fields form uniform radiation pattern
• Radiation Pattern: Maximum radiation perpendicular to dipole axis, zero radiation along axis

Mnemonic: “CORONA: Current Oscillates, Radiation Occurs, Near-far Areas”

Question 2(a) [3 marks]

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Differentiate the resonant and non-resonant antennas

Answer:

Table: Resonant vs Non-Resonant Antennas

Mnemonic: “RESI: Resonant Exhibits Standing-waves Impedance-real”

Question 2(b) [4 marks]

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Explain Yagi antenna and discuss its radiation characteristics

Answer:

Diagram: Yagi-Uda Antenna Structure

• Structure: Contains one reflector, one driven element, and multiple directors
• Directivity: High directivity in direction of directors (8-12dB)
• Gain: Higher gain with more directors (up to 15dB)
• Bandwidth: 2-5% of center frequency
• Applications: TV reception, point-to-point communication, amateur radio

Mnemonic: “DRAGONS: Directional Reflector And Gain-improving Directors Offer Narrow Signals”

Question 2(c) [7 marks]

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Describe radiation characteristics of resonant wire antennas and draw the current distribution of λ/2, 3λ/2 and 5λ/2 antenna

Answer:

Diagram: Current Distribution in Resonant Wire Antennas

• Half-Wave (λ/2): Current maximum at center, zero at ends; radiation pattern is figure-eight shaped
• Three Half-Wave (3λ/2): Three current maxima, phase reversal at λ/2 points; multiple lobes in radiation pattern
• Five Half-Wave (5λ/2): Five current maxima, more complex radiation pattern with multiple lobes
• Standing Waves: All resonant antennas exhibit standing wave current distribution
• Feed Point: Usually at current maximum for optimum impedance matching

Mnemonic: “NODE: Number Of Distributions Equals wavelength-multiple”

Question 2(a) OR [3 marks]

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Differentiate the broad side and end fire array antennas

Answer:

Table: Broadside vs End Fire Array Antennas

Mnemonic: “PEPS: Perpendicular Elements Produce Sideways radiation”

Question 2(b) OR [4 marks]

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Explain loop antenna and discuss its radiation characteristics

Answer:

Diagram: Loop Antenna

• Structure: Closed-loop conductor with circumference of one wavelength or less
• Types: Small loops (circumference < λ/10) and large loops (circumference ≈ λ)
• Polarization: Electric field polarized in plane of loop
• Radiation Pattern: Figure-eight pattern for small loops, more directional for large loops
• Applications: Direction finding, AM reception, RFID tags
• Impedance: High impedance for small loops, resonant for large loops

Mnemonic: “SPIRAL: Small Patterns In Receiving And Locating signals”

Question 2(c) OR [7 marks]

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Describe radiation characteristics of non resonant wire antennas and draw the radiation pattern of λ/2, 3λ/2 and 5λ/2 antenna

Answer:

Diagram: Radiation Patterns of Wire Antennas

• Non-Resonant Properties: Traveling waves rather than standing waves
• λ/2 Antenna: Simple bidirectional pattern, maximum radiation perpendicular to wire
• 3λ/2 Antenna: Multiple lobes, more complex pattern with side lobes
• 5λ/2 Antenna: Even more complex pattern with multiple main and side lobes
• Feed Point Impedance: Non-resonant, typically requires impedance matching
• Bandwidth: Wider than resonant antennas

Mnemonic: “TWIST: Traveling Waves Increase Side-lobe Transmission”

Question 3(a) [3 marks]

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Write short note on micro strip (patch) antenna

Answer:

Diagram: Microstrip Patch Antenna Structure

• Structure: Metal patch on dielectric substrate with ground plane below
• Size: Typically half-wavelength in size
• Profile: Low-profile, lightweight, easy to fabricate
• Radiation: Radiates from patch edges, omnidirectional or directional patterns
• Applications: Mobile devices, satellites, GPS receivers

Mnemonic: “PSALM: Patch Substrate Above Layer of Metal”

Question 3(b) [4 marks]

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Explain helical antenna and discuss its radiation characteristics

Answer:

Diagram: Helical Antenna

• Structure: Conducting wire wound in helix shape above ground plane
• Modes: Axial mode (end-fire) and normal mode (broadside)
• Axial Mode: When circumference ≈ λ, radiation along helix axis
• Normal Mode: When circumference « λ, radiation perpendicular to axis
• Polarization: Circular polarization in axial mode
• Applications: Satellite communication, space telemetry, radio astronomy

Mnemonic: “MOCHA: Mode Of Circular Helix Antennas”

Question 3(c) [7 marks]

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Explain horn antenna and discuss its radiation characteristics

Answer:

Diagram: Horn Antenna Types

• Structure: Waveguide with flared end to match impedance with free space
• Types: Pyramidal (rectangular), sectoral (E-plane or H-plane), and conical (circular)
• Directivity: 10-20 dB, higher than waveguide alone
• Bandwidth: Very wide bandwidth
• Radiation Pattern: Main lobe with small side lobes
• Applications: Microwave communications, radar, satellite tracking, EMC testing
• Advantages: High gain, simple construction, low VSWR

Mnemonic: “POWERS: Pyramidal Or Widening End Radiates Strongly”

Question 3(a) OR [3 marks]

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Write short note on slot antenna

Answer:

Diagram: Slot Antenna

• Structure: Rectangular/circular slot cut in conducting surface
• Babinet’s Principle: Complementary to dipole antenna
• Radiation Pattern: Similar to dipole but with E and H fields interchanged
• Polarization: Electric field perpendicular to slot length
• Impedance: High impedance compared to dipole
• Applications: Aircraft, spacecraft, base stations, flush mounting

Mnemonic: “CROPS: Complementary Radiation Opening Perpendicular to Surface”

Question 3(b) OR [4 marks]

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Explain parabolic reflector antenna and discuss its radiation characteristics

Answer:

Diagram: Parabolic Reflector Antenna

• Structure: Parabolic reflector with feed at focal point
• Working Principle: Parallel rays from reflector converge at focal point
• Directivity: Very high (30-40 dB)
• Beamwidth: Very narrow, inversely proportional to diameter
• Efficiency: 50-70% depending on feed design
• Applications: Satellite communications, radio astronomy, radar systems
• Types: Prime focus, Cassegrain, offset feed

Mnemonic: “DISH: Directing Incoming Signals to Hub”

Question 3(c) OR [7 marks]

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Describe V and inverted V antenna

Answer:

Diagram: V and Inverted V Antennas

Table: Comparison of V and Inverted V Antennas

Mnemonic: “VIVA: V Is Vertical Arrangement, Inverted V Aims downward”

Question 4(a) [3 marks]

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Define: (1) Reflection, (2) Refraction and (3) Diffraction

Answer:

Table: Wave Phenomenon Definitions

Mnemonic: “RRD: Rebounding, Redirecting, Detour”

Question 4(b) [4 marks]

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List HAM radio application for communication

Answer:

Table: HAM Radio Applications

Mnemonic: “EDSDMVP: Emergency DX Satellite Digital Morse Voice Public-service”

Question 4(c) [7 marks]

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Explain ionosphere’s layers and sky wave propagation

Answer:

Diagram: Ionospheric Layers and Sky Wave Propagation

graph TD
    A[Transmitter] -->|Sky Wave| B[F2 Layer: 250-400 km]
    A -->|Sky Wave| C[F1 Layer: 150-250 km]
    A -->|Sky Wave| D[E Layer: 90-150 km]
    A -->|Sky Wave| E[D Layer: 60-90 km]
    B -->|Reflection| F[Receiver at long distance]
    C -->|Reflection| F
    D -->|Reflection/Absorption| F
    E -->|Absorption| G[Signal Loss]

• D Layer (60-90 km): Exists during daylight, absorbs HF signals below 10 MHz
• E Layer (90-150 km): Reflects signals 3-5 MHz, stronger during day, sporadic-E in summer
• F1 Layer (150-250 km): Daytime only, merges with F2 at night
• F2 Layer (250-400 km): Main reflecting layer, enables long-distance HF communication
• Propagation Factors:
  • Virtual Height: Apparent height of reflection
  • Critical Frequency: Maximum frequency reflected vertically
  • MUF: Maximum Usable Frequency for a given distance
  • Skip Distance: Minimum distance for sky wave reception

Mnemonic: “DEFV: D-absorbs, E-reflects, F-provides Very-long-distance”

Question 4(a) OR [3 marks]

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Define: (1) MUF, (2) LUF and (3) Skip distance

Answer:

Table: Ionospheric Propagation Terms

Mnemonic: “MLS: Maximum-highest, Lowest-minimum, Skip-nearest”

Question 4(b) OR [4 marks]

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List HAM radio digital modes of communication

Answer:

Table: HAM Radio Digital Modes

Mnemonic: “FIRST PAD: FT8 Is RTTY SSTV Then Packet APRS Digital-voice”

Question 4(c) OR [7 marks]

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Explain space wave propagation

Answer:

Diagram: Space Wave Propagation

graph LR
    A[Transmitter] -->|Direct Wave| B[Receiver]
    A -->|Ground Reflected Wave| B
    A -->|Tropospheric Scatter| C[Extended Range Receiver]
    A -->|Ducting| D[Very Extended Range]

    subgraph Troposphere
    A
    B
    C
    D
    E[Temperature Inversion Layer]
    end
    
    A -->|Follows| E -->|Waveguide Effect| D

• Components: Direct wave, ground-reflected wave, tropospheric waves
• Line of Sight: Primary mechanism limited by Earth’s curvature
• Frequency Range: VHF, UHF, and microwave frequencies
• Tropospheric Scattering: Forward scattering extends range beyond horizon
• Duct Propagation:
  • Occurs in temperature inversion layers
  • Creates waveguide effect trapping signals
  • Enables very long distance VHF/UHF propagation
• Factors Affecting: Antenna height, terrain, atmospheric conditions
• Applications: TV broadcasting, microwave links, mobile communications

Mnemonic: “DRIFT: Direct Reflection Inversion Forward Tropospheric”

Question 5(a) [3 marks]

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Define: (1) Beam area (2) Beam efficiency, and (3) Effective aperture

Answer:

Table: Antenna Beam Parameters

Mnemonic: “BEA: Beam Efficiency Aperture”

Question 5(b) [4 marks]

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Describe need of smart antenna

Answer:

Diagram: Smart Antenna Benefits

graph TD
    A[Smart Antenna] -->|Provides| B[Increased Capacity]
    A -->|Provides| C[Enhanced Coverage]
    A -->|Reduces| D[Interference]
    A -->|Improves| E[Signal Quality]
    A -->|Saves| F[Battery Power]
    A -->|Enables| G[Spatial Multiplexing]

• Capacity Improvement: Serves more users in same bandwidth
• Coverage Enhancement: Extends range by focusing energy
• Interference Reduction: Nulls out unwanted signals
• Signal Quality: Better SNR through beam focusing
• Energy Efficiency: Lower transmit power requirements
• Spatial Multiplexing: Multiple data streams in same frequency
• Adaptive Operation: Dynamically adapts to changing environment

Mnemonic: “PRECISE: Power Reduction, Enhanced Coverage, Interference Suppression, Enhanced Signal”

Question 5(c) [7 marks]

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Draw the DTH Receiver indoor and outdoor black diagram and discuss its functions

Answer:

Diagram: DTH System Block Diagram

graph LR
    subgraph Outdoor Unit
    A[Dish Antenna] -->|Collects| B[LNB - Low Noise Block]
    end

    subgraph Indoor Unit
    C[Tuner] --> D[Demodulator]
    D --> E[Decoder]
    E --> F[MPEG Processor]
    F --> G[Video/Audio Output]
    H[Smart Card] --> E
    I[User Interface] --> E
    end
    
    B -->|Coaxial Cable| C

Outdoor Unit Components and Functions:

• Dish Antenna: Collects satellite signals, typically 45-90 cm diameter
• LNB (Low Noise Block):
  • Converts high frequency satellite signals (10-12 GHz) to lower IF frequencies (950-2150 MHz)
  • Amplifies weak signals with minimal noise
  • Contains local oscillator and polarization selection

Indoor Unit Components and Functions:

• Tuner: Selects desired transponder frequency
• Demodulator: Extracts digital signal from modulated carrier
• Decoder: Decrypts encrypted channels using smart card authorization
• MPEG Processor: Decompresses video/audio data streams
• User Interface: On-screen menus, program guide, channel selection
• Smart Card: Contains subscription details and decryption keys

Mnemonic: “COLD-TDUMS: Collection, Oscillator, Low-noise, Downconversion - Tuner Demodulator Unscrambler MPEG Smart-card”

Question 5(a) OR [3 marks]

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Define: (1) Antenna, (2) Folded dipole, and (3) Antenna array

Answer:

Table: Antenna Definitions

Mnemonic: “AFA: Antenna Folded Array”

Question 5(b) OR [4 marks]

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Describe application of smart antenna

Answer:

Table: Smart Antenna Applications

Mnemonic: “MBMRSWI: Mobile Base MIMO Radar Satellite Wi-Fi IoT”

Question 5(c) OR [7 marks]

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Explain Terrestrial mobile communication antennas and also discuss about base station and mobile station antennas

Answer:

Diagram: Mobile Communication Antenna Types

graph TD
    A[Terrestrial Mobile Antennas] --> B[Base Station Antennas]
    A --> C[Mobile Station Antennas]

    B --> D[Panel Antennas]
    B --> E[Sector Antennas]
    B --> F[Omnidirectional Antennas]
    B --> G[Smart Antennas]
    
    C --> H[Whip Antennas]
    C --> I[Helical Antennas]
    C --> J[Planar Inverted-F Antennas]
    C --> K[Internal PCB Antennas]

Base Station Antennas:

• Panel/Sector Antennas: 65°-120° coverage per sector, typically three sectors per site
• Characteristics:
  • High gain (10-18 dBi)
  • Vertical polarization
  • Downtilt capability (mechanical or electrical)
  • Multi-band operation
• Height: Mounted on towers 15-50m high for maximum coverage
• Pattern Control: Minimizes interference to adjacent cells

Mobile Station Antennas:

• External Antennas: Less common today, mainly for vehicles or rural areas
  • Whip antennas (¼λ monopoles)
  • Helical designs for flexibility
• Internal Antennas: Now dominant in handsets
  • PIFA (Planar Inverted-F Antenna)
  • PCB trace antennas
  • Characteristics:
    • Small size
    • Multi-band operation
    • Omnidirectional pattern
    • Lower efficiency (typically -3 to -6 dBi)

Mnemonic: “BEST-POMME: Base-station External Sector Tower - Portable Omnidirectional Multi-band Mobile Embedded”