Question 1(a) [3 marks]#
Write any three properties of Electromagnetic waves
Answer:
| Properties of Electromagnetic Waves |
|---|
| 1. EM waves can travel through vacuum or material media |
| 2. EM waves travel at the speed of light in free space (3×10⁸ m/s) |
| 3. EM waves exhibit transverse wave characteristics with oscillating electric and magnetic fields |
Mnemonic: “VTS” - Vacuum travel, Transverse nature, Speed of light
Question 1(b) [4 marks]#
Define: (1) Radiation resistance (2) Directivity (3) Gain
Answer:
| Term | Definition |
|---|---|
| Radiation resistance | The equivalent resistance that would dissipate the same amount of power as radiated by an antenna when the current at the feed point is equal to the antenna input current |
| Directivity | The ratio of maximum radiation intensity in a specific direction to the average radiation intensity in all directions |
| Gain | The product of directivity and radiation efficiency, measuring how efficiently an antenna converts input power into radio waves in a specific direction |
Mnemonic: “RDG” - Resistance dissipates power, Direction concentration, Gain includes efficiency
Question 1(c) [7 marks]#
Explain physical concept of generation of Electromagnetic waves with neat diagram
Answer:
Electromagnetic waves are generated when electric charges accelerate or oscillate, creating coupled oscillating electric and magnetic fields that propagate through space.
graph TD
A[Electric Current Flow] -->|Oscillation| B[Oscillating Electric Field]
B -->|Induces| C[Oscillating Magnetic Field]
C -->|Induces| D[Oscillating Electric Field]
D --> E[Self-sustaining wave propagation]
Diagram: Dipole Antenna EM Wave Generation
- Basic concept: When AC current flows in the antenna, electrons accelerate up and down
- Electric field: Created by charge separation in the antenna
- Magnetic field: Produced by the current flow, perpendicular to electric field
- Propagation: Fields detach from antenna and propagate outward at the speed of light
- Self-sustaining: Each field component regenerates the other as wave travels
Mnemonic: “COMAP” - Current Oscillations Make Alternating Propagations
Question 1(c) OR [7 marks]#
Design 4 Element Yagi Uda antenna for frequency of 435 MHz with necessary equations
Answer:
For a 4-element Yagi-Uda antenna at 435 MHz:
| Element | Length Formula | Spacing Formula | Calculated Value |
|---|---|---|---|
| Reflector | 0.5λ × 1.05 | - | 36.2 cm |
| Driven element | 0.5λ | - | 34.5 cm |
| Director 1 | 0.45λ | 0.2λ from driven | 31.0 cm at 13.8 cm spacing |
| Director 2 | 0.43λ | 0.25λ from Director 1 | 29.6 cm at 17.2 cm spacing |
Equations used:
- Wavelength: λ = c/f = 3×10⁸/435×10⁶ = 0.69 meters
- Half-wave dipole: L = 0.5λ = 34.5 cm
- Element spacing: S = 0.15λ to 0.25λ
graph LR
A[Reflector: 36.2cm] --- B[Driven Element: 34.5cm]
B --- C[Director 1: 31.0cm]
C --- D[Director 2: 29.6cm]
style A fill:#f9f,stroke:#333,stroke-width:2px
style B fill:#bbf,stroke:#333,stroke-width:2px
style C fill:#fbb,stroke:#333,stroke-width:2px
style D fill:#fbb,stroke:#333,stroke-width:2px
Mnemonic: “RDDS” - Reflector Driven Directors Shrink
Question 2(a) [3 marks]#
Explain Loop antenna with diagram
Answer:
Loop antenna is a radiating element formed by shaping a conductor into a loop.
- Small loops: Circumference < λ/10, radiation pattern similar to magnetic dipole
- Large loops: Circumference ≈ wavelength, bidirectional radiation pattern
- Applications: Direction finding, AM radio reception, RFID tags
Mnemonic: “SLC” - Size affects Loop Characteristics
Question 2(b) [4 marks]#
Explain Non Resonant wire antenna
Answer:
| Characteristic | Description |
|---|---|
| Definition | Antenna operating at frequencies where its physical length is not a multiple of half-wavelength |
| Impedance | Complex with both resistive and reactive components |
| Standing waves | Present along the antenna length |
| Example | Rhombic antenna, terminated with resistance at the end |
| Advantage | Wideband operation, suitable for multiple frequencies |
Mnemonic: “NITRO” - Non-resonance Incurs Termination for Resistance and Operation
Question 2(c) [7 marks]#
What is Radiation resistance of half wave dipole? Draw radiation patterns of Dipoles of length λ/2, λ and λ/4 antenna
Answer:
The radiation resistance of a half-wave dipole is approximately 73 ohms.
Radiation patterns:
| Dipole Length | Pattern Characteristics |
|---|---|
| λ/2 dipole | Figure-8 pattern; maximum radiation perpendicular to antenna axis; HPBW = 78° |
| λ dipole | Multi-lobed pattern; four main lobes at angles to antenna axis |
| λ/4 dipole | Broader pattern than λ/2; requires ground plane to complete the equivalent dipole |
Mnemonic: “SHORT” - Smaller Half-dipole Offers Rounded-Transmissions
Question 2(a) OR [3 marks]#
Explain Folded dipole antenna with figure
Answer:
Folded dipole is a variation of the half-wave dipole with ends folded back and connected to form a loop.
- Input impedance: Approximately 300 ohms (4 times that of simple dipole)
- Bandwidth: Wider than simple dipole
- Applications: TV reception, FM radio, balanced transmission lines
Mnemonic: “FIB” - Folded Increases Bandwidth
Question 2(b) OR [4 marks]#
Explain Rhombic antenna with figure
Answer:
Rhombic antenna consists of four wires arranged in a rhombus or diamond shape.
| Characteristic | Description |
|---|---|
| Shape | Diamond/rhombus with terminating resistor at far end |
| Operation | Non-resonant traveling-wave antenna |
| Directivity | High gain, unidirectional pattern |
| Bandwidth | Very wide frequency range |
| Applications | HF communications, point-to-point links |
Mnemonic: “TREND” - Terminated Rhombic Enables Numerous Directions
Question 2(c) OR [7 marks]#
Differentiate between Broadside array and End fire array with suitable diagram
Answer:
| Parameter | Broadside Array | End fire Array |
|---|---|---|
| Direction of maximum radiation | Perpendicular to array axis | Along array axis |
| Element phasing | Same phase (0°) | Progressive phase shift |
| Element spacing | λ/2 typically | λ/4 typically |
| Radiation pattern | Fan-shaped beam | Pencil-shaped beam |
| Applications | Broadcasting, base stations | Point-to-point links |
Diagram comparison:
Mnemonic: “PAPER” - Perpendicular And Parallel Emission Respectively
Question 3(a) [3 marks]#
Draw and Explain Inverted V antenna
Answer:
Inverted V antenna is a dipole with arms angled downward, resembling an inverted “V”.
- Angle: Arms typically form 90°-120° angle
- Impedance: Close to 50 ohms, lower than horizontal dipole
- Pattern: Omnidirectional, slightly broader than horizontal dipole
- Applications: Amateur radio, shortwave communications
Mnemonic: “AVS” - Angle Varies Signal
Question 3(b) [4 marks]#
Draw and explain parabolic reflector antenna
Answer:
| Component | Function |
|---|---|
| Parabolic reflector | Collects and focuses incoming signals or directs transmitted signals |
| Feed element | Located at focal point of parabola to collect/emit signals |
| Focal length | Distance from vertex to focus, determines beam characteristics |
| Applications | Satellite communications, radar, radio astronomy, microwave links |
Mnemonic: “FOLD” - Focus Of Large Dish
Question 3(c) [7 marks]#
Write down range of frequencies for HF, VHF and UHF. Write short note on Microstrip antenna.
Answer:
| Frequency Band | Range |
|---|---|
| HF (High Frequency) | 3 MHz - 30 MHz |
| VHF (Very High Frequency) | 30 MHz - 300 MHz |
| UHF (Ultra High Frequency) | 300 MHz - 3 GHz |
Microstrip Antenna:
- Structure: Conductive patch on dielectric substrate with ground plane
- Feeding methods: Microstrip line, coaxial probe, aperture-coupled
- Advantages: Low profile, lightweight, easy fabrication, compatible with PCB
- Limitations: Narrow bandwidth, low gain, low power handling
- Applications: Mobile devices, RFID, GPS, satellite communications
Mnemonic: “PATCH” - Planar Antenna That’s Cheaply Handled
Question 3(a) OR [3 marks]#
Write Morse code for word: “LINE OF SIGHT”
Answer:
| Letter | Morse Code |
|---|---|
| L | .-.. |
| I | .. |
| N | -. |
| E | . |
| (space) | / |
| O | — |
| F | ..-. |
| (space) | / |
| S | … |
| I | .. |
| G | –. |
| H | …. |
| T | - |
“LINE OF SIGHT” in Morse code: .-.. .. -. . / — ..-. / … .. –. …. -
Mnemonic: “Listen In Now, Every Other Frequency Supports Immediate Global Heightened Transmission”
Question 3(b) OR [4 marks]#
Draw and explain Turnstile & Super turnstile antenna
Answer:
Turnstile Antenna:
Super Turnstile Antenna:
| Type | Characteristics |
|---|---|
| Turnstile | Two horizontal dipoles at right angles, fed 90° out of phase |
| Super Turnstile | Modification with multiple elements forming rectangular loops |
| Pattern | Omnidirectional in horizontal plane, figure-8 in vertical |
| Polarization | Horizontal or circular polarization |
| Applications | TV broadcasting, FM broadcasting, satellite communications |
Mnemonic: “TOPS” - Turnstile Offers Perpendicular Symmetry
Question 3(c) OR [7 marks]#
What is Polarization? Explain Helical antenna in detail with diagram
Answer:
Polarization is the orientation of the electric field vector of an electromagnetic wave as it propagates through space.
Helical Antenna:
| Parameter | Description |
|---|---|
| Structure | Conductor wound in helical shape above ground plane |
| Diameter | Typically λ/π |
| Pitch | Spacing between turns, usually λ/4 |
| Turns | 3-10 turns depending on gain requirements |
| Modes | Normal mode (broadside) or Axial mode (end-fire) |
| Polarization | Circular polarization in axial mode |
| Applications | Satellite communications, space telemetry, tracking |
Mnemonic: “HASP” - Helical Antenna Supports Polarization
Question 4(a) [3 marks]#
Explain Tropospheric scattered propagation
Answer:
| Aspect | Description |
|---|---|
| Mechanism | Radio signals scatter from tropospheric irregularities and refractive index variations |
| Frequency | Typically VHF, UHF (100 MHz - 10 GHz) |
| Range | 100-800 km, beyond line-of-sight |
| Reliability | Less affected by weather than line-of-sight; more reliable than ionospheric |
| Applications | Military communications, remote areas where other systems aren’t practical |
Mnemonic: “STRIP” - Scatter Through Refractive Index Patterns
Question 4(b) [4 marks]#
Define: (1) Virtual Height (2) Maximum Usable Frequency - MUF (3) Critical Frequency
Answer:
| Term | Definition |
|---|---|
| Virtual Height | The apparent height of the ionosphere calculated from the time delay of a radio signal reflected back to Earth, as if reflection occurred at a single point |
| Maximum Usable Frequency (MUF) | The highest frequency that can be used for reliable communication via ionospheric reflection for a specified path and time |
| Critical Frequency | The highest frequency that can be reflected back when transmitted vertically to the ionosphere (when angle of incidence is 90°) |
Mnemonic: “VMC” - Virtual height Measures Critical reflection
Question 4(c) [7 marks]#
Explain effect of ground on electromagnetic wave propagation
Answer:
| Effect | Description |
|---|---|
| Ground reflection | Signal reflects off ground, causing multipath reception |
| Ground absorption | Part of signal energy absorbed by ground, reducing signal strength |
| Ground diffraction | Waves bend around obstacles, extending coverage beyond line-of-sight |
| Earth curvature | Limits line-of-sight distance based on antenna height |
| Ground conductivity | Higher conductivity (water, wet soil) allows better propagation than poor conductors (dry, rocky terrain) |
Wave behavior equation:
- Range (km) ≈ 4.12(√h₁ + √h₂) where h₁, h₂ are antenna heights in meters
Mnemonic: “RADAR” - Reflection Absorption Diffraction Affect Range
Question 4(a) OR [3 marks]#
Explain Duct Propagation
Answer:
Duct propagation occurs when radio waves become trapped in atmospheric layers with special refractive properties.
- Formation: Temperature inversions or moisture gradients create atmospheric ducts
- Effect: Signals trapped within duct, allowing propagation far beyond normal range
- Frequencies: Most common in UHF and microwave bands
- Applications: Extended over-water communications, radar anomalies
Mnemonic: “TIDE” - Trapped In Ducting Environment
Question 4(b) OR [4 marks]#
Explain different layers of Ionosphere
Answer:
| Layer | Altitude | Characteristics |
|---|---|---|
| D Layer | 60-90 km | Absorbs HF waves during daytime, disappears at night |
| E Layer | 90-150 km | Reflects frequencies up to 10 MHz, sporadic E phenomenon |
| F1 Layer | 150-210 km | Present during day, merges with F2 at night |
| F2 Layer | 210-400+ km | Main reflecting layer, highest electron density, present day and night |
Mnemonic: “DEAF” - D absorbs, E reflects, All merge, F2 persists
Question 4(c) OR [7 marks]#
Explain Ground wave and Sky wave propagation
Answer:
Ground Wave Propagation:
- Frequency range: LF, MF (30 kHz - 3 MHz)
- Components: Direct, ground-reflected, surface waves
- Range: Depends on frequency, ground conductivity, transmitter power
- Applications: AM broadcasting, navigation systems, maritime communications
Sky Wave Propagation:
- Mechanism: Waves refracted by ionosphere back to Earth
- Frequency: Mainly HF (3-30 MHz)
- Range: 100-10,000+ km, multiple hops possible
- Variability: Time of day, season, solar activity, frequency
- Applications: International broadcasting, amateur radio, military
Mnemonic: “GIST” - Ground-Interface Surface Transmission vs Ionospheric Sky Transmission
Question 5(a) [3 marks]#
Explain three different types of Satellites
Answer:
| Satellite Type | Characteristics |
|---|---|
| LEO (Low Earth Orbit) | Altitude: 160-2,000 km, Period: 90 min, Applications: Earth observation, communications |
| MEO (Medium Earth Orbit) | Altitude: 2,000-35,786 km, Period: 2-24 hours, Applications: Navigation (GPS) |
| GEO (Geostationary Orbit) | Altitude: 35,786 km, Period: 24 hours, Applications: TV broadcasting, weather monitoring |
Mnemonic: “LMG” - Low Medium Geostationary
Question 5(b) [4 marks]#
What are smart antennas? Write two applications of it
Answer:
Smart antennas are antenna systems that use digital signal processing algorithms to identify spatial signatures and dynamically adjust radiation patterns.
| Feature | Description |
|---|---|
| Types | Switched beam systems, Adaptive array systems |
| Operation | Uses multiple antenna elements and signal processing to adapt to changing conditions |
| Benefits | Increased capacity, improved coverage, reduced interference |
Applications:
- Mobile cellular networks (4G, 5G) for increased capacity and coverage
- Wireless LANs for improved throughput and reduced interference
Mnemonic: “SMART” - Signal Manipulation And Response Technology
Question 5(c) [7 marks]#
What is Satellite communication? Explain Data Communication
Answer:
Satellite Communication is the use of artificial satellites to provide communication links between various points on Earth.
Data Communication via Satellite:
| Component | Function |
|---|---|
| Earth Station | Transmits/receives signals to/from satellites |
| Transponder | Receives, amplifies and retransmits signals at different frequencies |
| Access methods | FDMA, TDMA, CDMA to allow multiple users to share satellite capacity |
| Protocols | TCP/IP adaptation for satellite latency, specialized protocols |
| Applications | Internet backhaul, VSAT networks, IoT, corporate networks |
| Advantages | Wide coverage area, independence from terrestrial infrastructure |
| Challenges | Signal delay (latency), power limitations, weather effects |
Mnemonic: “UPDATA” - Uplink Provides Data Access To All
Question 5(a) OR [3 marks]#
Write laws of Kepler for satellite
Answer:
| Kepler’s Laws | Description |
|---|---|
| First Law | Satellites orbit in elliptical paths with the Earth at one focus of the ellipse |
| Second Law | A line joining the satellite and Earth sweeps out equal areas in equal times (conservation of angular momentum) |
| Third Law | The square of the orbital period is proportional to the cube of the semi-major axis of the orbit |
Mnemonic: “ESP” - Elliptical orbits, Sweep equal areas, Period-distance relation
Question 5(b) OR [4 marks]#
Explain Base station and Mobile station antennas
Answer:
Base Station Antennas:
- Types: Omnidirectional, sector, panel antennas
- Gain: Typically 10-18 dBi
- Mounting: Tower or rooftop installation
- Features: Downtilt capability, multiple frequency bands
Mobile Station Antennas:
- Types: Internal PIFA, patch, monopole antennas
- Gain: Low gain (0-3 dBi)
- Size: Compact, often integrated inside device
- Characteristics: Omnidirectional pattern, multiple bands
Mnemonic: “BIMS” - Base stations Install Multiple Sectors, Mobile stations Stay small
Question 5(c) OR [7 marks]#
Explain DTH receiver system in detail
Answer:
DTH (Direct-to-Home) receiver system delivers television signals directly to users via satellite.
| Component | Function |
|---|---|
| Dish Antenna | Parabolic reflector to collect satellite signals (45-90 cm typical diameter) |
| LNB (Low Noise Block) | Converts high-frequency satellite signals to lower frequencies for transmission through coaxial cable |
| Coaxial Cable | Carries signals from LNB to set-top box |
| Set-top Box | Decodes/demodulates signals, provides user interface, conditional access |
| Conditional Access Module | Provides security and subscription management |
| Features | Electronic Program Guide, recording, interactive services |
Mnemonic: “DISCS” - Dish Intercepts Signals, Converter Sends to Set-top box