• 9. Antenna & Propagation
  • 9-1 Antenna basics
  • 9-2 Characteristics of electromagnetic waves
    • 1. Attenuation with distance
    • 2. Five characteristics of radio waves
    • 3. Fading
  • 9-3 Antenna performance parameters
    • 1. Radiation Pattern
    • 2. Directivity and gain
    • 3. Polarization

9. Antenna & Propagation

9-1 Antenna basics

• Definition: a device that converts between electrical signals and electromagnetic waves

[Transmitting antenna]
   Current of a specific frequency is input
        ↓
   Magnetic field generated (current → magnetic field)
        ↓
   Changing magnetic field → induces electric field
        ↓
   Changing electric field → induces magnetic field
        ↓
   (repeats this process while propagating through space)
        ↓
[Receiving antenna]
   Detects the change of the magnetic field over time
        ↓
   Induces current → converted into an electrical signal

The electric and magnetic fields induce each other and spread through space. This is the essence of an electromagnetic wave

9-2 Characteristics of electromagnetic waves

1. Attenuation with distance

• In free space, received power is inversely proportional to the square of the distance

Pt
Pr ∝ ─────
        R²

Pt: transmitted power
Pr: received power
R: distance

2. Five characteristics of radio waves

1. Linearity (rectilinear propagation) Travels the shortest path, the way light travels in a straight line Stronger at higher frequencies

2. Reflection Changes path when it hits a material Metal surfaces, walls, etc.

3. Refraction Direction and speed change when entering a different medium Atmospheric layers, glass, etc.

4. Diffraction Bends around the edges of obstacles Signals can reach even behind obstacles

5. Scattering Scattered in all directions by obstacles Rain, fog, dust, etc.

• Why communication is possible even over a mountain: even though high frequencies have strong linearity, the signal bends in behind obstacles thanks to the diffraction characteristic.

3. Fading

• Because of reflection, scattering and diffraction of radio waves, one signal arrives over multiple paths. Each path has a different length, so arrival times differ → interference occurs.
• This is called multipath interference

9-3 Antenna performance parameters

• There are 3 main parameters
  1. Radiation Pattern
  2. Directivity & gain
  3. Polarization

1. Radiation Pattern

• A graph showing in which direction and how much signal an antenna radiates
• No antenna radiates uniformly in all directions. Some directions are strong, some are weak. The radiation pattern visualizes this

2. Directivity and gain

1. Directivity

• The property of an antenna to concentrate radio waves in a particular direction

Physical definition
        max radiation intensity (U_max)
D = ─────────────────────────────
        avg radiation intensity (U_avg)

Ratio of maximum radiation intensity to average radiation intensity

High directivity = concentrates the signal in one direction = the main lobe is narrow and long.

• Omnidirectional antenna: dipole antenna, spreads out in all directions
• Directional antenna: parabolic antenna, concentrates in one direction

2. Antenna efficiency and gain

Antenna efficiency (η) = radiated power (Pr) / input power (Pi)
   "How much of the input power is actually radiated"

Antenna gain (G) = η × D
              = efficiency × directivity

Gain is expressed in dB

Meaning of gain: when radiating the same power in the same direction, how many times stronger it is than an omnidirectional antenna.

• Trade-off between gain and beam width
• High gain = narrow beam width = high directivity
• Gain and beam width are always inversely proportional

3. Polarization

• The direction in which the electric field (E) oscillates as an electromagnetic wave travels
• If the polarization directions of the transmitting and receiving antennas differ, transmission/reception performance degrades.
• If they are completely orthogonal, no signal can be exchanged
• Types: vertical polarization, horizontal polarization