Sky Wave vs. LOS Wave vs. Ground Wave Propagation

On Earth, radio waves propagate primarily through two methods: ground wave and sky wave propagation. In free space, however, radio waves travel in a straight line, known as Line of Sight (LOS) propagation. This article compares and contrasts these three propagation methods.

Ground Wave Propagation

Ground wave propagation

• Frequency Range: Below 2 MHz (ELF, SLF, ULF, VLF, LF, and MF bands)

• Characteristics:

  • The signal travels close to the Earth’s surface.
  • It follows the curvature of the Earth, allowing for considerable distances of propagation.
  • At lower frequencies, atmospheric noise is the primary source of interference.
  • Transmitter power is usually sufficient for a range of about 5000 miles.
  • The optimum antenna size is approximately $\lambda/2$, where $\lambda$ is the wavelength of the signal.
  • The received signal strength decreases exponentially with distance from the transmitter.
  • Often referred to as surface waves.

• Usage: Commonly used for AM radio broadcasting.

• Signal Strength: Decreases exponentially as the distance from the transmitter increases. This can be mathematically represented as:

  $$S(d) = S0 * e^{-αd}$$

  Where:

  • $S(d)$ is the signal strength at distance d
  • $S_0$ is the initial signal strength
  • $α$ is the attenuation constant
  • $d$ is the distance from the transmitter

Sky Wave Propagation

Sky wave propagation

• Frequency Range: 2 MHz to 30 MHz (ELF, SLF, ULF, VLF, LF, MF, and HF bands)
• Characteristics:
  • Relies on the reflective properties of the ionosphere.
  • Signals are reflected off the ionized layers of the atmosphere.
  • Signals can travel long distances via multiple “hops,” bouncing between the Earth and the ionosphere.
• Usage: Commonly used for shortwave (SW) radio.
• Day vs. Night:
  • Nighttime: The ionosphere layer is closer to the Earth’s surface, resulting in flatter angles of reflection and longer propagation distances. This leads to larger “skip zones” (areas with no reception).
  • Daytime: The ionosphere layer is farther from the Earth’s surface, resulting in steeper angles of reflection and shorter propagation distances. This leads to smaller skip zones.

Line of Sight (LOS) Wave Propagation

Line of sight (LOS) wave propagation

• Frequency Range: Above 30 MHz (VHF, UHF, SHF, and EHF bands)
• Characteristics:
  • Radio waves travel in a straight line.
  • Propagation follows the laws of free space.
  • Both the transmitting and receiving antennas must be within the Line of Sight of each other.
• Usage: Used in satellite communication and ground communication.
• VHF Band Consideration: In the VHF band, reflections from various objects on Earth can affect the LOS signal.