Terminology » RF Basics » Ground Wave vs. Sky Wave: Advantages and Disadvantages

Ground Wave vs. Sky Wave: Advantages and Disadvantages

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This document outlines the pros and cons of both ground wave and sky wave propagation methods.

Ground Wave Propagation

Introduction

Ground waves follow the Earth’s curvature, allowing them to propagate over considerable distances. This type of propagation is typically used at frequencies below 2 MHz. A common example is AM radio.

Ground wave propagation

Figure 1: Ground wave propagation

Advantages of Ground Wave

  • Reduced Interference: Lower frequencies are less susceptible to interference, mainly experiencing atmospheric noise.
  • Longer Distance Coverage: Absorption of electromagnetic waves is less at lower frequencies, enabling greater range. However, path loss increases with distance, requiring an optimal separation between the transmitter (Tx) and receiver (Rx).
  • Vertical Polarization: Ground waves are vertically polarized to prevent short circuits of the electric field (E) component.

Disadvantages of Ground Wave

  • Polarization Sensitivity: If the polarization is affected, the E-field components can be short-circuited by the ground.
  • Distance Limitations: The distance between the transmitter and receiver antennas should not be excessively large. Otherwise, signal strength diminishes due to ground and atmospheric absorption, hindering communication.
  • Cost: Repeaters may be necessary to maintain signal strength over longer distances, increasing the overall system cost.

Sky Wave Propagation

Introduction

Sky waves rely on reflection from the ionosphere, an ionized layer in the atmosphere. Signals can travel long distances through multiple “hops,” bouncing back and forth between the ionosphere and the Earth’s surface. Shortwave (SW) radio is a typical application.

Sky wave propagation

Figure 2: Sky wave propagation

Sky wave propagation occurs at frequencies ranging from 2 MHz to 30 MHz.

Advantages of Sky Wave

  • Simplicity: Sky wave propagation is a relatively simple mode of transmission, leveraging the reflective properties of the ionosphere.
  • Continuous Support: It can provide continuous communication support.

Disadvantages of Sky Wave

  • Distance Variability: The position of the ionosphere varies between day and night. During the day, the ionosphere is closer and during the night the ionosphere is far, leading to differing propagation distances.
  • Signal Strength Reduction: Signals often undergo multiple hops, which can significantly reduce signal strength, especially over long distances between the transmitter and receiver antennas.