Ground Wave vs. Sky Wave: Advantages, Disadvantages, Differences

Introduction : In radio communication, signal propagation can occur through various modes depending on frequency, terrain, and atmospheric conditions. Two fundamental types of propagation are Ground Wave and Sky Wave. Ground wave travels along the Earth’s surface and is commonly used for low frequency communications, while sky wave reflects off the ionosphere, enabling long distance transmission at higher frequencies. Understanding the characteristics, advantages, and limitations of each helps in selecting the appropriate method for specific communication needs.

Ground Wave Propagation

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.

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

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.

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.

Conclusion: Ground wave and sky wave propagation serve different purposes in radio communication, each with unique strengths. Ground waves offer reliable coverage over short to medium distances, especially for AM radio and maritime communication. Sky waves, on the other hand, are ideal for long range broadcasting without the need for satellites or repeaters. While ground waves are less affected by time of day variations, sky waves depend heavily on ionospheric conditions. In summary, choosing between ground wave and sky wave depends on the distance, frequency, and application requirements of the communication system.