Some newcomers to the hobby find HF ground, surface and sky wave propagation a little confusing. Hopefully, the information below will help to clarify different types of radio wave propagation.
I’m working on this page so do come back and check for additions. I’m also working on a video about propagation.
Ground Waves:
Ground wave propagation refers to radio waves that travel along or near the Earth’s surface, following its curvature, without relying on reflection from the ionosphere. The ground wave induces currents in the Earth’s surface, causing the wave to propagate along the ground. The wave remains close to the surface, bending slightly to follow the Earth’s curvature due to diffraction.
Ground waves are most effective at low frequencies. Below 2 MHz, such as AM radio in the 540–1600 kHz band. The signal weakens with distance due to ground absorption, which increases with higher frequencies and depends on terrain conductivity, Seawater is highly conductive, which reduces loss. Dry soil increases the loss. The ground wave is reliable for short to medium distances, especially over flat or conductive terrain. It’s not dependent on ionospheric conditions, making it stable day and night. It has a limited range compared to skywave propagation.
Ground waves also come into play for local contacts on bands such as 10 and 11 metres. Using a 1/2 or 5/8 wave vertical antenna, contacts can be made across town or further.
Surface Waves:
Surface wave propagation refers to the component of the ground wave that travels directly along the Earth’s surface, hugging the ground due to interaction with the Earth’s conductive surface. The radio wave interacts with the Earth’s surface, inducing currents in the ground that sustain the wave’s propagation. The wave’s electric field is perpendicular to the ground, and it propagates as a guided wave along the Earth-air interface. It diffracts around obstacles and follows the Earth’s curvature to some extent, allowing it to travel beyond the line of sight.
Surface waves are most effective very low frequencies, 30kHz to 3MHz. Higher frequencies are more rapidly attenuated. The range can extend for hundreds of kilometres, with better performance over highly conductive surfaces like seawater. Less affected by atmospheric conditions compared to skywave propagation.
Sky Waves:
When radio waves are transmitted from an antenna, some waves travel directly to the receiver, this is the ground wave, while others travel upward. If their frequency is suitable, these upward-traveling waves interact with the ionosphere. This is a region of the Earth’s upper atmosphere containing ionized particles created by solar radiation. The ionosphere can bend, refract, or reflect these radio waves back toward the Earth’s surface. When this happens, the signal can skip from the transmitter to a distant receiver, often far beyond the horizon. The wave can bounce up and down between the ground and the ionosphere to completely circle the Earth.
Summary:
Ground wave propagation is a broader term that includes surface waves, direct waves, and ground-reflected waves. Surface wave propagation specifically refers to the component traveling along the Earth’s surface. Components: Ground wave may involve line-of-sight and reflected paths, while surface wave is strictly the ground-hugging component.
Surface wave propagation is a specific type of ground wave that travels along the Earth’s surface. This is best suited for low frequencies with ranges up to hundreds of kilometres, such as BBC Radio 4 on 198kHz.
Sky wave propagation is where the wave is upward-traveling, at various angles, and is reflected or refracted by the ionosphere. This is best suited for higher HF frequencies with ranges of thousands of miles.
I’m working on this page so do come back and check for additions. I’m also working on a video about propagation.