Ground Wave Propagation

Tune to 1000 kHz on an AM radio during the day and you hear only local stations within a few hundred kilometers. Switch on at midnight and distant stations hundreds or even thousands of kilometers away suddenly crowd the dial. That dramatic difference between day and night on the AM broadcast band illustrates one of the most fundamental propagation phenomena in radio: ground wave. During the day, a local station's ground wave coverage dominates. At night, sky wave from distant stations competes — and often wins. Understanding ground wave explains not just AM broadcast behavior but the reliable short-range coverage that makes the 160-meter and lower HF bands valuable for emergency communications and regional nets.

Ground wave travels along the Earth's surface by diffraction, following the curvature of the Earth and inducing currents in the ground as it passes. Attenuation increases rapidly with both frequency and distance.

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The Diffraction Mechanism

If radio waves traveled only in straight lines, a transmitter at ground level would illuminate only the area within its line-of-sight — and the radio horizon for a 10-meter high antenna is only about 13 km. Yet medium-wave broadcast stations serve listeners 200 km or more away during the day. They do this through diffraction — the bending of waves around the curvature of the Earth.

Diffraction is a wave phenomenon that occurs at any frequency, but it is most effective when the wavelength is large relative to the obstacle or curvature being diffracted around. The Earth's curvature has a radius of about 6,371 km — an enormous obstacle compared to any practical radio wavelength. Yet even this enormous obstacle causes diffraction if the wavelength is long enough relative to the path length.

As a ground wave travels along the surface, the wave's lower boundary is in contact with the ground. The ground is not a perfect conductor — it absorbs some of the wave energy — but it also guides the wave along its surface. The induced currents in the ground that the wave creates are part of the wave system, not just losses. The overall effect is that the wave continuously diffracts around the horizon, following the Earth's curvature and extending coverage beyond the geometric radio horizon.

This is fundamentally different from sky wave propagation. Ground wave never leaves the lower atmosphere. It does not rely on the ionosphere at all. It works day and night, in any solar conditions, regardless of the K index or solar flux. This reliability is one of its key advantages — during geomagnetic storms that black out HF sky wave propagation, ground wave on 160 meters and medium wave can still provide regional coverage.

Attenuation with Distance and Frequency

Ground wave suffers two types of loss as it travels:

1. Geometric spreading: Like any radiation from a point source, the wave energy spreads over an increasing area as the wavefront expands. In the far field, power density falls roughly as 1/r².
2. Ground absorption: As the wave travels along the surface, it induces currents in the ground, converting some of its energy to heat. This absorption increases rapidly with frequency because higher-frequency waves penetrate less deeply into the ground (skin effect) and the surface resistance is effectively higher.

The combined effect of these two loss mechanisms means that ground wave attenuation increases dramatically with both distance and frequency. At low frequencies (below 1 MHz), ground wave provides useful signal over hundreds of kilometers. At 7 MHz (40 meters), the same transmitter power gives useful ground wave coverage only to perhaps 50–100 km. At 14 MHz (20 meters), ground wave coverage shrinks to under 50 km. By 30 MHz (10 meters), ground wave is essentially useless beyond local distances.

Why Vertical Polarization Is Required

Ground wave propagation requires vertical polarization — the electric field of the wave must be oriented vertically, perpendicular to the Earth's surface. This is not a convention or a design choice — it is a physical necessity dictated by the electromagnetic boundary conditions at the Earth's surface.

An electromagnetic wave has both an electric field (E-field) and a magnetic field (H-field). For a wave traveling horizontally along the surface, the E-field can be either vertical (perpendicular to the surface) or horizontal (parallel to the surface). A horizontal E-field is oriented along the Earth's surface. As the wave travels, this horizontal E-field attempts to drive currents along the surface. The Earth, being a conducting medium (even if imperfect), allows these currents to flow, and in doing so it absorbs energy from the wave extremely rapidly. A horizontally polarized wave at, say, 1 MHz is attenuated so quickly that it provides essentially no useful ground wave coverage beyond a few kilometers.

A vertical E-field, by contrast, is oriented perpendicular to the Earth's surface. The boundary condition at the Earth-air interface for the normal component of the electric field allows the vertical E-field to interact with the surface in a way that sustains the wave rather than simply absorbing it. The currents it drives flow downward into the Earth and back — these are the same currents that make the ground part of the antenna system for a vertical antenna. Energy is still absorbed, but at a much slower rate.

The practical consequence for ham radio is that any antenna intended for ground wave coverage — especially on 160 meters and 80 meters for regional emergency use — should be a vertical antenna. Horizontal dipoles at these frequencies produce sky wave coverage at night but are largely useless for reliable ground wave coverage during the day when sky wave is absorbed by the D layer.

The Effect of Soil Conductivity

The conductivity of the ground under and around the antenna has a profound effect on ground wave propagation range. High-conductivity ground supports lower-resistance current paths, which means less of the wave's energy is converted to heat per unit distance traveled, and the ground wave carries further.

The difference between the best and worst ground conditions is enormous. Seawater conductivity (about 5 S/m) is 5,000 times greater than rocky terrain. At 1 MHz, the ground wave range over the ocean may be 1,000 km or more, while over a rocky desert it might be only 50–100 km with the same transmitter power. This is why maritime HF and MF communication has always relied on ground wave for regional coverage — ships and shore stations separated by hundreds of kilometers of ocean can maintain reliable contact on the MF maritime bands.

For amateur operators, the soil conductivity at your location affects both your ground wave coverage and the efficiency of your ground system. In regions with poor soil conductivity (mountain-top locations, areas with granite bedrock), vertical antennas may be significantly less effective than expected, and building an extensive radial system to bypass the high-resistance local soil becomes even more important.

Practical Ranges at Amateur Frequencies

For the amateur bands most relevant to ground wave propagation — 160 meters and 80 meters — here are realistic range expectations under typical mid-continent conditions:

160 meters (1.8–2.0 MHz): Ground wave provides useful coverage to about 100–200 km during the day with a vertical antenna and a reasonable transmitter power (100 watts). Emergency nets in the eastern US often use 160 meters for reliable regional coverage within a state or multi-state area, knowing that this coverage exists regardless of ionospheric conditions. At night, sky wave begins to compete and can produce far longer paths, but it also creates interference from distant stations on the same frequency.

80 meters (3.5–4.0 MHz): Ground wave provides reliable coverage to about 50–100 km. During the day, 80 meters is primarily a ground wave band — sky wave is absorbed by the D layer. This makes 80 meters a good band for daytime regional contacts within a few hundred kilometers, free from the long-distance skip interference that appears at night when sky wave takes over.

40 meters (7 MHz): Ground wave is mostly limited to 30–50 km and is overshadowed by sky wave even during the day at most distances. For practical purposes, 40-meter contacts at distances beyond 50 km are sky wave, not ground wave.

Ground Wave in Ham Radio

Ground wave has specific, important roles in amateur radio despite its limited range compared to sky wave:

Emergency communications: ARES and RACES organizations value 160 and 80 meters for emergency use precisely because ground wave coverage is reliable regardless of solar conditions. A geomagnetic storm that wipes out HF sky wave propagation for 24–48 hours does not affect ground wave. Regional emergency nets can coordinate within a state or county when no other HF band is usable.

Daytime regional nets: 40-meter and 80-meter nets for rag-chewing, traffic handling, and news nets operate reliably during the day using ground wave for the short hops within their regional coverage area. Sky wave interference from distant stations is absent because the D layer prevents those distant signals from arriving — it simultaneously blocks ground wave from going very far, but also blocks incoming interference.

Near-vertical incidence sky wave (NVIS): Although technically sky wave, NVIS is used for similar short-range regional coverage (100–500 km) and is often confused with ground wave. NVIS uses a horizontal antenna close to the ground to radiate almost straight up, reflect off the ionosphere, and come back down close to the transmitter — it is sky wave at very low range. NVIS fills the gap between ground wave (under 100 km) and normal sky wave (over 500 km). NVIS is covered in more detail in the sky wave and ionosphere lessons.