Ground Wave Propagation: Range, Polarization and Best Bands

Most articles about HF propagation concentrate on the ionosphere: layers, hops and maximum usable frequencies. Yet there is a second path that has nothing to do with the sky. On the lowest amateur bands the signal that you hear from a station fifty kilometers away often did not go up at all. It followed the surface of the Earth. This path is called ground wave propagation, and it is the reason why a local net on 160 meters works in the middle of the day, why AM broadcast stations cover their city around the clock and why a simple vertical antenna can out-perform a dipole for local contacts on the low bands. This guide explains how a ground wave works, what limits its range, how to improve it and where it still matters in modern amateur radio.

What a Ground Wave Is

The Wikipedia article on the ground wave describes it as a propagation mode that is important below 30 MHz and generally insignificant at higher frequencies, where line-of-sight propagation dominates. It is used by AM and longwave broadcasting, by navigation systems, by low-frequency time signals, by non-directional beacons and by short-range HF communications. In the language of amateurs, a ground wave is a signal that travels along the surface between the transmitter and the receiver without being reflected by the ionosphere. The surface wave is guided by the boundary between the air and the ground, and long wavelengths are able to diffract over the horizon, following the curvature of the Earth to some extent.

It is useful to separate this path from the others. The direct wave travels in a straight line and is limited by the horizon, as discussed in our guide to the radio horizon. The sky wave, described in our article on HF propagation, is reflected back by the ionosphere and can cover thousands of kilometers. The ground wave sits between them: it reaches beyond the horizon but not as far as a sky wave, and, unlike the sky wave, it is available whether the ionosphere is working or not.

Why Frequency and Ground Matter

Two factors set the range of a ground wave: the frequency and the quality of the surface. The article cited above explains that lower frequencies below about 3 MHz travel efficiently as ground waves, that losses increase with frequency so that transmissions between 3 and 30 MHz have a more modest range, and that the ground wave becomes unimportant above 30 MHz. Surface conductivity is decisive: highly conductive surfaces such as seawater provide the best propagation, while dry ground and ice are poor. As the wave travels, the imperfect conductivity of the ground tilts it forward and dissipates energy into the ground, so the signal falls with distance and, once the curvature of the Earth becomes significant, the strength tends to decrease exponentially.

The pattern matches what operators experience. On 160 meters and 80 meters, a station within the range of the ground wave is heard at a steady level at any time of the day. On 40 meters the ground wave is weaker and the sky wave quickly takes over. The details differ from location to location, as soil in one valley may be much better than rock on a hill nearby, and the most reliable figure is the one that you measure with your own station.

Polarization: Why Verticals Win

Ground waves are normally vertically polarized. A vertically polarized wave loses less energy to the surface than a horizontally polarized one, which is attenuated strongly because the electric field lies parallel to the conducting ground. For that reason broadcast stations on medium wave use vertical masts, and amateurs who want to work local stations on the low bands do better with a vertical than with a low horizontal wire. Our article on vertical versus horizontal polarization explains the difference in more depth.

The vertical has a second advantage: its radiation at a low angle is exactly what a ground wave needs, since the wave travels along the surface rather than being launched at an angle into the sky. A low dipole, by contrast, sends most of its signal upward, which is good for NVIS and local sky-wave coverage as described in our guide to NVIS propagation, but poor for a ground-wave path. A station that wants both has two antennas: a vertical for the ground wave and the low-angle DX, and a low horizontal wire for the high-angle regional coverage.

The Ground System Is Part of the Antenna

A vertical monopole works against the ground, so the quality of the ground system decides how much of the transmitter power becomes radiation. The resistive losses in the Earth are often the largest loss factor on low-frequency antennas, a point made in the Wikipedia article on the monopole antenna. Radials that spread over the ground reduce the losses in the soil near the base, where the currents are highest. Our guide to vertical antenna radials gives practical numbers. For a ground-wave antenna on 160 meters, a large number of radials, even short ones, usually improves the signal more than a taller mast.

Environment affects the result as well. Buildings, trees and power lines absorb or reflect the wave, and the area close to the antenna matters most, because that is where the field is strongest. A vertical on the roof of a wooden house has a worse ground system than one in an open field, but it is higher and farther from the ground losses. In general, the open field on good soil is the best site for a ground-wave signal, and the sea shore, if you are lucky enough to live near it, is better still.

How Far Does It Go?

There is no single answer. Over seawater a ground wave on 160 meters can carry for hundreds of kilometers, and in the longwave range even farther. Over average soil, the range falls to a few tens of kilometers on 80 meters and less on 40 meters with typical amateur power. Over dry sand, rock or in a city, the range drops further. The Wikipedia article notes that attenuation over land is lowest in the winter in temperate climates and that hills, mountains, urban areas and forests can create areas of reduced signal strength, while the signal is relatively immune to fading. That last property is one of the reasons why ground wave is valued for local nets: the signal is steady and does not suffer the deep QSB of a sky-wave path, as described in our article on QSB and fading.

The simplest way to find the range of your own station is to ask. Arrange a test with friends at different distances, call at a set time and record the reports at various times of day. When the report is steady at noon and at midnight, you are probably seeing a ground wave. When it changes strongly with the time of day, the sky wave is mixing in. A map with a few reports soon shows the coverage area, and it also shows the directions in which your location is better or worse.

Day and Night: When the Sky Wave Joins In

On the lowest bands the ground wave is only half of the story. In the daytime, the D layer absorbs medium and low HF signals so strongly that the sky wave is almost absent on 160 meters, and the ground wave is the main path. That is why AM broadcast stations have a clean, local daytime coverage that is often described in terms of a protected service area. After sunset the D layer fades, and the sky wave returns with a vengeance. Signals from distant stations arrive on the same frequency, combine with the ground wave and produce the fading and interference zones that medium wave listeners know well. Within a certain distance of the transmitter the ground wave dominates; beyond a second distance the sky wave dominates; in between, the two are similar in strength and interfere.

For the amateur, the consequence is practical. If you want local contacts on 160 or 80 meters in the afternoon, the ground wave gives you a good chance, and a vertical with a good radial field is the right tool. If you want DX after dark, you need a low-angle antenna, but the same vertical is again a good choice. A station that uses the low bands seriously usually ends up with both types of antenna, and a switch between them to compare reports with the same station on the same frequency.

Measuring Your Own Ground Wave

You can learn a great deal with a simple experiment. Choose a quiet time in the daytime, when the sky wave on 160 meters is absent, and ask a friend at a distance of ten kilometers to transmit at a known power. Record the signal strength with a calibrated receiver or a signal report, then repeat with friends at twenty, thirty and fifty kilometers, and compare the numbers. In a plot of signal strength against distance, the ground wave shows a smooth decline. If you repeat the experiment at night and the signals at long distance become stronger than the daytime pattern predicts, you have found the sky wave. The test also shows whether the soil in some directions is much better than in others, which helps you to decide where to put a new antenna and how to orient a directional array of verticals.

Where Ground Wave Still Matters

• Local nets on 160 and 80 meters. Daytime coverage of a region without relying on the ionosphere, with steady signals.
• Emergency communications. In a disaster the ionosphere may be disturbed and the local path may be all that is needed. Ground wave on the low bands gives a robust regional link without towers or repeaters.
• Experimental LF and MF operation. On 2200 and 630 meters, the ground wave is the main mode for local and regional contacts; our guide to the 630 and 2200 meter bands describes the equipment.
• Beacons and timing. Non-directional beacons and LF time signals use ground waves, and amateurs can use them to check receiver performance and propagation.
• Understanding the low bands. The difference between a ground wave and a sky wave explains the skip zone, the deep nulls around the station and the local noise.

The Bottom Line

Ground wave propagation is the path along the surface of the Earth, important below 30 MHz and strongest at medium and low frequencies. Its range depends on the frequency and on the conductivity of the surface, with seawater best and dry ground and ice poor, and it needs vertical polarization and a good ground system to work well. It offers a steady signal without the fading of sky-wave paths, which is why it suits local nets, emergency communication and the experimental bands. Build a vertical with many radials if you want to use it on 160 or 80 meters, test your own coverage with friends and remember that above about 30 MHz the ground wave no longer matters.

Frequently Asked Questions

What is ground wave propagation?

The travel of a radio wave along the surface of the Earth, without reflection from the ionosphere. It is important below 30 MHz and strongest at the lowest frequencies.

Which bands use the ground wave?

Mainly 2200 m, 630 m, 160 m and 80 m. On 40 meters it is weak, and above about 30 MHz it is insignificant.

Why does seawater help?

Seawater is highly conductive, so the wave loses much less energy to the surface than over dry ground or ice, and the range is far greater.

Why use a vertical antenna for ground wave?

The wave is vertically polarized and travels at a low angle, and a vertical antenna produces exactly that kind of radiation, while a low horizontal wire radiates mostly upward.

Is ground wave signal stable?

Generally yes. It is relatively immune to fading compared with sky-wave paths, though changes in the ground and the weather cause some variation.

Can I work DX by ground wave?

Only over very good surfaces such as the sea, and mainly on the lowest bands. For DX on other paths, the sky wave is the usual mechanism.