Ground_wave - Wikiparency

# Ground wave **Ground wave** is a mode of radio propagation in which a wave travels close to the Earth's surface, diffracting around the planet's curvature with help from currents the wave induces in the ground itself, rather than needing a direct line of sight to the transmitter or a reflection from the [[Ionosphere|ionosphere]]. It carries [[AM_broadcasting|AM broadcasting]] and other services on frequencies below about 3 megahertz for tens to hundreds of kilometres beyond where [[Line-of-sight_propagation|line-of-sight]] transmission alone would already have failed, at the cost of an attenuation that grows quickly with both distance and frequency. A three.js sketch elsewhere on this site, an illustrative low- and medium-frequency variant of the shared [[Radio_propagation|radio-propagation]] scene, renders that ground-hugging attenuation in three dimensions. How far a ground wave reaches depends on the frequency in use and on the electrical properties of the ground it crosses, questions the Modeling section below works through in more detail; how that reach is put to work, from broadcasting to navigation, is the subject of Applications; and the wave's own somewhat contested name, more properly the Norton surface wave, is untangled in Related terms. ## Overview Below about 3 megahertz, and with rapidly diminishing effect up to perhaps 30 megahertz, a vertically polarised radio wave can follow the curve of the Earth for a considerable distance beyond the horizon that would otherwise stop it. The mechanism is not reflection or refraction from any layer above, as with [[Skywave|skywave]] propagation, but diffraction assisted by the ground itself: the wave's own electric field drives a small [[Electric_current|current]] into the Earth's surface as it passes, and because the ground is a lossy conductor rather than a perfect one, the wavefront tilts slightly forward, letting the wave bend downward and follow the planet's curvature rather than travel tangentially off into space. The lower the frequency, the longer the wavelength relative to the ground's own conductivity and permittivity, and the further this bending can carry a usable signal; by the time a wave's frequency has climbed into the shortwave bands, the ground has become effectively too lossy at that wavelength for the effect to matter, and whatever coverage exists beyond the horizon is left to skywave, or, at very high frequencies, to nothing at all. Ground wave and skywave often operate simultaneously on the same mediumwave transmission, one covering the area close to the transmitter reliably by day and by night, the other adding a fluctuating, interference-prone extension after dark once the ionosphere's absorbing lower layer clears; a receiver near the fringe of a station's ground-wave coverage after sunset can hear the same station twice, slightly out of step with itself, as the two modes beat against one another. ## Applications Ground wave is the propagation mode a listener actually depends on for reliable reception of an [[AM_broadcasting|AM broadcasting]] station within its stated daytime coverage area, since that coverage is calculated from ground-wave field strength alone, without counting on the ionosphere at all. Longwave broadcasting, still used in parts of Europe and Africa, relies on the same mechanism at even lower frequencies for an even steadier, if smaller, service area. Precision navigation systems built before satellite positioning leaned on ground wave specifically because its phase is stable and predictable in a way skywave's is not: a pulsed, low-frequency system such as LORAN measured the tiny timing differences between ground waves from several stations to fix a position, and treated any [[Skywave|skywave]] contamination of the same signal as the chief source of error to be filtered out rather than used. Non-directional beacons that guide aircraft, and low-frequency time-code transmitters that distribute a reference clock signal to any receiver within range, both depend on the same day-and-night reliability, since neither application can tolerate the fading and multipath that skywave would add. A ground wave's practical range is set as much by the receiving environment as by the transmitter: the man-made electrical noise a receiver picks up in a city can run many times higher than in open countryside at these same frequencies, so two receivers equally far from the same station, one urban and one rural, do not necessarily hear it equally well.[^ell-noise] Ships and short-range military stations use ground wave on the lower shortwave frequencies for much the same reason as the applications above: a shorter but dependable path is often worth more than a longer, fickle one. ## Modeling Arnold Sommerfeld first solved the problem mathematically in 1909, treating the ground as a flat, lossy dielectric beneath a vertical dipole and deriving how the resulting wave's field strength falls off with distance; later work refined and corrected details of that solution as engineers pushed it toward practical use.[^sommerfeld] The version actually used for broadcast engineering is a set of standardized curves, commonly credited to the American engineer Kenneth Norton, giving field strength as a function of distance, frequency, transmitter power, and the ground's own conductivity and relative permittivity, adopted internationally so a broadcaster in one country predicts coverage the same way as one anywhere else.[^norton] Ground conductivity varies enormously with terrain: seawater is an excellent conductor and carries a ground wave with comparatively little loss, moist or fertile soil is a middling conductor, and dry, sandy, or rocky ground is nearly an insulator at these frequencies, so two transmitters of identical power and frequency, one sited near a coastline and one inland, can serve very different areas by ground wave alone.[^conductivity] Because attenuation also increases with frequency, the same curves show a station near the top of the mediumwave band reaching a noticeably smaller ground-wave radius than a station near the bottom of it, power and ground held equal, one more reason low frequencies remain the ground wave's natural territory. ## Related terms The name ground wave is used two ways that are easy to conflate. In everyday broadcast-engineering usage, the sense used throughout this article, it means the combined signal that reaches a receiver along the ground at low and medium frequencies, dominated by the surface-following behaviour described above. In stricter usage, that same signal is itself the sum of two theoretically distinct parts: a space wave, essentially the same direct-plus-reflected combination behind ordinary [[Line-of-sight_propagation|line-of-sight]] transmission at higher frequencies, made of the direct ray plus whatever the ground reflects back up along essentially straight geometric paths, and a genuine surface wave, a wave whose field clings to the boundary between air and ground and cannot be described by ray geometry at all. It is this second, non-optical component that Kenneth Norton treated with enough rigor for engineering use, so the article's own alternative name, the Norton surface wave, or more precisely the Norton ground wave once the space-wave contribution is folded back in, credits that specific piece of the theory rather than the whole phenomenon loosely called ground wave elsewhere.[^norton] An earlier, more idealized version of the same surface-bound solution, over a perfectly flat and uniform ground, is sometimes called a Zenneck wave, after the physicist who first described it; it is a useful mathematical limit rather than something a real receiver picks up in isolation, since no real transmitter sits low enough over sufficiently uniform ground to separate it cleanly from the space wave riding on top of it.[^zenneck] ## Microsims This article carries no p5.js sketch of its own. The three.js companion named in the lead, an illustrative low- and medium-frequency variant of the shared radio-propagation scene, renders the ground-hugging attenuation described above in three dimensions rather than as a flat sketch. The clearest connection to a neighbouring sketch is a shared shape of loss rather than a shared mechanism. A radar echo's power falls with the fourth power of range, not the inverse-square law a single one-way transmission follows, because the wave spreads going out and again coming back; a ground wave's field strength falls off faster still than ordinary spherical spreading would predict, because the lossy ground constantly bleeds a little energy out of the wave as it travels, on top of whatever spreading loss the distance alone would cost. Both are examples of a signal losing ground faster than the simplest law suggests, for a reason specific to the mechanism rather than to distance itself. *Try:* in the [[Radar]] sketch, watch a distant echo on the A-scope fade sharply rather than gently as the target's range control is dragged outward, the same compounding of loss, for a different reason, that steepens a ground wave's own attenuation with distance. *Try:* in the same sketch, freeze the sweep with the space bar and compare a near echo's height with a far one's; the far echo has paid the same steep, faster-than-linear price in signal strength that a ground wave pays to the ground it travels over. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ground_wave) : [Wikitube](https://en.wikitube.io/wiki/Ground_wave) Skeleton mirrored at revision 1352178249. Prose, emphasis and the microsims are Wikitube's own. ## References The diffraction mechanism connecting frequency, ground conductivity, and attenuation is standard radio-propagation textbook material and is not separately footnoted here, per the Wikitube style guide's §6.1. Page numbers below are PDF pages of the open edition linked in Further reading. [^sommerfeld]: Citation needed: Arnold Sommerfeld's 1909 paper on the propagation of waves over a flat, lossy ground would confirm the date and the details of the original flat-Earth solution summarised here. [^norton]: Citation needed: Kenneth Norton's papers deriving the practical ground-wave field-strength curves used in broadcast engineering would confirm the dates and the exact form credited to him here. [^conductivity]: Citation needed: a current international table of ground-conductivity classes (seawater, fertile soil, dry or rocky ground) would confirm the relative figures described qualitatively here. [^zenneck]: Citation needed: Jonathan Zenneck's original description of the flat-ground surface-wave solution would confirm the date and the relationship between his result and Sommerfeld's. [^ell-noise]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 106-107 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. **Further reading** — the open textbook this article draws on, since the pair carries no separate Further reading heading of its own: - Steven Ellingson. *Radio Systems Engineering, Revised First Edition* (2023). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering **External links** — since the pair carries no separate External links heading of its own: this article carries no p5.js simulation of its own, and the three.js companion named in the lead is a preset of a scene shared with neighbouring articles rather than a separate resource linked here. ## See also - [[Radio_propagation]] - [[Skywave]] - [[Line-of-sight_propagation]] - [[Ionosphere]] - [[AM_broadcasting]] - [[Radar_horizon]]