Radio_propagation - Wikiparency

# Radio propagation **Radio propagation** is the study of what happens to a radio wave in the interval between leaving a transmitting antenna and reaching a receiving one, an interval that can span a fraction of a millisecond across a room or the better part of a second by way of a satellite. Nothing about a radio wave is unique to radio: it is [[Radio_wave|electromagnetic radiation]], governed by the same optics as visible light, so it bends wherever the medium it travels through changes, scatters off anything comparable in size to its own wavelength, and weakens wherever an obstacle blocks it outright or a rough surface breaks it into pieces headed in different directions. What changes enormously with frequency is which of these behaviours actually decides a given link: a [[Shortwave_radio|shortwave]] signal can bend back from the [[Ionosphere|ionosphere]] and reach another continent, while a [[Microwave]] link a thousand times higher in frequency needs a clear, unbroken [[Line-of-sight_propagation|line of sight]] and nothing more. Knowing in advance which mode will govern a given frequency, distance, terrain and state of the atmosphere is what lets an engineer choose a workable link rather than discover the choice was wrong only once the equipment is built. This article works through that general physics; the [[Ultra_high_frequency|UHF]], [[Very_high_frequency|VHF]] and shortwave articles on this site each apply one part of it to their own slice of the [[Radio_spectrum|spectrum]]. A three.js sketch elsewhere on this site renders a skywave ray refracting back from the ionosphere, showing how the skip zone it leaves behind opens and widens as frequency rises toward the maximum usable frequency, and closes again as frequency falls. ## Frequency dependence The physics governing every radio link is the same set of Maxwell's equations at every frequency, but the practical behaviour that follows from them changes enormously across the spectrum, because a wave's interaction with the ground, the atmosphere and any obstacle in its path depends on how the wavelength compares with the size of whatever it meets. At the low-frequency end, a wavelength of kilometres diffracts around the Earth's own curvature and follows the conductive ground as a surface wave with little loss; in the middle of the spectrum, [[Shortwave_radio|shortwave]] wavelengths of tens of metres are still long enough to refract back from the ionosphere's charged layers rather than pass through them; and from [[Very_high_frequency|VHF]] upward, wavelengths of metres or less mostly pass straight through the ionosphere and diffract too little around hills or buildings to be useful, leaving [[Line-of-sight_propagation|line of sight]] as the only reliable mode. [[Ultra_high_frequency|UHF]] and [[Microwave]] links sit firmly in this last regime, while shortwave sits in the middle one; the sections below work through each regime's own physics in turn, from the idealised free-space case with nothing at all in the way to the ground- and sky-bound modes that make real [[Radio|radio]] propagation different from empty space. ## Free space propagation In a vacuum with no ground, atmosphere or obstacle anywhere nearby, a transmitting antenna radiates power outward over an expanding sphere, so the power density a receiving antenna intercepts falls with the square of distance, exactly as it would for a point source of light. Expressed for a link between two antennas of known gain, this is the [[Friis_transmission_equation|Friis transmission equation]]; expressed instead as a loss the path alone imposes, it is [[Free-space_path_loss|free-space path loss]], which for a distance d and frequency f can be written `L_dB = 20*log10(d) + 20*log10(f) + K`, a constant K fixed only by the units chosen for d and f. Free-space loss is not a real loss of energy, since none of the transmitted power actually disappears; it is simply the geometric dilution of a fixed amount of power spread over an ever-larger sphere, the baseline every real link's [[Link_budget|link budget]] starts from before adding whatever the atmosphere, the ground or an obstacle takes away on top of it. ## Direct modes (line-of-sight) Line-of-sight propagation is free-space propagation's practical cousin: a direct path with nothing solid in the way, but with the Earth's curved surface eventually getting in the way regardless of how tall the antennas are built. Allowing for the standard four-thirds-Earth-radius model of atmospheric refraction, which bends a wave's path slightly toward the ground and effectively extends the geometric horizon, two antennas of height h1 and h2 above ground, in metres, share a radio horizon of about `d ≈ 4.12*(sqrt(h1) + sqrt(h2))` kilometres. This is the mode nearly every band above shortwave depends on: VHF and UHF broadcasting, cellular telephony, most satellite links, and [[Radar]] all rely on a clear or nearly clear path rather than on any bending of the wave back toward the ground. A path need not be perfectly unobstructed to work: some clearance around the direct line, not merely along it, is needed to avoid diffraction loss from an edge that intrudes on the wave's first Fresnel zone, which is why a link planner checks clearance over the whole width of that zone rather than only along the straight line between the antennas. ## Surface modes (groundwave) Below about the top of the medium-frequency band, a vertically polarised wave can follow the curvature of a conductive Earth as a [[Ground_wave|ground wave]], guided along the surface in much the way a wave guided along a lossy transmission line loses energy gradually rather than all at once. Ground-wave range depends strongly on ground conductivity: propagation over seawater reaches much farther than the same power over dry, poorly conducting soil, because a better conductor supports the wave with less of the resistive loss that saps its energy as it travels. This is the mode that gives AM broadcasting most of its reliable daytime coverage, since at those frequencies the ionospheric sky wave described below is largely absorbed by day and only the ground wave is available; the same station's after-dark range often grows past its daytime, ground-wave coverage once the sky wave returns, which is why some AM stations must reduce power or change their antenna pattern at night to avoid interfering with distant stations they cannot reach at all during the day.[^amnight] ## Non-line-of-sight modes The best-known non-line-of-sight mode is the ionospheric [[Skywave|sky wave]] that carries shortwave signals over the horizon: a wave launched upward at a shallow angle refracts back toward the ground from the ionosphere's charged layers rather than escaping into space, landing far beyond the point any ground or line-of-sight mode could reach. Which frequencies refract back, and at what angle, follows the secant law: a frequency that would pass straight through the layer at vertical incidence can still be returned at a shallower, oblique angle, up to a maximum usable frequency, or MUF, that rises with the secant of the incidence angle measured from vertical. Between the transmitter and the nearest point a sky wave returns to Earth lies a skip zone where neither the ground wave, already too weak, nor the sky wave, not yet back down, can be heard; the skip zone and the MUF both shift with the time of day, the season and the roughly eleven-year solar cycle, since all three change how strongly the upper atmosphere is ionised. A wave can also refract and reflect repeatedly, bouncing between the ionosphere and the ground for multi-hop paths that reach the far side of the planet. Diffraction bends a wave around an obstacle's edge without any layer to refract from, letting a signal reach partway into the shadow a hill or a building casts, though with substantial loss compared with a clear path. [[Multipath_propagation|Multipath]] propagation arises whenever a receiver picks up more than one version of the same signal, arriving by slightly different paths, a direct path plus one or more reflected or diffracted ones, with the versions adding constructively or destructively depending on the exact phase difference between them: an effect present to some degree in almost every non-free-space link, regardless of which other propagation mode dominates it. ## Measuring HF propagation Because ionospheric conditions vary continuously, HF propagation is measured rather than only predicted. An ionosonde transmits a swept-frequency pulse straight upward and times its return, building a plot of virtual reflection height against frequency that reveals each layer's critical frequency directly: the vertical-incidence measurement from which an oblique MUF for a specific path can then be estimated using the secant law described above.[^ionosonde] Oblique sounders instead transmit between two real, separated sites to measure a specific path's propagation directly rather than inferring it from an overhead sounding, at the cost of needing cooperating equipment at both ends. The technique descends from Edward Appleton's and, independently, Gregory Breit and Merle Tuve's mid-1920s pulse experiments, which first measured the ionosphere's height by timing a radio echo bounced off it, proving the layer's existence directly rather than only inferring it from Marconi's earlier long-distance results.[^appleton] ## Practical effects Below about 30 MHz, the frequencies where sky-wave propagation is most useful, atmospheric and man-made electrical noise routinely outweighs a receiver's own thermal noise by a wide margin, so a link's practical range is set by the noise the sky and the surrounding electrical environment contribute rather than by the receiver's own sensitivity. Median man-made noise temperatures near 30 MHz have been tabulated at roughly 1,120,000 kelvin in a city centre, 418,000 kelvin in a residential area and under 4,000 kelvin in a quiet rural area, against a receiver's own thermal-noise-equivalent temperature of at most a few hundred kelvin[^thermalnoise], a gap of two to three orders of magnitude that no amount of receiver design can close, since the noise arrives with the signal rather than being generated inside the set.[^noisetemp] At much higher frequencies the environment quiets considerably: the galactic background itself falls off steeply with frequency, and by microwave frequencies the [[Signal-to-noise_ratio|signal-to-noise ratio]] of a well-designed link is set mainly by the receiver's own noise figure rather than by the sky, the reverse of the HF case.[^noisetemp] Fading is the other practical constant: [[Fading]] from multipath interference, and its statistically well-characterised form, [[Rayleigh_fading|Rayleigh fading]], both cause a receiver's signal strength to vary over time and position even when the transmitter's power never changes, which is why a practical link budget includes a fade margin rather than relying on the nominal, unfaded signal strength alone. A [[Directivity|directive]] antenna or a [[Beamforming|beamforming]] or [[MIMO]] system can recover some of what fading takes away by favouring the strongest of several arriving paths, or combining several of them, rather than accepting whichever one a simple antenna happens to receive. ## Empirical models Because the physics above predicts a mode's existence more easily than it predicts its exact strength on a specific real path, propagation prediction leans heavily on empirical and semi-empirical models fitted to decades of measurement rather than on a closed-form calculation from first principles alone. International standards bodies publish curves for sky-wave field strength and for terrestrial broadcast and mobile coverage, and region- or service-specific models handle irregular terrain, urban clutter and other conditions a generic curve does not capture well.[^models] Every such model is only as good as the measurements and the terrain data behind it, and every one includes an explicit statement of the percentage of time or of locations for which its prediction is expected to hold, since ionospheric and tropospheric conditions are statistical rather than fixed quantities. ## Microsims This article carries no p5.js sketch of its own. A three.js sketch built for this article renders a skywave ray refracting back from the ionosphere, tracing how the skip zone described above opens and closes as frequency changes and the maximum usable frequency rises and falls with it. The interactive sketches that go with the specific ideas raised above belong to neighbouring articles, and this section points to three of them. *Try:* in the [[Radar]] sketch, drag the pulse-repetition frequency down until a distant target's echo folds into a closer-looking "ghost" — the same round-trip timing this article's Free space propagation section describes for an ordinary link, pushed into a regime where the timing itself becomes the measurement. *Try:* in the [[Doppler_effect]] sketch, set the source moving and watch the wavefronts bunch up ahead of it and spread out behind it; a receiver moving through a multipath field sees a smeared version of the same shift on every path at once, part of why a fading signal's envelope fluctuates the way it does. *Try:* in the [[Sonar]] sketch, drag the assumed sound speed away from the water's true speed and watch every measured range slide off its true value; a propagation model that assumes a fixed ionospheric height or a fixed atmospheric refractivity makes exactly the same kind of error whenever the real atmosphere departs from the assumption. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Radio_propagation) : [Wikitube](https://en.wikitube.io/wiki/Radio_propagation) Skeleton mirrored at revision 1362035544. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Skywave]] - [[Ground_wave]] - [[Ionosphere]] - [[Multipath_propagation]] - [[Free-space_path_loss]] - [[Line-of-sight_propagation]] - [[Radio_wave]] - [[Radio]] ## Footnotes Page numbers below are PDF pages of the open editions cited. Where a claim is well documented but could not be pinned here to a specific page or primary record, it is footnoted "Citation needed" with what would settle it, rather than omitted or guessed, per the Wikitube citation policy. ## References Standard propagation physics used here — free-space spreading, the radio-horizon geometry, ground-wave attenuation over a lossy Earth, and the secant law relating vertical and oblique critical frequencies — is textbook material used throughout radio engineering and is not separately footnoted, per the Wikitube style guide's §6.1. [^amnight]: Citation needed: a national regulator's own rules on AM broadcast night-time power reduction and directional-antenna patterns, to confirm the scope and rationale described here. [^thermalnoise]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 96-101 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^ionosonde]: Citation needed: a technical description of a specific ionosonde design and its virtual-height output, to confirm the details of the vertical-incidence sounding method described here. [^appleton]: Citation needed: Edward Appleton's and, separately, Gregory Breit and Merle Tuve's original mid-1920s papers reporting the pulsed measurement of ionospheric height, to confirm the dates and priority of each. [^noisetemp]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 104-107 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^models]: Citation needed: the current ITU-R Recommendations for HF sky-wave field-strength prediction and for VHF/UHF terrestrial field-strength prediction, to confirm the recommendation numbers and editions in current use. ## Further reading - Steven Ellingson. *Radio Systems Engineering - Revised First Edition*. 2023. CC BY-NC. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering - Christian Tiberius; Max Mulder. *Engineering Signal Analysis: From Fourier to filtering: Theory*. 2026. CC BY. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/engineering-signal-analysis-from-fourier-to-filtering-theory ## External links - Authoritative propagation-prediction references (for example the ITU-R's own propagation recommendations), to be pinned once specific editions are confirmed.