Skywave - Wikiparency

# Skywave **Skywave**, also called skip, is the propagation of a radio wave that leaves the ground steeply enough to strike the [[Ionosphere|ionosphere]] and is refracted back down to Earth far beyond the horizon, rather than being stopped by the planet's own curvature the way [[Line-of-sight_propagation|line-of-sight]] transmission is. Because the reflecting layer sets the range rather than the curvature of the ground, a single hop can cross a continent and several hops in succession can circle the globe, which is why [[Shortwave_radio|shortwave]] broadcasting and amateur radio have relied on skywave since the 1920s for the only over-the-horizon reach available before communications satellites. A three.js sketch elsewhere on this site, the night-time preset of a shared [[Radio_propagation|radio-propagation]] scene, renders the same reflecting layer in three dimensions. How far a given hop reaches, and whether it lands close enough to fill the gap a ground wave cannot cover, depends on the angle a wave leaves the ground and the frequency chosen for the ionosphere's condition at the time, the subject of the first section below. The reflecting layer itself is not fixed: it thins after sunset, shifts with the season, and swells or weakens over the roughly eleven-year solar cycle, so the frequencies that carry a signal furthest change through the day and the year. The mechanism was in everyday use for two decades before anyone could explain it, a puzzle the history section below closes with. ## Local and distant skywave propagation A radio wave that reaches the ionosphere does not bounce off a hard surface; it curves gradually back downward as it travels through a region where the density of free electrons, and so the local refractive index, changes with height, until the wave is briefly travelling horizontally and then begins to descend. Whether that turning point exists at all for a given wave depends on both its frequency and the angle at which it entered the layer, which is why the same ionosphere returns a low-angle signal from one transmitter while letting a near-vertical signal from another pass straight through into space. ### Low-angle skywaves A signal launched close to the horizon meets the ionosphere at a shallow, grazing angle, and grazing incidence lets a comparatively high frequency still be turned back to Earth; the shallower the angle, the higher the frequency that can still complete a hop, a relationship [[Shortwave_radio|shortwave]] operators exploit deliberately when reaching for the greatest possible distance. A simple tangent-line estimate, the same geometry used for the radio horizon but applied to a reflecting layer instead of an elevated antenna, puts the reach of one low-angle hop at roughly twice the horizon distance to the layer's height: about 2,400 km for a 110 km E layer and about 3,900 km for a 300 km F layer, the right order of magnitude for the single-hop ranges radio-propagation handbooks quote, even though the true geometry curves both the ray and the reflecting layer together rather than treating either as flat. Stringing two or more such hops together, each bouncing off the ionosphere and then off the ground in turn, is how a shortwave signal can reach all the way around the world rather than stopping at one continent. ### Near-vertical skywaves A signal sent nearly straight up needs a much lower frequency to make the same return trip, since near-vertical incidence is the least forgiving angle the relationship between frequency and incidence allows; above that frequency the wave simply continues on into space rather than curving back. What a near-vertical signal loses in distance it gains in coverage close to the transmitter: because the reflection point sits almost directly overhead, the signal comes back down within a radius of at most a couple of hundred kilometres, filling in the ground around the antenna that a low-angle hop, aimed at reaching far away, skips over entirely. This near-vertical-incidence technique is used deliberately for short-range military and emergency communication across terrain, such as mountains or dense jungle, where hills defeat both a ground wave and a direct line of sight. ### Intermediate distance coverage Between the near field a ground wave still reaches and the far field a single low-angle hop first touches down lies a skip zone: a band of distance too far for the ground wave to survive and too close for that hop's reflection to land, inside which a fixed frequency and a fixed antenna deliver no usable signal at all. Closing that gap is a matter of trade-offs rather than a single fix: a lower frequency shortens the hop and pulls its landing point in closer, a more steeply aimed antenna sends more power toward near-vertical paths that return nearby, and a broadcaster wanting continuous coverage from local to regional distances typically runs more than one frequency, or more than one antenna pattern, rather than asking either extreme to do both jobs. ### Fading Skywave rarely arrives by only one path at once. A one-hop and a two-hop signal, reflections from two different layers, or even the two independently polarised rays the ionosphere's own magnetic field splits a single wave into, can all reach the same receiver over paths of very slightly different length; because radio phase is extraordinarily sensitive to path length, these copies drift in and out of step as the reflecting layer's height flexes from one moment to the next, and the sum rises and falls with them, an instance of the general phenomenon of [[Fading|fading]] that radio engineers model statistically. Where many comparable, independently varying paths combine this way, the resulting envelope is well described by the same Rayleigh-fading statistics used throughout radio engineering for multipath channels generally, a model in which the received power can drop by tens of decibels for a fraction of a second before recovering.[^ell-fading] Because the depth of a fade also depends on frequency across a signal's own bandwidth, a wide or fast-modulated signal suffers selective fading, in which parts of the channel fade while others do not, distorting the signal rather than simply weakening it; that kind of fading is the more disruptive one, and a plain volume control cannot undo it. ## Other considerations How high in frequency a hop can reach, the maximum usable frequency, and how low it can go before the signal is simply absorbed, the lowest usable frequency, both move through the day and the year with the state of the [[Ionosphere|ionosphere]] itself. During daylight a lower layer forms and absorbs much of the energy in the lower shortwave bands before it ever reaches the reflecting layers above, which is why a station audible after dark on a lower shortwave band often vanishes by mid-morning, its signal absorbed rather than merely weakened; higher shortwave bands, less affected by that daytime absorption, take over the duty by day instead. The same nightly disappearance of daytime absorption is why a distant [[AM_broadcasting|AM broadcasting]] station many hundreds of kilometres away can suddenly be heard after sunset, arriving by the same skywave path a shortwave signal takes, on a band that during the day carries only local, ground-wave signals. The reflecting layers themselves also swell and weaken across the solar cycle, so the highest frequency that will still return to Earth on a given path can be noticeably greater at a solar maximum than at a solar minimum, a swing that shortwave broadcasters and radio amateurs plan their frequency choices around season by season. Occasionally a patch of unusually dense ionization forms much lower than the normal reflecting layers, in a phenomenon called sporadic E, and reflects even the very high frequencies that ordinarily pass straight through the ionosphere into space, letting a distant television or FM station be heard, briefly and unpredictably, as clearly as a local one. Because skywave paths circle the globe rather than stopping at a horizon, a receiver tuned to a clear channel after dark also picks up every other transmitter sharing that channel from thousands of kilometres away, so the crowding on a shortwave band is itself a nightly, worldwide phenomenon rather than a local one. ## History of discovery That a radio signal could travel far beyond the horizon at all, a foundational episode in the [[History_of_radio|history of radio]], was demonstrated before anyone could explain how. ### Marconi In December 1901, [[Guglielmo_Marconi|Guglielmo Marconi]] reported receiving the Morse letter S, transmitted from a station at Poldhu, Cornwall, at a receiving station in St. John's, Newfoundland, a distance of roughly 3,500 km that no ground wave at the frequencies then in use could plausibly have covered.[^marconi] The result met scepticism precisely because contemporary theory offered no mechanism for it: a radio wave was expected to travel in a straight line or hug the ground only a short way past the horizon, not span an ocean. Within a year, Arthur Kennelly and Oliver Heaviside independently proposed, neither aware of the other's work, that a conducting or ionized layer high in the atmosphere might be reflecting the wave back down, an idea that came to bear both their names.[^kennelly-heaviside] The hypothesis stood without direct proof for two more decades, until Edward Appleton's experiments in the 1920s, which swept a transmitter's frequency and measured the interference between the ground wave and the sky wave reaching the same receiver, fixed the reflecting layer's height and confirmed its existence outright, work recognised with the Nobel Prize in Physics in 1947.[^appleton] ## Microsims This article carries no p5.js sketch of its own. The three.js companion named in the lead, a night-time preset of the shared radio-propagation scene, is the site's interactive rendering of the reflecting layer described above; several neighbouring sketches, built for other articles, also touch on pieces of what happens here. A returning echo's timing is the closest analogy: much as the Radar sketch folds an echo that arrives after the next pulse has gone out back to a false, much shorter range, a two-hop skywave signal that arrives a little later than a one-hop signal on a nearby frequency can be mistaken for the wrong path unless the extra delay is accounted for. The Sonar sketch's resolution limit, in which two close echoes merge into one hump as the ping lengthens, is a fair stand-in for what happens when a one-hop and a two-hop signal of nearly equal strength and delay arrive together and interfere rather than resolving into two distinguishable signals, part of the fading described above. The Doppler effect sketch speaks to a subtler point: a reflecting layer that is slowly rising or falling, as the ionosphere does through the day, imposes a small Doppler shift on a fixed-frequency signal reflected from it, the same bunching or stretching of wavefronts the sketch shows for a moving source, and this shift is exactly what ionospheric sounders use to track how fast the layer is moving. *Try:* in the [[Radar]] sketch, push a target's range out past the dashed unambiguous-range ring and watch its echo fold back to a false, closer range, the timing confusion a late second hop can cause here. *Try:* in the [[Sonar]] sketch, lengthen the ping until the two close targets in the zoomed inset merge into one hump, the same merging that happens when two skywave paths of nearly equal delay interfere. *Try:* in the [[Doppler_effect]] sketch, raise the source speed and watch the wavefronts bunch ahead of it, the same bunching a rising or falling ionospheric layer imposes on a reflected signal's frequency. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Skywave) : [Wikitube](https://en.wikitube.io/wiki/Skywave) Skeleton mirrored at revision 1355180365. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Ionosphere]] - [[Shortwave_radio]] - [[Radio_propagation]] - [[Ground_wave]] - [[Line-of-sight_propagation]] - [[Fading]] - [[AM_broadcasting]] - [[Over-the-horizon_radar]] ## References The refraction geometry connecting frequency, incidence angle, and hop distance is standard ionospheric-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. [^marconi]: Citation needed: a primary account of Guglielmo Marconi's December 1901 Poldhu-to-St. John's transmission (his own report or a contemporary record naming the date and distance precisely) would confirm the details given here. [^kennelly-heaviside]: Citation needed: the original 1902 papers or notices by Arthur Kennelly and Oliver Heaviside proposing a reflecting atmospheric layer would fix the exact dates and venues of each independent proposal. [^appleton]: Citation needed: Edward Appleton's 1920s papers on the frequency-swept interference experiment, and the Nobel Foundation's 1947 physics prize citation, would confirm the dates and the experiment's details as summarised here. [^ell-fading]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 163-167 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. ## Further reading - Steven Ellingson. *Radio Systems Engineering, Revised First Edition* (2023). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering ## External links This article carries no p5.js simulation of its own. 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.