Shortwave_radio - Wikiparency

# Shortwave radio **Shortwave radio** owes its name to a historical accident of expectation: engineers in radio's first decades assumed that only the longest wavelengths could travel far, so they set aside wavelengths below about 100 metres as being of little use for distance, and called that leftover territory the "shortwave" band. The label stuck even after the assumption behind it was overturned. Shortwave has no single official boundary, but every definition of it includes the whole high-frequency (HF) range, 3 to 30 megahertz, a stretch of the wider [[Radio_spectrum]] wedged above the medium-frequency band that carries [[AM_broadcasting|AM broadcasting]] and below the first rung of [[Very_high_frequency|VHF]]. What actually sets shortwave apart is the [[Ionosphere|ionosphere]]: aimed upward at the right angle, a shortwave signal refracts back down from this charged layer far beyond the horizon that limits [[Line-of-sight_propagation|line-of-sight]] bands such as [[Ultra_high_frequency|UHF]] and [[Microwave|microwave]]. This [[Skywave|skywave]], or "skip," propagation can carry a signal across an ocean, or pass it from one ionospheric bounce to the next most of the way around the planet, all on a fraction of the transmitter power a line-of-sight service would need for the same distance. Shortwave has carried international broadcasting, amateur two-way contacts, and military, diplomatic and marine and aviation traffic for a century, and still does, even where satellites and the internet have taken over much of what it once carried alone. A three.js sketch elsewhere on this site renders a skywave ray bending back from the ionosphere at night, when the skip zone that shortwave broadcasters depend on opens widest. ## History ### Development For radio's first two decades, engineers assumed that reliable long-distance communication demanded the longest wavelengths a transmitter could generate: [[Guglielmo_Marconi|Marconi]]'s own transatlantic stations of the early 1900s ran on wavelengths of kilometres, and shortwaves, then anything below about 200 metres, were considered nearly useless and left to amateur experimenters as the only part of the spectrum regulators did not want. That assumption reversed within a single decade. Pushed toward shorter wavelengths by rules that reserved the long waves for commercial and government stations, amateur radio operators on both sides of the Atlantic found, in the early 1920s, that transmissions on 200 metres and below could be heard at transatlantic and even global distances on a fraction of the power the long-wave stations used.[^dev1921] Marconi himself turned to short waves for commercial service later in the decade, building directional "beam" stations that used the same skywave mechanism to link Britain with its empire on far less power and antenna than a long-wave station of comparable range would have needed.[^devbeam] By the early 1930s shortwave, not the long wave, was the medium of choice for long-distance point-to-point and international broadcast service, a reversal complete enough that long-wave transmission survives today only in a few specialised, low-data-rate niches. ### Amateur use of shortwave propagation Amateur radio operators did much of the early exploration of what shortwave could do, precisely because regulators had pushed them there expecting the band to be of little commercial value. Two-way contacts across the Atlantic on wavelengths near 100 metres, achieved by amateurs working with far less power and cruder equipment than any commercial station, were what actually demonstrated skywave's reach to a sceptical engineering community.[^hamtests] Amateurs went on to map out much of what is now standard propagation knowledge empirically, band by band and hour by hour, well before ionospheric physics could explain it: which bands opened at dawn and dusk, which stayed open all night, and how the roughly eleven-year sunspot cycle raised and lowered the ceiling on which frequency would still refract back to Earth. The hobby's traditional pursuit of contacting as many, and as distant, stations as possible (software-defined receivers have since joined the vacuum-tube and transistor sets amateurs used earlier) still produces some of the most systematic day-to-day observation of HF conditions available outside a research ionosonde network. ## Propagation characteristics Shortwave's defining property is that the ionosphere, layers of the upper atmosphere ionised chiefly by solar ultraviolet and X-ray radiation, refracts HF waves enough to bend them back toward the ground rather than letting them pass through into space. A wave launched upward at a shallow enough angle re-enters the atmosphere hundreds to thousands of kilometres from the transmitter, and can reflect off the ground and refract a second or third time off the ionosphere for multi-hop paths that reach the far side of the Earth. Which frequencies refract back depends on how densely ionised the layer is and how obliquely the wave strikes it: a frequency that passes straight through at vertical incidence, above the layer's critical frequency, 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: the secant law. Both quantities move with the time of day, the season and the sunspot cycle, since all three change how much ionising radiation the upper atmosphere receives; propagation examined in general, including the free-space and line-of-sight modes shortwave contrasts with, is the subject of [[Radio_propagation]]. Between the transmitter and the first point where a sky wave returns to Earth lies a skip zone in which the ground wave has already died away and the sky wave has not yet come down: a region where the station, despite ample power, cannot be heard at all, while listeners farther away receive it clearly. Skip distance and MUF both shrink after dark, when solar ionisation of the lower layers fades and low frequencies that were absorbed by day suddenly refract efficiently, which is why the same shortwave frequency can behave completely differently between afternoon and midnight, and why international broadcasters historically retuned to a lower frequency for their night-time service on the same daily schedule. ## Types of modulation ### Audio modes #### AM Ordinary double-sideband [[Amplitude_modulation|amplitude modulation]] has carried shortwave broadcasting since its beginning and remains the default mode for international broadcasters, chosen because a cheap receiver with no more than a diode detector can recover it. #### SSB [[Single-sideband_modulation|Single-sideband]] transmission drops the carrier and one of AM's two mirror-image sidebands, since neither carries information the other lacks, concentrating a transmitter's power into half the bandwidth and giving roughly the same effective range on a fraction of the transmitter power. Amateur, marine and aviation voice traffic on shortwave has used SSB almost exclusively since the mid-twentieth century. #### VSB Vestigial-sideband transmission is AM's compromise between the two: most of one sideband is filtered away but a vestige of it is kept to simplify receiver design, a technique more associated with analogue television video than with shortwave voice, though some shortwave data and facsimile services have used it. #### NFM Narrowband [[Frequency_modulation|frequency modulation]] appears on shortwave far less than on VHF and above, since a wide FM signal needs more bandwidth than a skywave channel reliably supports, but some short-range shortwave utility and amateur links use a narrow FM variant for its resistance to the amplitude fading that plagues AM and SSB on a fluctuating ionospheric path. #### DRM Digital Radio Mondiale digitises the broadcast far more thoroughly, coding audio and multiplexing it onto a set of orthogonal subcarriers so a receiver can reconstruct a clean signal from a shortwave channel too faded and noisy for analogue AM to survive, at the cost of a sharp cliff below which the signal is unusable rather than merely noisy.[^drm] ### Data modes #### CW Continuous-wave telegraphy (switching an unmodulated carrier on and off to send Morse code, a form of [[Amplitude-shift_keying|amplitude-shift keying]]) is shortwave's oldest data mode and still among its most robust: a trained ear can copy CW through fading and noise that would bury a voice signal completely, which keeps it in use among amateurs and some utility and military stations long after more data-efficient modes became available. #### RTTY, FAX, SSTV Radioteletype (RTTY) sends text by switching between two tones, a form of [[Frequency-shift_keying|frequency-shift keying]] that early mechanical teleprinters could decode directly; radiofax sends a slowly scanned image, historically weather charts transmitted to ships at sea, as a tone whose pitch tracks brightness; and slow-scan television (SSTV) applies the same slow-scan idea to still photographs exchanged mostly by amateurs. All three trade speed for a signal narrow and robust enough to survive an HF skywave path. ## Users Amateur radio operators remain shortwave's largest population of everyday users, but far from the only one. International broadcasters use the band to reach audiences across borders that satellite and internet distribution cannot, or that a government would rather not have to negotiate access through. Military and diplomatic services value shortwave precisely because it needs no satellite, cable or local infrastructure at all: a transmitter and a wire antenna are enough to reach the other side of the world, which is also why over-the-horizon backscatter [[Over-the-horizon_radar|radar]] (bouncing a shortwave pulse off the ionosphere to see targets beyond the geometric horizon rather than listening for one directly) was built on the same propagation described above. Marine and long-distance aviation crews used shortwave voice and data for decades before satellite links became affordable, and utility and numbers stations of uncertain origin still occupy parts of the band, a minor mystery that shortwave listeners enjoy chasing for its own sake. ## Shortwave broadcasting ### Frequency allocations International broadcasters are assigned a handful of shortwave bands set aside by international agreement, conventionally named for their approximate wavelength in metres (the 49, 41, 31, 25, 19, 16, 13 and 11 metre bands among them), chosen to sit in parts of the HF spectrum that reliably support skywave propagation at different times of day and phases of the solar cycle.[^swbands] A broadcaster typically holds several frequencies across different bands and switches between them through the day and the season as the MUF and absorption conditions described above shift which band actually reaches the target audience. ### Advantages Shortwave's chief advantage is reach for cost: a single transmitter and antenna, without a chain of relay towers, a satellite transponder or undersea cable capacity, can reach an entire continent, which made it for decades the only practical way to broadcast into a country that would not permit a local relay. It is also comparatively resistant to the kind of infrastructure failure or deliberate disruption that can take a terrestrial or internet-delivered service off the air within its own borders, since the transmitter and the audience's receivers can sit entirely outside the jurisdiction being reached. ### Disadvantages The same ionosphere that gives shortwave its reach also gives it its unreliability: fading, atmospheric and man-made noise, and a MUF that can close a path for hours are all normal, and a broadcaster has no equivalent of a line-of-sight link's steady signal. Audio quality is correspondingly modest even under good conditions, adequate for speech and tolerable for music but well short of an FM or digital broadcast, and a worthwhile signal at the receiver still typically needs a transmitter power, and an antenna, well beyond what a domestic FM station requires for comparable local coverage. ## Shortwave listening Shortwave listening, or SWLing, is the hobby of receiving rather than transmitting: tuning across the broadcast and utility bands to log distant stations, a pursuit that predates amateur transmitting licences and needs none of its own. Listeners traditionally confirmed a reception by writing to the station and receiving a QSL card in return, a paper record that still carries some prestige among collectors even as broadcasters increasingly confirm receptions by e-mail instead. Chasing distant, weak or rare stations under difficult conditions, DXing, rewards exactly the propagation quirks described above — skip zones, sporadic band openings, a frequency that unexpectedly comes alive after dark — that make shortwave frustrating for a broadcaster trying to guarantee steady coverage, and the same logbook discipline that once served DXers alone now feeds informal, crowd-sourced propagation reports that amateurs and broadcasters both consult. ## Shortwave broadcasts and music International broadcasting built much of its early audience on music and cultural programming as much as on news, since a shared symphony or folk broadcast crossed language barriers that spoken commentary could not, and state broadcasters used shortwave music programming as soft-power outreach throughout the [[Cold_War|Cold War]][^coldwarsw] alongside more overtly political content. Religious broadcasters have long been among the heaviest users of international shortwave capacity, reaching audiences with neither reliable terrestrial radio nor an internet connection of their own. The medium's own sound, the flutter of multipath fading and the rise and fall of signal strength as the ionosphere shifts beneath a broadcast, became a recognisable aesthetic in its own right for listeners who grew up with it, closer to a weather report on the state of the sky than a flaw in the transmission. ## Shortwave's future International shortwave broadcasting contracted sharply from the 1990s onward as the Cold War audiences that had justified much of it disappeared and satellite and internet distribution offered cheaper, higher-quality alternatives for reaching listeners who had access to either.[^swdecline] The band remains relevant precisely where those alternatives do not reach: regions with limited internet infrastructure, disaster and emergency communication when local networks fail, and the amateur and utility services that were never dependent on a broadcasting audience in the first place. Digital modes such as DRM aim to keep shortwave broadcasting competitive on audio quality without giving up its reach, though converting an existing audience of simple analogue receivers to digital-capable ones has been the slower half of that bargain, and the [[Radio_receiver|receiver]] population, more than the transmitter network, is what will decide how much of shortwave broadcasting survives the transition. ## Microsims This article carries no p5.js sketch of its own. A three.js sketch elsewhere on this site renders a night-time HF skywave: a ray leaving the ground, bending back from the ionosphere, and landing far enough away to show how the skip zone described above opens and closes as the frequency changes. 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 and watch a distant target's echo fold into a closer-looking "ghost" — the same pulse-timing arithmetic that an over-the-horizon radar applies to a skywave path bounced off the ionosphere instead of a straight line through the air. *Try:* in the [[Doppler_effect]] sketch, set the source moving and watch the received frequency shift ahead of it and behind it; a slowly heaving ionospheric layer imposes a much smaller version of the same shift on a shortwave signal, a technique used to sound the ionosphere's own motion. *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 shortwave receiver that assumes a fixed ionospheric height makes exactly the same kind of error whenever the layer is actually higher or lower than assumed. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Shortwave_radio) : [Wikitube](https://en.wikitube.io/wiki/Shortwave_radio) Skeleton mirrored at revision 1368309552. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Skywave]] - [[Ionosphere]] - [[Radio_propagation]] - [[Radio_spectrum]] - [[Ultra_high_frequency]] - [[Amplitude_modulation]] - [[Single-sideband_modulation]] - [[Guglielmo_Marconi]] ## References Standard propagation physics used here (ionospheric refraction, the secant law relating vertical and oblique critical frequencies, and the skip zone) is textbook material used throughout radio engineering and is not separately footnoted, per the Wikitube style guide's §6.1; it is developed in more depth in this portal's Radio propagation and Ionosphere articles. Page numbers below are PDF pages of the open editions linked from External links and this portal's book shelf. [^dev1921]: Citation needed: a primary account (an ARRL Transatlantic Tests report or a participant's log) of the amateur transatlantic reception tests of December 1921, to confirm the date, the wavelength and the stations involved. [^devbeam]: Citation needed: a Marconi Company record of the shortwave "beam wireless" stations built to link Britain with its empire in the mid-1920s, to confirm the dates and route. [^hamtests]: Citation needed: a primary account of the first two-way amateur transatlantic contacts on wavelengths near 100 metres, generally dated to late 1923, to confirm the stations, date and wavelength. [^drm]: Citation needed: the current Digital Radio Mondiale technical standard, to confirm the coding and multiplexing details quoted here. [^swbands]: Citation needed: the ITU Radio Regulations' table of shortwave broadcasting bands, to confirm the exact metre-band and frequency boundaries quoted here. [^coldwarsw]: Citation needed: a history of Cold War international broadcasting confirming the role and scale of music and cultural programming alongside news and political content. [^swdecline]: Citation needed: a broadcaster's or regulator's own record of shortwave transmitter hours or station closures from the 1990s onward, to confirm the scale of the contraction described here. ## External links - Authoritative shortwave band-plan and international-broadcasting references (for example the ITU Radio Regulations and a current frequency-guide publication), to be pinned once specific editions are confirmed.