Radio - Wikiparency

# Radio **Radio** is the technology of sending and receiving information by means of [[Radio_wave|radio waves]]: electromagnetic waves generated by an electronic transmitter and launched into space by an [[Antenna_(radio)|antenna]], travelling largely unhindered until a second antenna intercepts them and delivers a matching signal to a receiver. That one physical fact, that a wave can carry a pattern across empty space at the speed of light, underlies broadcasting, two-way radio, cellular telephony, wireless networking, satellite links, radar and radio navigation alike; what differs between them is only how the transmitter varies, or modulates, some property of the wave to impress information on it, and how much of the spectrum each is allotted to do so. A three.js companion to this article, a variant of the sketch built for [[Radio_spectrum]], renders that whole range as one scene with the broadcast bands lit, showing at a glance where the uses described below actually sit. The word names a set of engineering choices as much as a physical phenomenon. The sections below trace the term's own history, the nineteenth-century physics that made radio possible before anyone had built a transmitter, the principles common to every radio link, the systems built from those principles, and the regulatory structure that keeps millions of transmitters from talking over one another. ## Etymology The word borrows the Latin *radius*, a ray or spoke, the same root that gives radiation its name: a transmitter's antenna sends its signal outward in every direction at once, the way light spreads from a point source. For the technology's first three decades the common English name was "wireless telegraphy," later shortened to "wireless," a name that described what the new medium replaced (a telegraph wire) rather than what it actually was. Some early technical writers preferred "Hertzian wave telegraphy," after Heinrich Hertz, before that phrase gave way in turn. "Radio" displaced "wireless" first in American usage, spreading through the navies and licensed radio services of the 1910s before crossing into everyday speech once broadcasting made the word a household one in the 1920s; British and Commonwealth usage kept "wireless" in common speech for decades longer, and the word survives today mainly as a period piece, in the name of a "wireless set" from an old film or a museum placard.[^etym] ## History The physics came before the machine. [[James_Clerk_Maxwell|James Clerk Maxwell]]'s [[Maxwell's_equations|electromagnetic theory]] predicted in the 1860s that an oscillating electric current should radiate a wave travelling at the speed of light,[^maxwell] a prediction Heinrich Hertz confirmed in the laboratory some two decades later by building a spark transmitter and detecting the waves it threw off across a room.[^hertz] What Hertz treated as a demonstration of theory, [[Guglielmo_Marconi|Guglielmo Marconi]] treated as a business: through the 1890s he lengthened the range of spark transmitters and improved their antennas and receivers, and by the turn of the century was sending messages across the Atlantic, a feat many of his own contemporaries had judged impossible on the reasoning that radio waves, like light, should travel in straight lines and be blocked by the Earth's own curvature.[^marconi] The reflecting layer that actually carried the wave over the horizon, later named the ionosphere, was not understood until years afterward; Marconi's transatlantic signal worked before anyone could fully explain why it should. [[History_of_radio|Wireless telegraphy]] remained a dots-and-dashes medium for its first decade. Voice and music followed once a transmitter could vary a wave continuously rather than merely switch it on and off, a change usually credited to Reginald Fessenden's Christmas Eve broadcast of speech and music in 1906.[^fessenden] Commercial broadcasting arrived after the First World War, and the [[Electronics|electronics]] industry it created was rebuilt twice more during the twentieth century: once around the vacuum tube, and again after the [[Semiconductor_device|semiconductor]] made a radio receiver small enough to carry in a pocket. The Second World War pushed the technology in the opposite direction, toward higher frequencies and narrower beams, work that produced [[Radar|radar]] as radio's most consequential wartime application and left behind the microwave engineering that underlies satellite and cellular systems today. ## Principles Every radio link runs the same physics in one direction and then the other. A transmitter drives an [[Alternating_current|alternating current]] through an antenna at the frequency the wave is meant to carry; the accelerating [[Electron|electrons]] in that antenna radiate an electromagnetic wave exactly as Maxwell's equations describe, and the wave propagates away from the antenna carrying energy but no matter. At the far end the same process runs backward: the wave's oscillating field pushes electrons in a receiving antenna, inducing a tiny alternating voltage that the [[Radio_receiver|receiver]] amplifies, filters and decodes. Nothing about this exchange requires a wire, and nothing about it is unique to any one frequency; what changes across the radio spectrum is the wave's [[Radio_propagation|propagation]]: how far it travels and how readily it bends around obstacles, and how much bandwidth is available to modulate. Modulation is the half of the process that actually carries information: the transmitter varies the amplitude, frequency or phase of an otherwise steady [[Carrier_wave|carrier wave]] in step with a signal, and the receiver undoes the same variation to recover it. A receiver's usefulness is set as much by how quietly it does this as by how strongly it receives: every stage after the antenna adds some noise of its own, but textbook analysis of a receiver chain shows that the first amplifier's noise dominates almost the whole result. In one worked example, a first stage with 25 dB of gain and a noise figure of 2.7 dB leaves a lossy 4.1 dB cable connected right after it contributing only about a tenth of a decibel to the overall noise figure, which is why a sensitive receiver's design effort concentrates on the very first component the antenna feeds.[^friis] The same chain sets a hard limit on how much information a given channel can carry no matter how cleverly it is modulated, a limit expressed through the channel's [[Signal-to-noise_ratio|signal-to-noise ratio]] and named for [[Claude_Shannon|Claude Shannon]]. ## Communication systems A radio communication system is built from a transmitter, a channel and a receiver connected by a [[Communication_protocol|protocol]] that both ends agree on: how a carrier is modulated, how a channel is shared, and how errors are found and corrected. Two-way systems either take turns on one frequency or use a slightly different frequency in each direction; broadcast systems accept that the channel runs only one way and spend their design effort instead on reaching as many receivers as cheaply as possible. Standards bodies such as the [[Institute_of_Electrical_and_Electronics_Engineers|IEEE]] publish the protocols that let equipment from different manufacturers interoperate on the same band. ### Bandwidth Every modulated signal occupies a range of frequencies around its carrier, not a single one, and that range, its bandwidth, is what a receiver's filter must pass and what a regulator must set aside. Bandwidth and noise together set a hard ceiling on how much information a channel can carry: a channel of a given bandwidth and signal-to-noise ratio has a capacity in bits per second per hertz of `C = log2(1 + SNR)`, and no amount of clever modulation raises that ceiling, only how close to it a real system gets.[^shannon] A worked comparison makes the gap concrete: a 6 MHz television channel at 15 dB signal-to-noise ratio has a Shannon capacity of roughly 30 megabits per second, yet the ATSC digital television standard that actually uses a 6 MHz channel carries about 19.4 megabits per second: respectable, but well short of the theoretical ceiling, the shortfall being the price of practical, low-error modulation and coding rather than an idealized one.[^shannon] Some services shave the bandwidth they need further still: ordinary amplitude modulation spreads a signal into two mirror-image [[Sideband|sidebands]] plus the carrier, and [[Single-sideband_modulation|single-sideband]] transmission simply drops the carrier and one of the two sidebands, since neither carries information the other lacks, roughly halving the bandwidth a voice channel needs. Digital broadcasting recovers bandwidth a different way, by compressing what it sends before modulating it at all: digital radio and television both lean on [[Data_compression|data compression]], and digital video specifically on transform coding methods such as the [[Discrete_cosine_transform|discrete cosine transform]] that also underlies ordinary [[Image_compression|image compression]], to fit a usable programme stream into a channel that could never carry it unabridged. ### ITU frequency bands The radio spectrum is divided by international agreement into decade-wide bands with their own names and letter codes, from the very lowest frequencies used for submarine communication up through [[Very_high_frequency|VHF]], [[Ultra_high_frequency|UHF]] and beyond; each band's name reflects roughly what kind of propagation and antenna size is practical there rather than any difference in the physics of the wave itself. The full ladder of bands, their boundaries and what typically lives in each one is set out in [[Radio_spectrum]]; what matters here is only that a communication system's choice of band is rarely free — it is assigned, licensed or shared according to the rules in the next section. ## Regulation No two transmitters can use the same frequency in the same place without one drowning out the other, so radio has been a regulated activity from its earliest commercial days. Internationally, the International Telecommunication Union coordinates which services may use which bands and adjudicates interference between neighbouring countries; nationally, an agency licenses transmitters, assigns call signs and enforces power and emission limits within the bands the international process has allotted to that country. Licensing is only part of the job: regulators also police how much stray radio-frequency energy leaks from equipment that is not a radio transmitter at all — a motor, an ignition system, a switching power supply, because that leakage is heard by every receiver nearby as noise. The scale of the problem is not small: one widely used engineering model puts man-made background noise at 30 MHz at under 4,000 kelvin in a quiet rural area against more than a million kelvin in a city centre, a gap of better than two-hundredfold attributed mostly to electrical machinery and vehicle ignition rather than to any natural source.[^itunoise] Emission limits on ordinary electrical equipment exist largely to keep that number from climbing further. ## Applications Broadcasting was radio's first mass application and remains among its largest: [[AM_broadcasting|AM broadcasting]] and its FM and digital successors deliver audio to an unlimited number of receivers from a single transmitter, while [[Shortwave_radio|shortwave]] stations exploit the ionosphere's reflection to reach listeners a continent away on comparatively little power. Two-way systems run the same principle in both directions, from a handheld radio to a satellite uplink, and [[NASA]] and other space agencies depend on it for every command sent to, and every measurement returned from, a spacecraft beyond the reach of any wire. [[Radar]] turns the same transmitter-and-antenna arrangement into a ranging instrument by timing the echo of the radio wave the antenna sends out, an idea developed alongside broadcasting but for a wholly different purpose. At the receiving end, whatever the application, the signal a receiver recovers is usually still meant for a human ear or eye, which is why a [[Loudspeaker|loudspeaker]] or another [[Transducer|transducer]] remains the last link in the chain even in a system as sophisticated as satellite radio or wireless computer networking, all of which sit inside the wider apparatus of modern [[Telecommunications|telecommunications]] built on the same transmitter–antenna–receiver chain described above. ## Microsims This article carries no p5.js sketch of its own. A three.js companion, a variant of the sketch built for [[Radio_spectrum]], renders the whole radio spectrum as a single scene with the broadcast bands lit, placing everything discussed above — from a shortwave broadcast band to a VHF one — on one shared scale. 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 the dashed unambiguous-range ring shrinks inside a target's true range and a folded "ghost" echo appears — the same round-trip timing this article's Principles section describes for an ordinary link, pushed into a regime where the timing itself becomes the message. *Try:* in the [[Doppler_effect]] sketch, set the source moving and watch the wavefronts bunch up ahead of it and spread out behind it; the same shift in frequency is what a radar or a cellular network measures on a moving target or a moving phone. *Try:* in the [[Sonar]] sketch, compare how readily a two-way delay converts into a range reading with how radio would perform the same trick: sound crawls through water at roughly 1,500 metres per second, while a radio wave covers 1,500 metres in about five microseconds — one reason radio ranging, as in radar, needs far faster electronics than sonar does over the same distance. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Radio) : [Wikitube](https://en.wikitube.io/wiki/Radio) Skeleton mirrored at revision 1375420338. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radio_wave]] - [[Radio_spectrum]] - [[Radio_propagation]] - [[Antenna_(radio)]] - [[Radio_receiver]] - [[Single-sideband_modulation]] - [[History_of_radio]] - [[Very_high_frequency]] ## References [^maxwell]: Maxwell, J. C. "A Dynamical Theory of the Electromagnetic Field." *Philosophical Transactions of the Royal Society of London*, vol. 155, 1865, pp. 459-512. [^etym]: Citation needed: a primary record (dictionary citation or contemporary trade-press usage) fixing when "radio" overtook "wireless" in American usage. [^hertz]: Citation needed: the specific paper and date of Heinrich Hertz's first published demonstration of electromagnetic waves (1887-1888). [^marconi]: Citation needed: a primary record (Marconi Company report or contemporary press account) of the December 1901 transatlantic transmission, to confirm the date and the signal received. [^fessenden]: Citation needed: a primary or eyewitness account of Reginald Fessenden's 1906 Brant Rock broadcast, to confirm the date and its content. [^friis]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 102-103 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^shannon]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 160-161 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^itunoise]: 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 - Steven Ellingson. *Radio Systems Engineering - Revised First Edition*. 2023. CC BY-NC. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering - Don Johnson. *Fundamentals of Electrical Engineering I*. 2014. CC BY. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 ## External links - Authoritative frequency-allocation and spectrum-regulation references (for example the ITU Radio Regulations and national regulators' band plans), to be pinned once specific editions are confirmed.