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An antenna farm hosting various radio antennas on Sandia Peak near Albuquerque, New Mexico, United States

Radio is the technology of communicating using radio waves.[1][2][3] Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves. They can be received by other antennas connected to a radio receiver; this is the fundamental principle of radio communication. In addition to communication, radio is used for radar, radio navigation, remote control, remote sensing, and other applications.

In radio communication, used in radio and television broadcasting, cell phones, two-way radios, wireless networking, and satellite communication, among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by modulating the radio signal (impressing an information signal on the radio wave by varying some aspect of the wave) in the transmitter. In radar, used to locate and track objects like aircraft, ships, spacecraft and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, and the reflected waves reveal the object's location to a receiver that is typically colocated with the transmitter. In radio navigation systems such as GPS and VOR, a mobile navigation instrument receives radio signals from multiple navigational radio beacons whose position is known, and by precisely measuring the arrival time of the radio waves the receiver can calculate its position on Earth. In wireless radio remote control devices like drones, garage door openers, and keyless entry systems, radio signals transmitted from a controller device control the actions of a remote device.

The existence of radio waves was first proven by German physicist Heinrich Hertz on 11 November 1886.[4] In the mid-1890s, building on techniques physicists were using to study electromagnetic waves, Italian physicist Guglielmo Marconi developed the first apparatus for long-distance radio communication,[5] sending a wireless Morse Code message to a recipient over a kilometer away in 1895,[6] and the first transatlantic signal on 12 December 1901.[7] The first commercial radio broadcast was transmitted on 2 November 1920, when the live returns of the 1920 United States presidential election were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA.[8]

The emission of radio waves is regulated by law, coordinated by the International Telecommunication Union (ITU), which allocates frequency bands in the radio spectrum for various uses.

Etymology

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The word radio is derived from the Latin word radius, meaning "spoke of a wheel, beam of light, ray." It was first applied to communications in 1881 when, at the suggestion of French scientist Ernest Mercadier [fr], Alexander Graham Bell adopted radiophone (meaning "radiated sound") as an alternate name for his photophone optical transmission system.[9][10]

Following Hertz's discovery of the existence of radio waves in 1886, the term Hertzian waves was initially used for this radiation.[11] The first practical radio communication systems, developed by Marconi in 1894–1895, transmitted telegraph signals by radio waves,[4] so radio communication was first called wireless telegraphy. Up until about 1910 the term wireless telegraphy also included a variety of other experimental systems for transmitting telegraph signals without wires, including electrostatic induction, electromagnetic induction and aquatic and earth conduction, so there was a need for a more precise term referring exclusively to electromagnetic radiation.[12][13]

The French physicist Édouard Branly, who in 1890 developed the radio wave detecting coherer, called it in French a radio-conducteur.[14][15] The radio- prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe. For example, in early 1898 the British publication The Practical Engineer included a reference to the radiotelegraph and radiotelegraphy.[14][16]

The use of radio as a standalone word dates back to at least 30 December 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions."[14][17] This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included a Service Regulation specifying that "Radiotelegrams shall show in the preamble that the service is 'Radio'".[14]

The switch to radio in place of wireless took place slowly and unevenly in the English-speaking world. Lee de Forest helped popularize the new word in the United States—in early 1907, he founded the DeForest Radio Telephone Company, and his letter in the 22 June 1907 Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced."[18] The United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms wireless telegraph and wireless telegram, by 1912 it began to promote the use of radio instead. The term started to become preferred by the general public in the 1920s with the introduction of broadcasting.

History

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See also: History of radio, Invention of radio, Timeline of radio, and History of broadcasting

Electromagnetic waves were predicted by James Clerk Maxwell in his 1873 theory of electromagnetism, now called Maxwell's equations, who proposed that a coupled oscillating electric field and magnetic field could travel through space as a wave, and proposed that light consisted of electromagnetic waves of short wavelength. On 11 November 1886, German physicist Heinrich Hertz, attempting to confirm Maxwell's theory, first observed radio waves he generated using a primitive spark-gap transmitter.[4] Experiments by Hertz and physicists Jagadish Chandra Bose, Oliver Lodge, Lord Rayleigh, and Augusto Righi, among others, showed that radio waves like light demonstrated reflection, refraction, diffraction, polarization, standing waves, and traveled at the same speed as light, confirming that both light and radio waves were electromagnetic waves, differing only in frequency.[19] In 1895, Guglielmo Marconi developed the first radio communication system, using a spark-gap transmitter to send Morse code over long distances. By December 1901, he had transmitted across the Atlantic Ocean.[4][5][6][7] Marconi and Karl Ferdinand Braun shared the 1909 Nobel Prize in Physics "for their contributions to the development of wireless telegraphy".[20]

During radio's first two decades, called the radiotelegraphy era, the primitive radio transmitters could only transmit pulses of radio waves, not the continuous waves which were needed for audio modulation, so radio was used for person-to-person commercial, diplomatic and military text messaging. Starting around 1908 industrial countries built worldwide networks of powerful transoceanic transmitters to exchange telegram traffic between continents and communicate with their colonies and naval fleets. During World War I the development of continuous wave radio transmitters, rectifying electrolytic, and crystal radio receiver detectors enabled amplitude modulation (AM) radiotelephony to be achieved by Reginald Fessenden and others, allowing audio to be transmitted. On 2 November 1920, the first commercial radio broadcast was transmitted by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA featuring live coverage of the 1920 United States presidential election.[8]

Technology

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Radio waves are radiated by electric charges undergoing acceleration.[21][22] They are generated artificially by time-varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna.[23][24]

As they travel farther from the transmitting antenna, radio waves spread out so their signal strength (intensity in watts per square meter) decreases (see Inverse-square law), so radio transmissions can only be received within a limited range of the transmitter, the distance depending on the transmitter power, the antenna radiation pattern, receiver sensitivity, background noise level, and presence of obstructions between transmitter and receiver. An omnidirectional antenna transmits or receives radio waves in all directions, while a directional antenna transmits radio waves in a beam in a particular direction, or receives waves from only one direction.[25][26][27][28]

Radio waves travel at the speed of light in vacuum[29] and at slightly lower velocity in air.[30]

The other types of electromagnetic waves besides radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays, can also carry information and be used for communication. The wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their longer wavelength.[24] Radio waves have the ability to pass through the atmosphere in any weather, foliage, and at longer wavelengths through most building materials. By diffraction, longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength.

Communication systems

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Radio communication. Information such as sound is converted by a transducer such as a microphone to an electrical signal, which modulates a radio wave produced by the transmitter. A receiver intercepts the radio wave and extracts the information-bearing modulation signal, which is converted back to a human usable form with another transducer such as a loudspeaker.
Comparison of AM and FM modulated radio waves

In radio communication systems, information is carried across space using radio waves. At the sending end, the information to be sent is converted by some type of transducer to a time-varying electrical signal called the modulation signal.[24][31] The modulation signal may be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal consisting of a sequence of bits representing binary data from a computer. The modulation signal is applied to a radio transmitter. In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the carrier wave because it serves to generate the radio waves that carry the information through the air. The modulation signal is used to modulate the carrier, varying some aspect of the carrier wave, impressing the information in the modulation signal onto the carrier. Different radio systems use different modulation methods:[32]

Many other types of modulation are also used. In some types, the carrier wave is suppressed, and only one or both modulation sidebands are transmitted.[34]

The modulated carrier is amplified in the transmitter and applied to a transmitting antenna which radiates the energy as radio waves. The radio waves carry the information to the receiver location.[35] At the receiver, the radio wave induces a tiny oscillating voltage in the receiving antenna – a weaker replica of the current in the transmitting antenna.[24][31] This voltage is applied to the radio receiver, which amplifies the weak radio signal so it is stronger, then demodulates it, extracting the original modulation signal from the modulated carrier wave. The modulation signal is converted by a transducer back to a human-usable form: an audio signal is converted to sound waves by a loudspeaker or earphones, a video signal is converted to images by a display, while a digital signal is applied to a computer or microprocessor, which interacts with human users.[32]

The radio waves from many transmitters pass through the air simultaneously without interfering with each other because each transmitter's radio waves oscillate at a different frequency, measured in hertz (Hz), kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The receiving antenna typically picks up the radio signals of many transmitters. The receiver uses tuned circuits to select the radio signal desired out of all the signals picked up by the antenna and reject the others. A tuned circuit acts like a resonator, similar to a tuning fork.[31] It has a natural resonant frequency at which it oscillates. The resonant frequency of the receiver's tuned circuit is adjusted by the user to the frequency of the desired radio station; this is called tuning. The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by the tuned circuit and not passed on.[36]

Bandwidth

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Frequency spectrum of a typical modulated AM or FM radio signal. It consists of a component C at the carrier wave frequency with the modulated information contained in two narrow bands of frequencies called sidebands (SB) just above and below the carrier frequency. The bandwidth (BW) is the amount of spectrum occupied by the sidebands.

A modulated radio wave, carrying an information signal, occupies a range of frequencies. The information in a radio signal is usually concentrated in narrow frequency bands called sidebands (SB) just above and below the carrier frequency. The width in hertz of the frequency range that the radio signal occupies, the highest frequency minus the lowest frequency, is called its bandwidth (BW).[32][37] For any given signal-to-noise ratio, a given bandwidth can carry the same amount of information regardless of where in the radio frequency spectrum it is located; bandwidth is a measure of information-carrying capacity. The bandwidth required by a radio transmission depends on the data rate of the information being sent, and the spectral efficiency of the modulation method used; how much data it can transmit in each unit of bandwidth. Different types of information signals carried by radio have different data rates. For example, a television signal has a greater data rate than an audio signal.[32][38]

The radio spectrum, the total range of radio frequencies that can be used for communication in a given area, is a limited resource.[37][3] Each radio transmission occupies a portion of the total spectrum available. Radio spectrum is regarded as an economic good which has a monetary cost and is in increasing demand. In some parts of the radio spectrum, the right to use a frequency band or even a single radio channel is bought and sold for millions of dollars. So there is an incentive to employ technology to minimize the spectrum used by radio services.[38]

A slow transition from analog to digital radio transmission technologies began in the late 1990s.[39][40] Part of the reason for this is that digital modulation can transmit more information in a given bandwidth than analog modulation; the modulation itself is more efficient and loss compression further improves efficiency. Digital modulation also has greater noise immunity than analog, associated digital signal processors have more power and flexibility than analog circuits, and a wide variety of information can be transmitted using the same digital modulation.[32]

Because it is a fixed resource which is in demand by an increasing number of users, the radio spectrum has become increasingly congested in recent decades, and the need to use it more effectively is driving many additional radio innovations such as trunked radio systems, spread spectrum (ultra-wideband) transmission, frequency reuse, dynamic spectrum management, frequency pooling, and cognitive radio.[38]

ITU frequency bands

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The ITU arbitrarily divides the radio spectrum into 12 bands, each beginning at a wavelength which is a power of ten (10n) metres, with corresponding frequency of 3 times a power of ten, and each covering a decade of frequency or wavelength.[3][41] Each of these bands has a traditional name:[42]

Band name Abbreviation Frequency Wavelength
Extremely
low frequency		 ELF 3–30 Hz 100,000–10,000 km
Super
low frequency		 SLF 30–300 Hz 10,000–1,000 km
Ultra
low frequency		 ULF 300–3,000 Hz 1,000–100 km
Very
low frequency		 VLF 3–30 kHz 100–10 km
Low
frequency		 LF 30–300 kHz 10–1 km
Medium
frequency		 MF 300–3,000 kHz 1,000–100 m
Band name Abbreviation Frequency Wavelength
High
frequency		 HF 3–30 MHz 100–10 m
Very
high frequency		 VHF 30–300 MHz 10–1 m
Ultra
high frequency		 UHF 300–3,000 MHz 100–10 cm
Super
high frequency		 SHF 3–30 GHz 10–1 cm
Extremely
high frequency		 EHF 30–300 GHz 10–1 mm
Tremendously
high frequency		 THF 300–3,000 GHz
(0.3–3.0 THz)		 1.0–0.1 mm

It can be seen that the bandwidth, the absolute range of frequencies, contained in each band is not equal but increases exponentially as the frequency increases; each band contains ten times the bandwidth of the preceding band.[43]

Though not defined by the ITU,[42] the term tremendously low frequency (TLF) has been used for wavelengths from 1–3 Hz (300,000–100,000 km).[44]

Regulation

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Further information: Radio regulation

The airwaves are a resource shared by many users. Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly.[37] Interference with radio transmissions can not only have a large economic cost, but it can also be life-threatening (for example, in the case of interference with emergency communications or air traffic control).[45][46]

To prevent interference between different users, the emission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU), which allocates bands in the radio spectrum for different uses.[37][3] Radio transmitters must be licensed by governments, under a variety of license classes depending on use, and are restricted to certain frequencies and power levels. In some classes, such as radio and television broadcasting stations, the transmitter is given a unique identifier consisting of a string of letters and numbers called a call sign, which must be used in all transmissions.[47] In order to adjust, maintain, or internally repair radiotelephone transmitters, individuals must hold a government license, such as the general radiotelephone operator license in the US, obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation.[48]

Exceptions to the above rules allow the unlicensed operation by the public of low power short-range transmitters in consumer products such as cell phones, cordless phones, wireless devices, walkie-talkies, citizens band radios, wireless microphones, garage door openers, and baby monitors. In the US, these fall under Part 15 of the Federal Communications Commission (FCC) regulations. Many of these devices use the ISM bands, a series of frequency bands throughout the radio spectrum reserved for unlicensed use. Although they can be operated without a license, like all radio equipment these devices generally must be type-approved before the sale.[49]

Radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals. Jamming devices are called "signal suppressors" or "interference generators" or just jammers.[9]

During wartime, militaries use jamming to interfere with enemies' tactical radio communication. Since radio waves can pass beyond national borders, some totalitarian countries which practice censorship use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries. Jamming is usually accomplished by a powerful transmitter which generates noise on the same frequency as the target transmitter.[10][11]

US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices, including ones that interfere with GPS, cellular, Wi-Fi and police radars.[15]

Applications

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Main article: Radio applications
See also: Radio spectrum § Applications, and Radio receiver § Applications

Radio has many practical applications, which include broadcasting, voice communication, data communication, radar, radiolocation, and remote control.

See also

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References

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General references

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Radio

View on Grokipedia
from Grokipedia
Radio is a technology for transmitting and receiving , such as , images, and , using radio waves—a type of with the longest wavelengths in the , typically ranging from about 1 millimeter to 100 kilometers. These waves, with frequencies from 3 kilohertz (kHz) to 300 gigahertz (GHz), propagate through the atmosphere and , enabling communication over vast distances without physical connections. In a basic radio system, a transmitter modulates a with the signal, an antenna radiates it, and a receiver demodulates the wave to extract the original content. The development of radio began in the late 19th century, building on James Clerk Maxwell's 1860s predictions of electromagnetic waves. experimentally confirmed their existence in 1887–1888 through spark-gap transmissions, proving waves could be generated, detected, and reflected like light. advanced practical , patenting a system in 1896 and achieving the first transatlantic signal in 1901, which revolutionized maritime communication—most notably saving lives during the 1912 Titanic disaster. Key innovations followed, including Reginald Fessenden's 1906 amplitude-modulation (AM) voice broadcast and Lee de Forest's 1906 for amplification, paving the way for widespread audio transmission. Edwin Armstrong's inventions in the 1910s–1930s, such as the and (FM), improved signal quality and reduced interference, while regulatory bodies like the U.S. (established 1927) managed spectrum allocation. Radio's applications span , where AM and FM stations deliver news, music, and to billions; for , maritime, and services; mobile technologies like cellular phones, first demonstrated in and commercialized in ; and wireless networks including and . It also powers for and detection, systems for global positioning (GPS), and scientific tools like radio telescopes for astronomy. In space exploration, radio enables deep-space probes to send data back to Earth, as with NASA's missions. Despite digital advancements, radio remains essential for its reliability, low cost, and ability to penetrate obstacles, evolving into for efficient spectrum use in modern networks.

History

Early discoveries and experiments

The foundations of radio technology trace back to key electromagnetic discoveries in the . In 1831, demonstrated , showing that a changing could induce an electric current in a nearby conductor, laying the groundwork for later developments in electrical signaling. This principle, verified through experiments with coils wrapped around an iron ring, established the reciprocal relationship between and essential for wireless transmission. Building on Faraday's work, James Clerk Maxwell theoretically predicted the existence of electromagnetic waves in 1865. In his paper "A Dynamical Theory of the Electromagnetic Field," Maxwell unified and into a set of four equations that described how electric and magnetic fields propagate through space at the , implying the possibility of self-sustaining waves. These equations are: ∇⋅D=ρ,∇⋅B=0,∇×E=−∂B∂t,∇×H=J+∂D∂t.\begin{align} \nabla \cdot \mathbf{D} &= \rho, \\ \nabla \cdot \mathbf{B} &= 0, \\ \nabla \times \mathbf{E} &= -\frac{\partial \mathbf{B}}{\partial t}, \\ \nabla \times \mathbf{H} &= \mathbf{J} + \frac{\partial \mathbf{D}}{\partial t}. \end{align} Maxwell's formulation suggested that varying electric currents could generate propagating waves, a concept that directly foreshadowed radio waves, though experimental confirmation was still decades away. The first experimental verification came in 1887 from , who confirmed Maxwell's predictions by generating and detecting electromagnetic waves in his laboratory. Using a —consisting of an to create high-voltage sparks across a gap in a wire loop—and a simple loop receiver tuned to resonate at the same , Hertz produced waves with wavelengths of about 1 to 10 meters and observed their reflection, , and interference, mirroring light's properties. These experiments, conducted over distances of up to 12 meters, proved that electromagnetic disturbances could travel through air without wires, validating the wave nature of electricity. The Alexander Graham Bell's invention of the telephone in 1876, which transmitted voice over wires using varying currents, further inspired ideas for wireless communication by highlighting the potential of electrical signals to carry information. Guglielmo Marconi advanced these concepts into practical wireless telegraphy starting in 1894, when he began experiments transmitting Morse code signals over short distances using improved spark transmitters and grounded antennas. By 1896, Marconi secured patents for his apparatus in Britain (No. 12039, filed June 2, 1896) and Italy, enabling reliable signaling up to several kilometers. His breakthrough culminated in 1901 with the first transatlantic transmission of the letter "S" in Morse code from Poldhu, Cornwall, to St. John's, Newfoundland, covering 3,400 kilometers despite early challenges like signal attenuation, which weakened waves over long distances due to spreading and absorption. This achievement marked the shift from laboratory curiosity to viable long-range communication, though attenuation limited early systems to line-of-sight or short-hop relays. However, the invention of radio involved contributions from multiple inventors. developed key principles of tuned circuits in his 1897 US patent application (No. 645,576, granted 1900), which the recognized in 1943 as predating Marconi's claims for the fundamental radio . also demonstrated wireless transmission in 1894 and patented a syntonic system in 1897, influencing later developments in selective tuning.

Commercialization and expansion

The commercialization of radio began in the late with Guglielmo Marconi's efforts to transform experimental into a practical . In 1900, Marconi secured British patent No. 7777 for tuned electrical circuits (syntonic system), which improved selectivity and range in wireless transmissions, enabling more reliable communication over distances. That same year, he founded the Wireless Telegraph and Signal Company in , later reorganized as the Marconi International Marine Communication Company, to market wireless equipment primarily for maritime use. A key technological enabler was Lee de Forest's invention of the in 1906, a that provided the first practical amplification of weak radio signals, dramatically extending reception capabilities and paving the way for voice broadcasting. A pivotal milestone in radio's expansion occurred on December 24, 1906, when achieved the first (AM) voice transmission from his station in Brant Rock, , broadcasting speech and music that was received over 1,000 miles away. This demonstrated radio's potential for entertainment beyond . The technology's life-saving role was underscored by the 1912 RMS Titanic disaster, where operators sent distress signals that alerted nearby ships, saving hundreds of lives despite the tragedy's 1,500 fatalities. In response, the International Radiotelegraphic Convention of 1912 mandated continuous radio watch on large passenger ships and established distress frequencies, while the U.S. required licensed operators and equipment on vessels over 300 tons. The 1920s marked the explosive growth of as a mass medium. On November 2, 1920, Westinghouse's KDKA in aired the first scheduled commercial broadcast, covering the Harding-Cox presidential election results to an audience of enthusiasts, initiating regular programming that included news, music, and sports. This spurred the formation of national networks: the launched in November 1926 under RCA ownership, linking 20 stations via telephone lines for simultaneous coast-to-coast broadcasts; the followed in September 1927, initially as a of 16 stations to compete with NBC's dominance. By the decade's end, these networks had standardized programming, with and affiliates reaching millions through sponsored shows. Advancements continued with AT&T's inauguration of the first commercial transatlantic telephone service on January 7, 1927, using beams between New York and , costing $75 for three minutes and enabling real-time voice communication across oceans. During the Great Depression of the 1930s, radio became an indispensable source of affordable entertainment, with receiver prices dropping to under $10 by 1935, allowing 70% of U.S. households to tune into free programs like soap operas, comedies, and FDR's , which provided and information amid economic hardship. This era solidified radio's societal role, with networks expanding to over 600 stations by 1935.

Digital and modern advancements

In the post-World War II era, radio technology saw significant advancements beginning with Edwin Armstrong's invention of (FM) in 1933, which provided superior audio quality and resistance to static compared to (AM). FM's adoption accelerated in the 1940s, with the first commercial FM station launching in 1939 and the Yankee Network expanding its use across by 1940, leading to widespread regulatory approval and infrastructure growth for high-fidelity broadcasting. The marked a pivotal shift with the transistor's replacement of vacuum tubes in radio devices, enabling compact, portable designs that consumed less power and offered greater reliability. This innovation, exemplified by the 1954 Regency TR-1—the first commercial —revolutionized consumer access, making radios affordable and mobile for everyday use. Digital radio standards emerged in the late to enhance audio fidelity and efficiency. In , Digital Audio Broadcasting (DAB) was first demonstrated in 1985 and publicly in 1988, with commercial rollout in the mid-1990s, such as the UK's launch in 1995, offering CD-quality sound and multiplexed channels over terrestrial networks. In the United States, HD Radio debuted in the early 2000s, providing in-band digital sidebands alongside analog FM for improved reception and datacasting without requiring new spectrum allocations. Concurrently, satellite-based systems like Sirius (licensed in 1997 and launched in 2002) and XM (launched in 2001), which merged as SiriusXM, delivered nationwide via geostationary satellites, expanding coverage to remote areas. Software-defined radio (SDR) arose in the 1990s, leveraging (DSP) to reconfigure radio functions via software, allowing flexible modulation schemes and multi-standard operation without hardware changes. This technology, rooted in military applications, enabled adaptive systems that could switch frequencies and protocols dynamically, paving the way for versatile communication devices. In the 21st century, radio integrated with cellular networks through 5G New Radio (NR), which rolled out commercially in 2019, supporting high-speed wireless data rates up to 20 Gbps via millimeter-wave bands and massive MIMO for enhanced capacity. Cognitive radio further advanced spectrum utilization with the IEEE 802.22 standard ratified in 2011, enabling dynamic access to unused TV white spaces for rural broadband while minimizing interference through spectrum sensing. By 2025, prototypes for networks incorporated AI-enhanced to optimize signal directionality in real-time, using for predictive and reducing latency in dense environments. Low-Earth orbit (LEO) constellations, such as , expanded global radio coverage with over 8,000 satellites by late 2025, providing broadband speeds of 100-200 Mbps and direct-to-device connectivity in underserved regions. Persistent challenges like scarcity have prompted reallocations by regulatory bodies; the FCC has auctioned mid-band frequencies for since 2020 to meet demand, while Canada's CRTC and ISED have prioritized rural sharing to support indigenous and remote needs.

Fundamental Principles

Electromagnetic radiation

Radio waves are a form of characterized by their low frequencies, ranging from 3 kHz to 300 GHz, which positions them at the longest-wavelength end of the . These waves consist of oscillating electric and magnetic fields that propagate perpendicular to each other and to the direction of travel, forming transverse waves that travel at the in . Key properties of radio waves include their , calculated as λ=cf\lambda = \frac{c}{f}, where cc is the (3×1083 \times 10^8 m/s) and ff is the , resulting in wavelengths from thousands of kilometers at low frequencies to millimeters at high frequencies. Radio waves also exhibit polarization, describing the orientation of their oscillations, which can be linear (oscillating in a single plane) or circular (rotating in a helical pattern). Radio waves are generated by accelerating electric charges, particularly through oscillating electric currents in conductors such as antennas, which produce time-varying electric and magnetic fields according to , including the application of Faraday's law of . This process creates transverse electromagnetic fields that detach from the source and propagate as self-sustaining waves. The radio spectrum is classified into bands based on frequency ranges, from very low frequency (VLF: 3–30 kHz) for applications like submarine communication (noting that extremely low frequency (ELF: 3–30 Hz) is occasionally used for similar purposes), to (EHF: 30–300 GHz) for high-data-rate wireless links. For example, the (MF) band (300 kHz–3 MHz) includes the (AM) radio range of 540–1600 kHz, commonly used for . At the quantum level, radio waves can be described as streams of photons, each with energy E=hfE = hf, where hh is Planck's constant (6.626×10−346.626 \times 10^{-34} J·s), though their low photon energies (on the order of 10−910^{-9} to 10−410^{-4} eV) make classical wave treatments sufficient for macroscopic phenomena in radio .

Wave propagation

Radio waves propagate from a transmitter to a receiver through various modes depending on , atmospheric conditions, and terrain. The primary modes include , sky wave, and . propagation involves signals traveling along the Earth's surface, following its curvature due to and induction, and is effective for medium frequencies (MF) like AM up to about 2 MHz, where the wave is guided by the ground's conductivity. Sky wave propagation relies on reflection from the , enabling long-distance communication by multiple hops between the Earth and ionized layers, particularly useful for high-frequency (HF) bands from 3 to 30 MHz in international shortwave broadcasts. Line-of-sight (LOS) propagation occurs directly between antennas with minimal obstruction, dominant above 30 MHz for VHF and higher frequencies, limited by the radio horizon approximated as d≈3.57hd \approx 3.57 \sqrt{h}
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