Radio

Summary

Radio is a technology that involves the use of electromagnetic waves to transmit and receive electric impulses. Since its inception as a method of wirelessly transmitting Morse code, radio communications technology has had a tremendous impact on society. Although television has supplanted radio to a significant extent for public broadcasting, radio continues to play an important role in this arena. Radio broadcasts may be delivered via technologies such as satellites, cable networks, and the Internet. Although the term “radio” commonly brings to mind listening to news broadcasts, music, and other audio signals, electromagnetic-wave transmission encompasses other fields, such as radar, radio astronomy, radio control, and microwave technology.

Definition and Basic Principles

In contrast to sound waves, which require a medium such as air or water for propagation, electromagnetic waves can travel through a vacuum. In a vacuum such as outer space they travel at the speed of light (299,800 kilometers per second). In space, electromagnetic waves conform to the inverse-square law: the power density of an electromagnetic wave is proportional to the inverse of the square of the distance from a point source. Thus, all radio waves weaken as they travel a distance. When traveling through air, then, the intensity of the waves is weakened. At some point, depending on the strength of the signal, the electromagnetic wave will no longer be discernible. Interference can also weaken or destroy a radio signal. Other radio transmitters and accidental radiators (such as automobile ignition systems) produce interference and static. Frequency modulation (FM) radio signals are much more resistant to interference and static than amplitude modualtion (AM) radio signal. Radio waves travel in a straight line; therefore, the curvature of the earth limits their range. However, radio waves can be reflected by the ionosphere, which extends their range.

The following steps occur in radio transmission: A transmitter modulates (converts) sound to a specific radio frequency; an antenna broadcasts the electromagnetic wave; the wave is received by an antenna; and a receiver tuned to the radio frequency demodulates the electromagnetic energy back into sound. Radio waves range from 3 kilohertz (kHz) to 300 gigahertz (GHz) and are categorized as: very low frequency (VLF; 3 to 30 kHz); low frequency (LF; 30 to 300 kHz); medium frequency (MF; 300 to 3,000 kHz); high frequency (HF; 3 to 30 megahertz [MHz]); very high frequency (VHF; 30 to 300 MHz); ultra high frequency (UHF; 300 to 3,000 MHz); super high frequency (SHF; 3 to 30 GHz); and extremely high frequency (EHF; 30 to 300 GHz). Each frequency range has unique characteristics and unique applications for which they are best suited.

Background and History

Electromagnetic waves were discovered in 1877 by the German physicist Heinrich Hertz, whose name is used to describe radio frequencies in cycles per second. Eight years later, American inventor Thomas Alva Edison obtained a patent for wireless telegraphy by discontinuous (intermittent) wave. A far superior system was developed in 1894 by the Italian inventor Guglielmo Marconi. Initially, he transmitted telegraph signals over a short distance on land. Subsequently, an improved system was capable of transmitting signals across the Atlantic Ocean. At the start of the twentieth century, Canadian inventor Reginald Aubrey Fessenden began experimenting with voice transmission via discontinuous waves while employed by the United States Weather Bureau, and he eventually pioneered radio broadcasting. In 1902, he switched to continuous waves and successfully transmitted voice and music. In 1906, history was made when Fessenden transmitted voice and music from Massachusetts that was heard as far away as the West Indies. The same year, Lee de Forest developed the Audion tube (later known as the triode vacuum tube), which became a key component of radios.

In 1920, the first radio news program was broadcast by station 8MK in Detroit. Also in 1920, the station WRUC in New York began broadcasting a series of Thursday night concerts, the initial range of which was 100 miles. However, it was soon expanded to 1,000 miles. WRUC also began sports broadcasts in the same year. In the early 1930s, frequency modulation (FM) and single sideband shortwave radio were invented by amateur radio operators. In 1954, the companies Texas Instruments and Regency embarked on a joint venture to launch the Regency TR-1, the first portable transistor radio, which was powered by a 22.5-volt battery.

Radio also played a role in other fields. Radio astronomy began in the 1930s and expanded greatly after World War II. The broadcast of television over radio waves, first demonstrated in the 1920s, became increasingly popular in the 1950s. The cellular radio-telephone was invented in 1947 by Bell Laboratories, although it was not until the late 1990s and early 2000s that cell phones became a major cultural influence. Radio technology played a significant role in the space race, and satellites themselves increased the broadcasting of signals. In the later twentieth and early twenty-first centuries, digital technology shaped the radio industry.

How It Works

Basic Example of a Radio Receiver. The humble crystal set, which first appeared at the close of the nineteenth century, is a radio receiver in its simplest form. It consists of an antenna, a tuned circuit, a crystal detector, and earphones. The tuned circuit consists of a tuning coil (a sequentially wound coil that can be tapped at any point) connected to a capacitor. This pair of components allows tuning of the receiver to a specific frequency, known as the resonant frequency. Only signals at the resonant frequency pass through the tuned circuit; other frequencies are blocked. The crystal is a semiconductor, which extracts the audio signal from the radio frequency carrier wave. This is accomplished by allowing current to pass in just one direction, blocking half the oscillations of the radio wave. This rectifies, or changes, the wave into a pulsing direct current, which varies with the audio signal. The earphones then convert the direct current into sound. The sound power is solely derived from the radio station that originated it. The electrical power and circuitry of more complex receivers serve to amplify this extremely weak signal to one that can power loudspeakers.

Amplitude Modulation (AM). With AM radio, the amplitude (height) of the transmitted signal is made proportional to the sound amplitude captured (transduced) by the microphone. The transmitted frequency remains constant. AM transmission is degraded by static and interference because sources of electromagnetic transmission such as lightning and automobile ignitions, which are at the same frequency, add their amplitudes to that of the transmitted signal. AM radio stations in the United States and Canada are limited to 50 kilowatts (kW). In the early twentieth century, U.S. stations had powers up to 500 kW, some of which could be heard worldwide. Conventional AM transmission involves the use of a carrier signal, which is an inefficient use of power. The carrier can be removed (suppressed) from the AM signal. This produces a reduced-carrier transmission, which is termed a double-sideband suppressed-carrier (DSB-SC) signal. A sideband refers to one side of the mirror-image radio signal. DSB-SC has three times more power efficiency than an unsuppressed AM signal. Radio receivers for DSB-SC signals must reinsert the carrier. Another AM refinement is single-sideband modulation in which both one sideband and the carrier are stripped out. This modification doubles the effective power of the signal.

Frequency Modulation (FM). With FM, the variation in amplitude from the microphone causes fluctuations in the transmitter frequency. FM broadcasts have a higher fidelity than AM broadcasts and are resistant to static and interference. FM requires a wider bandwidth to operate and is transmitted in the VHF (30 to 300 MHz) range. These high frequencies travel in a straight line, and the reception range is generally limited to about 50 to 100 miles. FM signals broadcast from a satellite back to Earth do not have this distance limitation. An FM transmission can contain a subcarrier in which secondary signals are transmitted in a piggyback together with the main program. The subcarrier allows stereo broadcasts to be transmitted. The subcarrier can also transmit other information, such as station identification and the title of the current song being played.

Digital Radio. Digital audio transmission consists of converting the analogue audio signal into a digital code of zeros and ones in a process known as digitizing. This technology allows for an increase in the number of radio programs in a given frequency range, improved fidelity, and reduction in fading (signal loss). Satellite radio is a digitized signal and can cover a distance in excess of 22,000 miles.

Applications and Products

Radio applications are widespread and include commercial radio, amateur radio, marine radio, aviation radio, radar, navigation, and microwave.

Commercial Radio. Radio broadcasts are available throughout the globe. They offer a variety of products, such as news, music, and political opinion. Many broadcasts are free of charge and available to receivers in range of the station. The station derives its revenue from advertising. An hour of broadcasting time typically contains ten to twenty minutes of advertising. Cable television and internet services rebroadcast these stations either at no charge or with a fee. Cable television services and satellite radio broadcast a variety of commercial-free programs. In these cases, the subscription fee covers the cost of the broadcast.

Amateur Radio. Amateur radio (also known as ham radio) is the licensed use of designated radio bands for noncommercial exchange of messages, private recreation, emergency communication, and experimentation. Ham operators maintain and operate their own equipment. Through the years, they have made numerous contributions to radio technology, including FM and single sideband. They also perform a number of public services at no cost. For example, during the Vietnam War, the Military Auxiliary Radio System (MARS) allowed military personnel to call friends and relatives in the United States. A broadcast emanating from Vietnam was received by a US-based amateur operator who patched the communication into the phone lines. MARS is still active. However, many of its services have been supplanted by the internet and mobile devices.

Shortwave Radio. Shortwave radio, which was first theorized in 1919, has many applications for long-range communication. The term “shortwave” refers to the wavelength of the frequency spectrum in which it operates: high frequency (3,000 to 30,000 kHz). The high-frequency wavelength is shorter than the ones first used for radio communications: medium frequency and low frequency. Shortwave broadcasts are readily transmitted over distances of several thousand kilometers, allowing intercontinental communication and ship-to-shore communication. Low-cost shortwave radios are available worldwide and facilitate the transfer of information to individuals where other forms of media are controlled for political reasons. A major disadvantage of shortwave radio is that it is subject to significant interference problems, such as atmospheric disturbances, electrical interference, and overcrowding of wave bands. The internet and satellite radio have impacted shortwave radio, but it is still useful in areas where those services are unavailable or too expensive.

Marine Radio. All large ships and most small oceangoing vessels are equipped with a marine radio. Its purposes included calling for aid, communicating with other vessels, and communicating with shore-based facilities, such as harbors, marinas, bridges, and locks. Marine radio operates in VHF, from 156 to 174 MHz. Channels from 0 to 88 are assigned to specific frequencies. Channel 16 (156.8 MHz) is designated as the international calling and distress channel.

Aircraft Band. The aircraft band (or air band) operates in the VHF range. Different sections of the band are used for commercial and general aviation aircraft, air traffic control, radio-navigational aids, and telemetry (remote transmission of flight information). VHF omnidirectional range (VOR) or an instrument landing system operates in the aircraft band. A VOR is a ground-based station that transmits a magnetic bearing of the aircraft from the station. Unmanned aircraft (drones) are navigated by a radio system in which a pilot sits at a console on the ground and directs the flight.

Radio Telescope. A radio telescope is a large, directional, parabolic (dish) antenna that collects data from space probes and Earth-orbiting satellites. The first purpose-built example was created in 1937 by Grote Reber, an engineer and amateur astronomer. Many astronomical objects emit electromagnetic radiation in the radio frequency range, therefore radio telescopes can image astronomical objects such as galaxies. In 2019, a series of radio telescopes known as the Event Horizon Telescope, captured the first-ever image of a distant black hole, the remains of a massive dead star. The black hole was 55 million light-years from Earth and was about 6.5 billion times more massive than the sun. Some researchers are using radio telescopes to search for intelligent life forms in the universe.

Navigation. Radio navigation encompasses a number of applications including radio direction finding (RDF), radar, LORAN, and Global Positioning System (GPS).

An RDF system homes in on a radio transmission, which can be a specialized antenna that directs aircraft or commercial transmitters that have a known location.

Radar is an acronym for radio detecting and ranging. The device consists of a transmitter and receiver. The transmitter emits radio waves, which are deflected from a fixed or moving object. The receiver, which can be a dish or an antenna, receives the wave. Radar circuitry then displays an image of the object in real time. The screen displays the distance of the object from the radar. If the object is moving, consecutive readings can calculate the speed and direction of the object. If the object is airborne, and the radio is so equipped, the altitude is displayed. Radar is invaluable in foggy weather when visibility can be severely reduced.

LORAN is an acronym for long-range navigation. The system relies on land-based low-frequency radio transmitters. The device calculates a ship's position by the time difference between the receipt of signals from two radio transmitters. The device can display a line of position, which can be plotted on a nautical chart. Most LORAN convert the data into longitude and latitude. Since GPS became available, the use of loran has markedly declined.

GPS is a space-based global navigation satellite system that provides accurate location and time information at any location on Earth where there is an unobstructed line of sight to four or more GPS satellites. GPS can function under any weather condition and anywhere on the planet. The technology depends on triangulation, just as a land-based system such as LORAN employs. GPS is composed of three segments: the space segment; the control segment; and the user segment. The US Air Force operates and maintains both the space and control segments. The space segment is made up of satellites, which are in medium-space orbit. The satellites broadcast signals from space, and a GPS receiver (user segment) uses these signals to calculate a three-dimensional location (latitude, longitude, and altitude). The signal also transmits the current time, accurate within nanoseconds.

Careers and Course Work

Many technical and nontechnical careers are available in radio. The technical fields require a minimum of a bachelor's degree in engineering or other scientific field. Many also require a postgraduate degree, such as a master's or doctorate. Course work should include mathematics, engineering, computer science, and robotics. Positions are available in both the government and private sector. According to the US Bureau of Labor Statistics, radio technicians made a median 2021 salary of $49,050 per year. Technicians in more specialized fields, such as radio astronomers, made higher salaries commensurate with skill level and job description. The ability to be a team player is often of value for these positions because ongoing research is often a collaborative effort. Less technical positions, such as operation or maintenance of broadcasting equipment, can be achieved with less training, which can be obtained at a technical college or trade school. A bachelor's degree in communications or related fields can qualify one for employment in radio or television broadcasting.

Social Context and Future Prospects

In terms of continuous advances in radio technology, further advances are extremely likely. Radio and related technologies are an integral component of everyday life in most societies. Radio has political implications, particularly in nations governed by tyrants and dictators, where radio may be used as a propaganda tool and use of shortwave radio and the Internet is strongly discouraged. The Voice of America (VOA), which began broadcasting in 1942, is an international multimedia broadcasting service funded by the US government. The VOA broadcasts about 1,500 hours of news, information, educational, and cultural programming each week to an estimated worldwide audience of 123 million people. In some repressed nations, the broadcast is jammed, which involves broadcasting noise at the same frequency to prevent reception. Even in the United States, political movements exist that wish to ban certain types of radio broadcasts (such as talk radio), which are deemed too conservative or too liberal.

Many associate the term microwave with a useful appliance to heat a frozen meal, but microwave is in the radio wavelength (0.3 GHz to 300 GHz) and is also used in communication. The microwave oven illustrates the fact that electromagnetic waves are a form of energy capable of heating—and damaging—tissue. For example, standing near or touching a powerful radio transmitter can result in severe burns. Microwave ovens are shielded to prevent exposure; other devices are not. The heating effect of an electromagnetic wave varies with its power and the frequency. The heating effect is measured by its specific absorption rate (SAR) in watts per kilogram. Many national governments as well as the Institute of Electrical and Electronics Engineers (IEEE) have established safety limits for exposure to various frequencies of electromagnetic energy based on their SAR.

In 2024, the future of AM radio in the United States was widely debated. While many politicians and citizens believe AM radio is necessary, critics claim it is an antiquated means of communication. The National Association of Broadcasters (NAB), which was founded in 1923, had been pushing for Congress to pass the AM Radio for Every Vehicle Act, which would mandate that all new vehicles have access to AM radio in the US. The bill was a response to many automakers, such as BMW, Mazda, Volkswagen, Volvo and Tesla, phasing out AM radio access. Carmakers, who are critics of the bill, claim that consumers can access pertinent information in a variety of ways, negating the need for AM radio. Proponents of the bill claim that access to AM radio during emergenies can be crucial.

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