Radio and television physics

Type of physical science: Classical physics

Field of study: Electromagnetism

Radio and television communication employs a form of electromagnetic energy that can be described in terms of waves. Radio and television frequencies occupy part of the electromagnetic spectrum, as specified by national and international laws and agreements.

Overview

Radio and television make use of energy belonging to the same class of electromagnetic energy as visible light, X rays, and cosmic rays. Collectively, all forms of electromagnetic energy share three characteristics: They all travel at the same high velocity; in traveling, they assume the properties of waves; and they radiate outward from a source without benefit of any discernible physical vehicle.

Electromagnetic energy can be envisioned as oscillating motion, depicted as waves. For example, visible light waves, fundamentally the same as radio waves, travel at a velocity of 300 million meters per second. (Light-wave energy has much shorter waves and higher frequency.)

The number of separate wavelike motions produced in a second determines a wave's frequency.

Also, the large number of frequencies--visualized in numerical order--constitutes the electromagnetic spectrum.

The electromagnetic spectrum offers a vast number of possibilities, of which radio and television form but a handful. Visualize, for example, the keyboard of a piano. This keyboard presents one version of a spectrum, starting with keys that produce low sound frequencies at the left end of the keyboard and progressing through higher and higher frequencies toward the right end. Radio and television constitute merely a few keys on the electromagnetic spectrum.

Frequencies at the lower end of the electromagnetic spectrum are used for radio and television.

Note that wavelength decreases as frequency increases. Microwave waves, infrared radiation, visible light, ultraviolet light, X rays, γ rays, and cosmic rays can be found at higher frequencies. As the frequencies rise, the electromagnetic spectrum contains practical constraints for human use, but scientists continue to push the usable upper bounds.

Like sound waves, radio and television waves attenuate, can be absorbed or reflected, and can produce echoes. Visual echoes resulting from reflected signals account for the ghosts that sometime mediate out of television pictures. Note, however, that the frequency range for radio and television waves is vast compared to sound waves. Radio and television waves travel at the speed of light, about 900,000 times the speed of sound in air. Indeed, radio and television waves need no intervening medium (whereas sound uses air and water, for example) in which to travel. They travel best in a total vacuum; this principle was utilized in the historic transmissions of television images from the Moon in 1969.

To describe and classify radio and television waves, the phrase "cycles per second" has been shortened to the term hertz, meaning one cycle per second. The numbers of hertz in the higher-frequency radio and television waves can rise into the billions; therefore, the terms kilo- (thousand), mega- (million), and giga- (billion) prefix hertz frequently.

The radio and television terms known to everyone reflect these millions of hertz: VHF (very high frequency), or 30 to 300 megahertz, and UHF (ultra high frequency), or 300 to 3,000 megahertz. For radio and television, waves begin at 535 kilohertz and progress (in terms of frequency range) from 535 to 1,605 kilohertz (amplitude modulation--AM--radio), from 3 to 26 megahertz (short-wave radio), from 54 to 216 megahertz (VHF television and frequency modulation--FM--radio), and from 470 to 806 megahertz (UHF television). The gaps not cited fall into the nonbroadcast and auxiliary broadcast applications of radio and television communication.

Radio and television wave production depends on the vibration of an electrical current.

An oscillating current can be envisioned as power surging back and forth in a wire, rising to a maximum in one direction (one phase), then to a minimum in the other direction (the opposite phase). A radio or television broadcaster transmits or generates radio frequency energy into space by way of an antenna. The basic emission, the transmitter's carrier wave, oscillates at the station's allotted frequency, radiating radio or television energy at that frequency continuously, even though no sound or picture may be going out at any single moment.

Because of the fixed relationship between wavelength and frequency, one can identify the position of a radio station within its band by either the length or frequency of its basic carrier wave. Usually, the frequency is used. The number 600 on the AM dial identifies a carrier frequency of 600 kilohertz. An FM radio station's dial number gives the frequency used in megahertz; therefore, 98.9 means a carrier wave of 98.9 megahertz. Television stations, however, are identified by channel number because of their dual bandwidth. For example, it is easier to speak of "channel 6" than of a station having a video carrier wave frequency of 83.5 megahertz and a sound carrier wave frequency of 88.75 megahertz.

The single radio or television frequency that identifies a carrier wave can carry only a very small amount of information each second as energy spills out. Modulation affects adjacent frequencies, both above and below the specific carrier frequency. These additional groups of frequencies constitute sidebands. The more information conveyed, the larger the sideband.

Although a single sideband is all that is needed, it is expensive to suppress the second sideband.

As a result, AM and FM radio use the cheaper double sideband transmission, while television suppresses one. The existence of sidebands requires the allotment of a bundle of frequencies to each radio and television station. This bundle is referred to as a station's channel. The two chief methods for imposing patterns on broadcast carrier waves focus on varying either the amplitude or frequency. AM radio modulates amplitude, and FM radio modulates frequency. Television broadcasting uses both methods--AM for the video signal and FM for the audio signal.

AM is vulnerable to electrical interference. AM radio receivers pick up random bits of modulated energy, such as those caused by nature (for example, lightning) or people (for example, electrical machinery). Listeners to AM radio perceive such distortions as irritating static, resulting in less than optimal listening to music. In contrast, FM radio carriers, relying on frequency rather than amplitude modulating patterns, are relatively immune to electrical interference and, hence, provide a better outlet for musical reproduction.

Applications

Physical facts dictate where and how far radio and television electromagnetic signals can travel and how much information they can carry. An antenna radiates the radio and television signals into surrounding space. The traveling of signals outward from the antenna is called signal propagation. In particular, propagation overflow limits how many radio and television transmitters can operate in any one locality. To avoid interference, a system must be devised to enable cooperation among users. Such a system has been developed and is enforced in the United States by the independent agency, the Federal Communications Commission (FCC).

One series of constraints begins by noting that in traveling, radio and television energy attenuates, growing weaker and weaker as it covers a larger and larger area. Under ideal conditions, a radio or television transmitter would cover a circular geographic area. Assuming an omnidirectional antenna, the radio or television energy radiates evenly in all directions. Yet, a number of factors prevent this. Conditions influencing coverage patterns include weather, physical obstructions (both natural and artificial), the nature of the soil, the time of day, and even seasonal sunspot activity. Radio and television waves can also be refracted, reflected, or absorbed. As a result of these factors, the coverage of a transmitted radio energy assumes an irregular shape. Attenuation also limits the distance that different types of radio and television waves can be picked up. In particular, radio and television waves fall into certain categories when transmitted. Line-of-sight waves (called direct waves) occur in the VHF frequencies and above. They follow a straight path from the transmitting antenna to the receiver antenna, lasting only to the horizon. Beyond the horizon, most of the energy of direct waves flies off into space.

The actual line-of-sight to the horizon depends on the height of the antenna; the higher the sending antenna, the farther the direct wave will go, all other things being equal. Likewise, the height of the receiving antenna can also extend the horizon. FM radio and both VHF and UHF television use direct waves and therefore cover areas limited by the horizon distance.

Obstructions in the paths of radio and television waves cast "shadows" if the objects are wider than the length of the waves. Waves used for television broadcasting, for example, have such short lengths than even relatively small objects can interfere with their propagation. AM radio signals occur at medium waves and are called ground waves. Because ground waves propagate along the surface of the earth, they follow the curvature and therefore can travel well beyond the horizon. As a result, ground waves have the potential to cover more territory than direct waves. In practice, the ground wave depends on the transmitter power, the conductivity of the soil surrounding the antenna, and the amount of interference from distant stations on the same channel. Dry, sandy soil, for example, conducts radio energy poorly; stations seek to position their transmitters on damp, loamy soil.

To force an AM radio signal to travel over the longest possible distance, one seeks to take advantage of sky waves. Most radio waves, when allowed to radiate upward, lose much of their energy because of atmospheric absorption. Waves in medium frequency bands, however, tend to bend back, forming sky waves. Under the right conditions of the ionosphere, these refracted waves bounce off the surface of the earth, bend again, and so on, for thousands of kilometers past the horizon. The ionosphere refracts medium-frequency AM radio waves only after sundown. At night, therefore, AM stations reach out well beyond the extent of their daytime coverage contours. Unless protected from cochannel interference, however, AM radio stations do not necessarily get improved nighttime coverage from sky waves. In the 1930's, the FCC established clear channel bands for certain AM stations so rural-based citizens could listen to distant radio stations at night.

International radio services depend primarily on sky waves. They use a shortwave (high-frequency) band, whose waves tend to be refracted by the ionosphere both day and night.

Unlike domestic AM radio stations, shortwave international services are allowed to switch frequency from time to time throughout the day. They need this latitude because the ionosphere rises and falls with temperature changes. Angles of refraction change, and different ionosphere layers become refractive under the sun's influence. By switching frequencies, shortwave stations can provide continuous sky-wave service to distant areas for listeners willing to retune their sets as frequencies change.

The optimal height of the transmitter antenna can be mathematically calculated; the usual rule is one-half to one-quarter of the wavelength. Thus, the 556-meter waves of a 540-kilohertz signal could be radiated best by a tower 139 meters in height. The entire steel tower acts as the radiating element. Good soil conductivity, free from surrounding sources of electricial interference, that is well grounded, can do the best job. For FM radio and television antennas, engineers pick the highest possible locations, such as mountaintops and the roofs of tall buildings. Rather than acting as radiating elements, direct-wave antenna towers simply support small radiating elements, in keeping with the shortness of the VHF and UHF waves. Antennas serve to launch signals at the transmitting end and to pick them up at the receiving end. Small, built-in receiving antennas in radios and televisions are adequate to pick up strong signals, but the higher the frequency, the more elusive the signal and the more essential it becomes to have an efficient, well-placed outdoor antenna. For example, indoor "rabbit-ear" antennas may be acceptable for powerful, VHF television signal reception, while an outdoor, fine-tuned antenna is necessary for proper, clear UHF television reception.

For television, the standard of picture fidelity possible within the information capacity of the 6-megahertz channel is not high in terms of photographic reproduction. In practice, the average home receiver produces about 150,000 picture elements per frame. The best-quality 35-millimeter film produces 1 million elements per frame. The search for sufficient space for television's huge 6-megahertz channels ended in four blocks, because when the time came to allocate frequencies, the lower bands had long been allocated. Channels 2 to 4 in one VHF block, channels 5 and 6 in a higher VHF block, channels 7 to 13 in a much higher VHF block, and channels 14 and above in a UHF block was the solution. It also meant that pairs of 4/5, 6/7, and 13/14 could be assigned to the same community without causing interference because they are adjacent in name only.

Context

The discovery of the nature of electricity and electromagnetic energy by nineteenth century European scientists provided the bedrock for the twentieth century use of radio and television. In England, Michael Faraday was able to construct the first electric motor. James Clerk Maxwell described electromagnetic radiation in mathematical terms as a wave motion. In the 1870's, Maxwell's equations were developed. Heinrich Rudolph Hertz found that waves emitted by sparking an induction coil could be detected at a distance. As the nineteenth century drew to a close, scientists began to understand the nature of the electromagnetic spectrum. In time, other scientists perfected the use of the spectrum and the principles of electricity. In the United States, Thomas Alva Edison helped to create the first electron tube; Lee de Forest developed the actual tube. Later, Edison, de Forest, and other inventors spent two decades in court claiming the rights to the technology that gave rise to radio transmission and reception.

The actual experimentation of sending and receiving radio signals began with Guglielmo Marconi at the beginning of the twentieth century. Inspired by Marconi, a host of scientists, engineers, and amateur "ham" radio operators in the United States developed a mass medium of radio that became a business in the 1920's. By the mid-1920's, with the creation and building up of the government-sponsored Radio Corporation of America (RCA), radio broadcasting became a mass industry, based on advertisements. Networks, such as the National Broadcasting Company (NBC) and the Columbia Broadcasting System (CBS), developed into dominant forces in the industry.

After World War II came television transmission, based on the same model as radio.

Television networks included NBC, CBS, and the American Broadcasting Company (ABC). By the late 1950's, nearly every household in the United States had a television as well as a radio.

The world of printed culture gave way to a world of aural and visual entertainment. By the end of the twentieth century, both these industries had become cornerstones of American mass culture.

The future seems bright for continued use of radio and television. There is little doubt that radio and television will continue to dominate mass entertainment and information. The basic principles of the physics of radio energy will be refined, squeezing more and more use from the precious spectrum space. Digital transmission will make sounds and images clear and interference free. High-definition television will enable more and more visual information to be transmitted across the airwaves.

Both AM and FM radio signal patterns are based on analog variation because they reproduce music and spoken language by means of continuous change, analogous to the variations in the original generation of sounds. This continuous analog pattern is inherently fragile, susceptible to various sources of distortion, the static of AM radio or the "hiss" long associated as background for even the highest-quality tape recording. In contrast, digital signal processing breaks down the signals into a stream of separate, individual pulses of energy. Rapid sampling of the analog incoming signal (so fast, it seems continuous) enables the conversion of the signal into a stream of simple pulses of energy. The extreme simplicity of the binary digital encoding and decoding protects them from the many forms of distortion that affect analog signals. Repeated recording, relaying, and other manipulations of analog signals cause accumulating distortions. Nevertheless, digital signals--being simply numbers 1 or 0--are immune to distortion as long as the elementary difference between "on" and "off" is maintained.

The use of digital radio and television signals is becoming standard.

Also increasing is the transmission of radio and television energy through fiber-optic systems, which have a vast capacity to carry hundreds of signals at the same time. Fiber-optic systems minimize the problems caused by interference and other radio and television energy broadcast constraints. So-called broadband networks can carry traditional radio and television entertainment and information into and out of homes, businesses, and government institutions.

Coupled with computers, fiber-optic radio and television transmission can provide interaction, replacing one-way transmissions. As such, radio and television can play an even greater role in the economic, societal, and cultural life of all persons around the world.

Principal terms

AMPLITUDE MODULATION (AM): the process of altering the amplitude of a radio wave in accordance with the input signal

DIGITAL SIGNALS: a signal format in which the information is transmitted by a series of binary coded pulses rather than by analog fashion

FREQUENCY MODULATION (FM): the process of altering the frequency of a radio wave in accordance with the input signal

GROUND WAVE: a vertically polarized medium wave that is guided along the earth's surface as the result of its electrical conductivity

HERTZ: the basic unit of frequency; one hertz equals one cycle per second

SKY WAVE: electromagnetic radiation that is transmitted by means of reflection from the ionosphere

SPECTRUM: the range of electromagnetic phenomena; only the portion from about 10,000 cycles to 3,000 billion cycles is the radio spectrum

ULTRA HIGH FREQUENCY (UHF): electromagnetic radiation in the frequency range of 300 to 3,000 megahertz; television channels 14 and above are in the UHF range

VERY HIGH FREQUENCY (VHF): electromagnetic radiation in the frequency range of 30 to 300 megahertz; FM broadcasting and television channels 2 to 23 are in the VHF band of the radio spectrum

Bibliography

Aitken, Hugh G. J. SYNTONY AND SPARK: THE ORIGINS OF RADIO. Princeton, N.J.: Princeton University Press, 1976. This well-researched, scholarly book examines the scientific work that led to modern radio and the innovations that gave rise to radio as a mass industry. The science of Maxwell, Hertz, and Marconi is explored in depth. A topflight technological history.

Benson, K. Blair. TELEVISION ENGINEERING HANDBOOK. New York: McGraw-Hill, 1986. An engineering summary of the basics of the radio and television industry. Somewhat advanced in its language for the general reader, but it can function as a valuable reference text.

Fink, Donald G., and David M. Luytens. THE PHYSICS OF TELEVISION. Garden City, N.Y.: Anchor Books, 1960. Although somewhat dated, this book remains the standard text on the basic physical principles of television and radio. Geared for the general reader.

Head, Sydney W., and Christopher H. Sterling. BROADCASTING IN AMERICA: A SURVEY OF TELEVISION, RADIO, AND NEW TECHNOLOGIES. 5th ed.

Boston: Houghton Mifflin, 1988. One of the most readable and comprehensive textbooks that outline for college students the basic physical concepts of electromagnetic energy that enable the extensive use of television and radio. This tome has long been the leading book in its field.

Inglis, Andrew. BEHIND THE TUBE: A HISTORY OF BROADCASTING TECHNOLOGY AND BUSINESS. Boston: Focal, 1990. Details the basics of radio and television technology and how it led to the modern radio and television industry. Written in clear, straightforward language.

Langley, Graham. TELECOMMUNICATIONS PRIMER. London: Pitman Books, 1983. A basic dictionary of the fundamental terms of the physics of television and radio. A must for any reference shelf.

Levin, Harvey J. THE INVISIBLE RESOURCE: USE AND REGULATION OF THE RADIO SPECTRUM. Baltimore: The Johns Hopkins University Press, 1971. Examines how and why decisions are made to divide up and use the radio electromagnetic spectrum. Explains the tradeoffs that are necessary in the employment of radio and television among other uses of the spectrum. Levin is the leading expert in this area of study.

Pierce, John R. SIGNALS: THE TELEPHONE AND BEYOND. San Francisco: W. H. Freeman, 1981. A fine introduction to basic physics; it assumes no prior knowledge. Pierce places the physics of radio and television within the grasp of the general reader.

Radio frequencies

Generating and Detecting Electromagnetic Waves

Producing and Detecting Sounds

X-Ray and Gamma-Ray Astronomy