Radio

Radio is the technology and practice that enables the transmission and reception of information carried by long-wave electromagnetic radiation . Radio makes it possible to establish wireless two-way communication between individual pairs of transmitter and receiver, and it is used for one-way broadcasts to many receivers. Radio signals can carry speech , music, telemetry , or digitally-encoded entertainment. Radio is used by the general public, within legal guidelines, or it it is used by private business or governmental agencies. Cordless telephones are possible because they use low-power radio transmitters to connect without wires. Cellular telephones use a network of computer-controlled low power radio transmitters to enable users to place telephone calls away from phone lines.

The history of radio

In the nineteenth century, in Scotland, James Clerk Maxwell described the theoretical basis for radio transmissions with a set of four equations known ever since as Maxwell's Field Equations. Maxwell was the first scientist to use mechanical analogies and powerful mathematical modeling to create a successful description of the physical basis of the electromagnetic spectrum . His analysis provided the first insight into the phenomena that would eventually become radio. He deduced correctly that the changing magnetic field created by accelerating charge would generate a corresponding changing electric field. The resulting changing electric field would, he predicted, regenerate a changing magnetic field in turn, and so on. Maxwell showed that these interdependent changing electric and magnetic fields would together be a part of a self-sufficient phenomenon required to travel at the speed of light .

Not long after Maxwell's remarkable revelation about electromagnetic radiation, Heinrich Hertz demonstrated the existence of radio waves by transmitting and receiving a microwave radio signal over a considerable distance. Hertz's apparatus was crude by modern standards but it was important because it provided experimental evidence in support of Maxwell's theory.

Guglielmo Marconi was awarded the Nobel Prize in physics in 1909 to commemorate his development of wireless telegraphy after he was able to send a long-wave radio signal across the Atlantic Ocean.

The first radio transmitters to send messages, Marconi's equipment included, used high-voltage spark discharges to produce the charge acceleration needed to generate powerful radio signals. Spark transmitters could not carry speech or music information. They could only send coded messages by turning the signal on and off using a telegraphy code similar to the landline Morse code.

Spark transmitters were limited to the generation of radio signals with very-long wavelengths, much longer than those used for the present AM-broadcast band in the United States. The signals produced by a spark transmitter were very broad with each signal spread across a large share of the usable radio spectrum . Only a few radio stations could operate at the same time without interfering with each other. Mechanical generators operating at a higher frequency than those used to produce electrical power were used in an attempt to improve on the signals developed by spark transmitters.

A technological innovation enabling the generation of cleaner, narrower signals was needed. Electron tubes provided that breakthrough, making it possible to generate stable radio frequency signals that could carry speech and music. Broadcast radio quickly became established as source of news and entertainment.

Continual improvements to radio transmitting and receiving equipment opened up the use of successively higher and higher radio frequencies. Short waves, as signals with wavelengths less than 200m are often called, were found to be able to reach distant continents. International broadcasting on shortwave frequencies followed, allowing listeners to hear programming from around the world.

The newer frequency-modulation system, FM, was inaugurated in the late 1930s and for more than 25 years struggled for acceptance until it eventually became the most important mode of domestic broadcast radio. FM offers many technical advantages over AM, including an almost complete immunity to the lightning-caused static that plagues AM broadcasts. The FM system improved the sound quality of broadcasts tremendously, far exceeding the fidelity of the AM radio stations of the time. The FM system was the creation of E. H. Armstrong, perhaps the most prolific inventor of all those who made radio possible.

In the late 1950s, stereo capabilities were added to FM broadcasts along with the ability to transmit additional programs on each station that could not be heard without a special receiver. A very high percentage of FM broadcast stations today carry these hidden programs that serve special audiences or markets. This extra program capability, called SCA for Subsidiary Communications Authorization, can be used for stock market data, pager services, or background music for stores and restaurants.

Radio and the electromagnetic spectrum

Radio utilizes a small part of the electromagnetic spectrum, the set of related wave-based phenomena that includes radio along with infrared light, visible light, ultraviolet light, x rays , and gamma rays. Picture the electromagneticspectrum as a piano keyboard: radio will be located where the piano keys produce the low frequency musical notes. Radio waves have lengths from many miles down to a fraction of a foot.

Radio waves travel at the velocity of electromagnetic radiation. A radio signal moves fast enough to complete a trip around the earth in about 1/7 second.

How radio signals are created

Jiggle a collection of electrons up and down one million times a second and a 1-MegaHertz radio signal will be created. Change the vibration frequency and the frequency of the radio signal will change.

Radio transmitters are alternating voltage generators. The constantly changing voltage from the transmitter creates a changing electric field within the antenna . This alternating field pushes and pulls on the conduction electrons in the wire that are free to move. The resulting charge acceleration produces the radio signal that moves away from the antenna. The radio signal causes smaller sympathetic radio frequency currents in any distant electrical conductor that can act as a receiving antenna.

Modulation

A radio signal by itself is like a mail truck without letters. A radio signal alone, without superimposed information, is called a carrier wave. An unmodulated radio signal conveys only the information that there was once a source for the signal picked up by the receiver. Adding information to a carrier signal is a process called modulation. To modulate a radio carrier means that it is changed in some way to correspond to the speech, music, or data it is to carry.

The simplest modulation method is also the first used to transmit messages. The signal is turned on and off to transmit the characters of an agreed code. Text messages can be carried by the signal modulated in this way. Unique patterns stand for letters of the alphabet, numerals, and punctuation marks.

The least complicated modulation method capable of transmitting speech or music varies the carrier signal's instantaneous power. The result is called amplitude modulation, or AM. Another common system varies the signal's instantaneous frequency at an informational rate . The result is frequency modulation, FM.

If radio is to transmit speech and music, information must be carried that mimics the pattern of changing air pressure the ear would experience hearing the original sound. To transmit sounds these air-pressure changes are converted into electrical signals, amplified electronically, then used to modulate the carrier.

Amplitude modulation was the first process to have the capability of transmitting speech and varied the radio signal's instantaneous power at a rate that matched the original sound vibrations in the air. A better modulation technology followed that varied the instantaneous frequency of the radio signal but not the amplitude. Frequency modulation, or FM, has advantages compared to AM but both AM and FM are still in use.

Sound can be converted to digital data, transmitted, then used to reconstruct the original waveform in the receiver. It seems likely that a form of digital modulation will eventually supplant both FM and AM.

Demodulation

Radio receivers recover modulation information in a process called demodulation or detection. The radio carrier is discarded after it is no longer needed. The radio carrier's cargo of information is converted to sound using a loudspeaker or headphones or processed as data.

Wavelengths, frequencies, and antennas

Each radio signal has a characteristic wavelength just as is the case for a sound wave. The higher the frequency of the signal, the shorter will be the wavelength. Antennas for low-frequency radio signals are long. Antennas for higher frequencies are shorter, to match the length of the waves they will send or receive.

It is a characteristic of all waves, not just radio signals, that there is greater interaction between waves and objects when the length of the wave is comparable to the object's size. Just as only selected sound wavelengths fit easily into the air column inside a bugle, only chosen frequencies will be accepted by a given antenna length. Antennas, particularly transmitting antennas, function poorly unless they have a size that matches the wavelength of the signal presented to them. The radio signal must be able to fit on the antenna as a standing wave. This condition of compatibility is called resonance . If a transmitter is to be able to "feed" energy into an antenna, the antenna must be resonant or it will not "take power" from the transmitter. A receiver antenna is less critical, since inefficiency can be compensated by signal amplification in the receiver, but there is improvement in reception when receiving antennas are tuned to resonance.

If an antenna's physical length is inappropriate, capacitors or inductors may be used to make it appear electrically shorter or longer to achieve resonance.

Near 100 MHz, near the center of the FM broadcast band in most of the world, signals have a wavelength of approximately three meters. At 1 MHz, near the center of the U.S. AM broadcast band, the signal's wavelength is 327 yds (300 m), about three times the length of a football field. One wavelength is about 1 ft (0.3 m) at the ultra-high frequency used by cellular telephones.

Radio signals and energy

Energy is required to create a radio signal. Radio signals use the energy from the transmitter that accelerates electric charge in the transmitting antenna. A radio signal carries this energy from the transmitting antenna

to the receiving antenna. Only a small fraction of the transmitter's power is normally intercepted by any one receiving antenna, but even a vanishingly-small received signal can be amplified electronically millions of times as required.

Radio signal propagation

Radio signals with very short wavelengths generally follow straight line paths much as do beams of light, traveling from transmitter to receiver as a direct wave. Radio signals with very long wavelengths follow the curvature of the earth, staying close to the surface as signals called ground waves.

Radio signals with intermediate wavelengths often reflect from layers of electrically-charged particles high above the earth's surface. These signals are known as skywaves. The layers of electrically-charged particles found between 25-200 mi (40-322 km) above the earth are collectively known as the ionosphere. The ionosphere is renewed each day when the sun's radiation ionizes atoms in the rarefied air at this height. At higher altitudes the distance between ions causes the ionization to persist even after the sun sets.

A good way to become familiar with radio propagation is to listen for distant AM-broadcast radio at various times of the day. A car radio works well for this experiment because they often have better sensitivity and selectivity than simpler personal radios.

During the daylight hours, on the standard-broadcast band, only local stations will normally be heard. It is unlikely that you will hear stations from more than 150 mi (241 km). As the sun sets you will begin to hear signals from greater distances.

AM-broadcast reception is generally limited to ground-wave radio signals when the sun is high in the sky. There is a very dense layer of the ionosphere at a height of approximately 25 mi (40 km) that is continually created when the sun is high in the sky. This D layer, as it is called, absorbs medium wavelength radio signals so that skywave signals cannot reflect back to earth. The D layer dissipates quickly as the sun sets because the sun's rays are needed to refresh the ionization of this daytime-only feature of the ionosphere. After dark, when the D layer has disappeared, you will hear strong signals from far away cities.

After the D layer has disappeared, skywave signals reflect from a much higher layer of the ionosphere called the F layer. The F layer acts as a radio mirror, bouncing skywaves back to earth far from their source. The F layer degrades in darkness as does the D layer, but since the ions are separated more widely at higher altitude, the F layer functions as a significant radio mirror until dawn. Toward morning stations at intermediate distances fade, leaving only skywave signals that reflect from the thinning ionosphere at a very shallow angle.

Signal absorption by the D layer is less at shorter wavelengths. Stations using higher frequencies can use skywave in the daytime. High frequencies pass through the D layer. Skywave radio circuits are usually best in the daytime for higher frequencies, just at the time that the standard-broadcast band is limited to groundwave propagation.

Forecasting long distance radio signal propagation conditions depends upon predicting conditions on the sun. It is the changing radiation from the sun that affects long distance radio circuits when the ionosphere changes as the earth rotates. On the sunlit side of the earth the ionosphere is most strongly ionized. On the night side of the earth the radio ionosphere begins to dissipate at sunset until it is almost insignificant as a radio mirror in the early morning hours. When the ionosphere is at its best as a reflector it can support communication between any locations on the earth.

When the ionosphere is more densely ionized it will reflect radio signals with a shorter wavelength than when the ionization is weaker. At any one time, between any two distant locations on the earth, there is a limiting upper frequency that can be used for radio communication. Signals higher in frequency than this maximum-usable frequency, F layer called the MUF, pass through the ionosphere without returning to earth. Slightly lower than the MUF, signals are reflected with remarkable efficiency. A radio signal using less power than a flashlight can be heard on the opposite side of the earth just below the MUF. The MUF tends to be highest when the sun is above the midpoint between two sites in radio communication.

The 11-year solar sunspot cycle has a profound effect on radio propagation. When the average number of sunspots is large, the sun is more effective in building the radio ionosphere. When the sun's surface is quiet the maximum-usable frequency is usually very low, peaking at less than half the MUF expected when the sun surface is covered with sunspots.

From time to time, the sun bombards the earth with charged particles that disrupt radio transmissions. When solar flares are aimed toward the earth, the earth's magnetic field is disturbed in a way that can cause an almost complete loss of skywave radio propagation. Microwave radio signals are not significantly disturbed by the magnetic storms since microwaves do not depend upon ionospheric reflection.

FM-broadcast signals are seldom heard reliably further than the distance to the horizon. This is because the frequencies assigned to these services were deliberately chosen to be too high to expect the ionosphere to reflect them back to earth. FM signals are received as direct waves, not skywaves. The limited range of FM stations is an advantage because frequency assignments can be duplicated in cities that are in fairly close proximity without encountering unacceptable interference . This protection is much harder to achieve where skywave propagation may permit an interfering signal to be heard at a great distance.

Shortwave radio

Shortwave radio services may change frequency often as the ionosphere's reflectivity varies. Unlike domestic broadcast stations that stay on a single assigned frequency, shortwave broadcast stations move frequency to take advantage of hour-to-hour and season-to-season changes in the ionosphere. As the 11-year sunspot cycle waxes and wanes, shortwave stations the world around move to shorter wavelengths when there are more sunspots and to longer wavelength bands when sunspots are minimal.

Listening to shortwave radio requires more effort than istening to local domestic radio. The best frequencies to search change from one hour to the next throughout the day. Due to the effect of the sun, shortwave signals sometimes may disappear for days at a time, then reappear with astounding strength. Many shortwave stations do not broadcast at all hours of the day. In addition, a station must be targeting your part of the world specifically; otherwise the signal will probably be weak.

Regulation of radio transmissions

The part of the electromagnetic spectrum that can be used for radio communication cannot accommodate everyone who might wish to use this resource. Access is controlled and technical standards are enforced by law. With few exceptions, radio transmissions are permitted only as authorized by licenses.

Since 1934 in the United States, licensing and equipment approval has been the responsibility of the Federal Communications Commission. Similar regulation is the rule in other countries. Technical standards are required by radio regulation. Just as traffic laws improve highway safety, laws and regulations that encourage the fair use of the limited radio spectrum help to avoid conflicts between users.

The future of radio

Radio broadcasting now includes a newer, better digital system known as DAB, digital-audio broadcasting. Early tests indicate that a switch to digital imparts compact-disc quality to radio programming. There are two possible modes for DAB. In Europe , completely new stations on a different band of frequencies is favored. In the United States it seems probable that digital information will transmitted as information superimposed on the programming modulation now used. The digitized audio can be so much lower in power than the "main" programming that it will be inaudible to listeners with analog receivers. Early program tests of this system have been successful and increases in audio quality have been significant.

Resources

books

The 1995 ARRL Handbook. The American Radio Relay League, 1995.

Hobson, Art. Physics: Concepts and Connections. New York: Prentice-Hall, Inc., 1995.

Jacobs, George, and Theodore J. Cohen. The Shortwave Propagation Handbook. Cowan Publishing Corp., 1970.

Ostdiek, Vern J., and Donald J. Bord. Inquiry Into Physics. St. Paul: West Publishing Company, 1995.

other

Now You're Talking. The American Radio Relay League, 1994.

Donald Beaty

KEY TERMS

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Antenna

—An electrical conductor used to send out or receive radio waves.

Capacitor

—Electrical component that cancels magnetic property of wire.

D Layer

—Arbitrary designation for the lowest layer of the ionosphere.

Electric field

—The concept used to describe how one electric charge exerts force on another, distant electric charge.

Electron tube

—Active device based on control of electrons with electric fields.

F layer

—Arbitrary designation for the highest layer of the ionosphere.

Gamma ray

—Electromagnetic radiation with the shortest wavelengths.

Inductor

—Electrical component that adds magnetic property to wire.

Infrared light

—Light with wavelengths longer than those of visible light.

Ionized

—Missing one or more electrons, resulting in a charged atom.

Magnetic field

—Effect in space resulting from the motion of electric charge.

MegaHertz

—One million cycles per second; MHz.SI abbreviation for MegaHertz.

Morse code

—Dot and dash code used to send messages over telegraph wires.

Resonance

—The enhancement of the response of a system to a force, when that force is applied at a particular frequency known as the resonant frequency.

Selectivity

—Receiver property enabling reception of only wanted signals.

Sensitivity

—Receiver property enabling reception of weak signals.

Standing wave

—A stationary pattern of activity resulting from interference.

Sunspot

—Cooler and darker areas on the surface of the sun. They appear dark only because they are cooler than the surrounding surface. Sunspots appear and disappear in cycles of approximately 11 years.

Telemetry

—Engineering and scientific measurements transmitted by radio.

X ray

—Electromagnetic radiation of very short wavelength, and very high energy.