Radio is the transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. Information is carried by systematically changing (modulating) some property of the radiated waves, such as amplitude, frequency, or phase. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.
Waves in general are measured or specified by two properties: the physical length of the wave, and the number of times the wave cycle is repeated in a given period of time. Because of their varying characteristics, radio waves of different lengths are employed for different purposes but are usually identified by their frequency (cycles per second).
In honor of the German radio pioneer Heinrich Hertz, his name has been given to the cycle per second (hertz, Hz); 1 kilohertz (kHz) is 1000 cycles per sec, 1 megahertz (MHz) is 1 million cycles per sec, and 1 gigahertz (GHz) is 1 billion cycles per sec. Radio waves range from a few kilohertz up to 30 gigahertz (30,000,000,000 Hz).
The higher the frequency, the smaller the wavelength, since all the waves travel at the speed of light (about 186,000 mi / 300,000 km per second). The shortest waves have the highest frequency, or number of cycles per second; the longest waves have the lowest frequency, or fewest cycles per second.
In a vacuum, all electromagnetic waves travel at a uniform speed of about 300,000 km (about 186,000 mi) per second.
Radio waves are a form of electromagnetic radiation, created whenever a charged object accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. This is the frequency range 3 kHz to 40,000 MHz, wavelength range 1 cm to more than 1 km (0.4 inch to 0.6 mile). The radio-frequency spectrum is arbitrarily divided into a number of wavebands, from very low frequencies (long wavelengths) to ultra-high and microwave frequencies (short wavelengths). Sections of the spectrum have been allocated by international agreement to use for telegraph, telephonic speech, and radio and television broadcasting.
In radio transmission a microphone converts sound waves (pressure variations in the air) into electromagnetic waves that are then picked up by a receiving aerial and fed to a loudspeaker, which converts them back into sound waves. To carry the transmitted electrical signal, an oscillator produces a carrier wave of high frequency; different stations are allocated different transmitting carrier frequencies. The transmitter generates electrical oscillations at a radio frequency called the carrier frequency.
A modulator superimposes the audiofrequency signal on the carrier. Information is imposed on a carrier wave by varying its amplitude, frequency, or duration in a process called modulation. There are two main ways of doing this:
- Amplitude modulation (AM)
- used for long- and medium-wave broadcasts, in which the strength of the carrier is made to fluctuate in time with the audio signal; Amplitude modulation (AM) creates "side-band" frequencies at the upper and lower limits of a carrier wave, which register variations in the strength of the wave. An amplitude-modulated signal consists of the carrier frequency plus two sidebands resulting from the modulation.
- Frequency Modulation (FM)
- as used for VHF broadcasts, in which the frequency of the carrier is made to fluctuate. The transmitting aerial emits the modulated electromagnetic waves, which travel outwards from it. Frequency modulation (FM) varies the number of cycles the wave goes through, instead of its amplitude. Frequency modulation produces more than one pair of sidebands for each modulation frequency. These changes in the otherwise constant frequency of the wave carry the desired information from transmitter to receiver.
Duration modulation is a simpler system, in which a constant tone is transmitted for a certain period of time, shut off, and then transmitted again. Morse code utilizes this system.
When radio waves pass an electrical conductor, the oscillating electric or magnetic field (depending on the shape of the conductor) induces an alternating current and voltage in the conductor. This is the principle of the antenna or aeriel. This can be transformed into audio or other signals that carry information. In radio reception a receiving aerial picks up minute voltages in response to the waves sent out by a transmitter. A tuned circuit selects a particular frequency, usually by means of a variable capacitor connected across a coil of wire. A demodulator separates the audio signal from the carrier, and an amplifier boosts the audio signal for feeding to the loudspeaker.
In a superheterodyne receiver, the incoming signal is mixed with a signal of fixed frequency generated within the receiver circuits. The resulting signal, called the intermediate-frequency (i.f.) signal, has a frequency between that of the incoming signal and the internal signal. The intermediate frequency is near the optimum frequency of the amplifier to which the i.f. signal is passed. This arrangement ensures greater gain and selectivity. The superheterodyne system is also used in basic television receivers.
Shortwave Radio is a means of radio broadcast, similar to that of mediumwave, but which travels more reliably for longer distances. It is therefore used as a means of international broadcasting for a number of purposes. International Broadcasting, as the name implies is transmitting trans-nationally, although the term usually refers to speech 'broad'casters. Transmission and reception of information by means of electromagnetic waves about 10 to 80 m (33 to 262 feet) in length having frequencies of approximately 29.7 to 3.5 megahertz.
From a purely technical point of view, shortwave radio refers to those frequencies between 3 and 30 MHz. Their main characteristic is their ability to "propagate" for long distances, making possible such worldwide communications as international broadcasting and coordination of long-distance shipping. During the early 1920s attempts were made to transmit radio signals over long distances by bouncing them off the layers of charged particles in the Earth's ionosphere. The success of these experiments prompted the establishment of worldwide shortwave communication by the late 1930s.
From a social point of view, shortwave radio is a way to find out what the rest of the world thinks is important. Many countries broadcast to the world in English, making it easy to find out what a given country's position is on those things it finds important. Shortwave radio can also provide a way to eavesdrop on the everyday workings of international politics and commerce. Shortwave broadcasts provide the major source of news and popular entertainment in much of the world except in highly developed regions such as western Europe, North America, and Japan, where government or commercial programming is transmitted within other bands of frequencies. Among the world's most powerful shortwave broadcasting stations are Radio Peking, Radio Moscow, the British Broadcasting Corporation, and the Voice of America.
Besides their use in international broadcasting, shortwave radio frequencies and techniques are utilized to relay telephone and telegraph communications over great distances. Amateur radio stations and portable two-way radios also operate at shortwave frequencies.
Interest in amateur radio arose around the turn of the century, shortly after the Italian inventor Guglielmo Marconi successfully sent the first transatlantic wireless signal in 1901. The interference of amateur broadcasts with commercial and military transmissions led to the institution of government control in 1911. After World War I, amateurs became active in radio experimentation, contributing to developments in long-distance broadcasting and becoming the first radio operators successfully to exploit the upper medium-frequency and lower high-frequency radio bands. Over the years, amateur radio operators have also provided emergency communications during forest fires, floods, hurricanes, and other disasters. They serve as an important link between stricken communities and the outside world until normal communications are reestablished.
Amateur radio operators in the United States are subject to international and federal regulations. There are five classes of licenses. Competence in the use of the International Morse Code and a knowledge of radio theory and regulation are required to obtain the advanced-level licenses. Amateur radio is allocated frequencies at the extreme high-frequency end of the medium-wave band, five groups of frequencies in the shortwave band, two groups in the very-high-frequency band, three in the ultrahigh-frequency band, and seven in the superhigh-frequency band for telegraphic and telephonic communication using amplitude and frequency modulation. There are restrictions on the power of the transmitters, and certain of the frequencies must be shared with due regard for the needs of other users.