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radio wave[′rād·ē·ō ‚wāv]
an electromagnetic wave of wavelength greater than 500 micrometers (μm) or of frequency less than 6 × 1012 hertz (Hz). Radio waves have many applications, including radio broadcasting, radiotelephone communications, television, radar, and radio meteorology. In these cases radio waves are a means of wireless transmission of information, which may be in the form of speech, telegraph signals, or images. Radio waves are used to determine the direction and, in range only radar, the distance to various objects. Their other uses include the obtaining of information about the structure of the upper atmospheric layers, the sun, and the planets.
|Table 1. Subdivisions of radio spectrum|
|Subdivision1||Wavelength range (m)||Frequency range (Hz)|
|1These subdivisions are discussed in individual articles of the encyclopedia|
|Superlong waves ......||greater than 104||less than 3 × 104|
|Long waves..........||104–103||33 × 104–3 × 105|
|Medium waves........||103–102||3 × 105–3 × 106|
|Short waves..........||102–10||3 × 106–3 × 107|
|Meter waves .........||10–1||3 × 107–3 × 108|
|Decimeter waves ......||1–0.1||3 × 108–3 × 109|
|Centimeter waves......||0.1–0.01||3 × 109–3 × 1010|
|Millimeter waves......||0.01–0.001||3 × 1010–3 × 1011|
|Submillimeter waves ......||103–5 × 10–5||3 × 1011–6 × 1012|
The first experimental transmissions of signals by means of radio waves were carried out by A. S. Popov between 1895 and 1899. Popov used wavelengths of 200 to 500 m—that is, frequencies of 1.5 × 106 to 0.6 × 106 Hz. The subsequent development of radio engineering resulted in the employment of a wider spectrum of electromagnetic waves. The lower limit for the spectrum of radio waves radiated by radio-transmitting devices is of the order of 103–104 Hz.
Many sources of radio waves exist in nature. Examples are the stars, including the sun, the galaxies, the metagalaxy, and the planets. Radio astronomy’s investigation of radio waves from extraterrestrial sources has permitted us to broaden our knowledge of the universe. Some processes that occur within the earth’s atmosphere are also accompanied by the generation of radio waves. For example, radio waves result from lightning discharges (seeATMOSPHERICS) and from the excitation of oscillations in the ionospheric plasma. Such processes can cause the excitation of radio waves of frequencies as low as fractions of a hertz.
Since radio waves of different frequencies propagate differently within the limits of the earth and in outer space, they find different applications in radio communications and in scientific investigations. On the basis of the characteristics of progaga-tion, generation, and—to some extent—processes, the radio spectrum is commonly divided into a number of subdivisions, which are given in Table 1. The basic classification of radio waves into bands for radio communications purposes has been established by the international Radio Regulations. Bands 4 through 11 in Table 2 are used by the various radio services; the designations of the other bands listed are standard Soviet usage.
M. B. VINOGRADOVA
RF(Radio Frequency) The range of electromagnetic frequencies above the audio range and below infrared light (from 10 kHz to 300 GHz). RF transmission includes AM and FM radio, analog and digital TV, satellite communications, cordless phones, cellular and Wi-Fi. Signals can be focused in one direction (directional), or they can transmit in all directions (omnidirectional).
Frequency and Power
The range of frequencies and power output determine how well RF signals can penetrate objects. Signals up to 5 GHz can generally transmit through wooden and plaster walls in their path, but the higher the frequency, the more an unobstructed view from transmitter to receiver is required. See line of sight, RF modulation, RF shielding, RF remote control, RFI, microwave and ISM band.
|Radio frequency ports (right) are typically coaxial cable connectors for antennas and TVs. See coaxial cable.|