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Data can be transmitted over various media. The most common are cable, radio (also called wireless), satellite links (a specialized form of wireless), and fiber optics. Cable, radio/TV, and satellite communications use the RF spectrum. Fiber optics uses IR or visible light energy.
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Cable The earliest cables were simple wires that carried dc. Nowadays, data transmission cables more often carry ac at radio frequencies. One advantage of using RF is the fact that the signals can be amplified at intervals on a long span. This greatly increases the distances over which data can be sent by cable. Another advantage of using RF is the fact that numerous signals can be carried over a single cable, with each signal on a different frequency. Cables can consist of pairs of wires, somewhat akin to lamp cords. But more often coaxial cable, of the type described and illustrated at the end of Chap. 10, is used. This has a center conductor surrounded by a cylindrical shield. The shield is grounded, and the center conductor carries the signals. The shield keeps signals confined to the cable, and also keeps external EM fields from interfering with the signals. Radio All radio and TV signals are electromagnetic waves. The radio or TV transmitter output is coupled into an antenna system located at some distance from the transmitter. The energy follows a transmission line, also called a feed line, from the transmitter output to the antenna itself. Most radio antenna transmission lines are coaxial cables. There are other types, used in special applications. At microwaves, hollow tubes called waveguides are used to transfer the energy from a transmitter to the antenna. A waveguide is more efficient than coaxial cable at the shortest radio wavelengths. Radio amateurs sometimes use parallel-wire transmission lines, resembling the ribbon cable popular for use with consumer TV receiving antennas. In a parallel-wire line, the RF currents in the two conductors are always 180 out of phase, so that their EM fields cancel each other. This keeps the transmission line from radiating, guiding the EM field along toward the antenna. The energy is radiated when it reaches the antenna. The RF bands are generally categorized from very low frequency (VLF) through extremely high frequency (EHF), according to the breakdown in Table 25-1. As noted in Chap. 24, the exact lower limit of the VLF range is a matter of disagreement in the literature. In Table 25-1, it is defined as 3 kHz, which is consistent with defining the frequency boundaries between RF bands by order of magnitude.
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424 Wireless Transmitters and Receivers
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Table 25-1.
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Frequency designation Very Low (VLF) Low (LF) Medium (MF) High (HF) Very High (VHF) Ultra High (UHF) Super High (SHF) Extremely High (EHF)
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Bands in the RF spectrum.
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Frequency range 3 kHz 30 kHz 30 kHz 300 kHz 300 kHz 3 MHz 3 MHz 30 MHz 30 MHz 300 MHz 300 MHz 3 GHz 3 GHz 30 GHz 30 GHz 300 GHz Wavelength range 100 km 10 km 10 km 1 km 1 km 100 m 100 m 10 m 10 m 1 m 1 m 100 mm 100 mm 10 mm 10 mm 1 mm
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Satellite Systems At very high frequencies (VHF) and above, many communications circuits use satellites in geostationary orbits around the earth. If a satellite is directly over the equator at an altitude of 22,300 mi (36,000 km) and orbits from west to east, it follows the earth s rotation, thereby staying in the same spot in the sky as seen from the surface, and is thus a geostationary satellite. A single geostationary satellite is on a line of sight with about 40 percent of the earth s surface. Three such satellites, placed at 120 (1 3 circle) intervals around the planet, allow coverage of all populated regions. A dish antenna can be aimed at a geostationary satellite, and once the antenna is in place, it need not be turned or adjusted. Another form of satellite system uses multiple satellites in low orbits that take them over the earth s poles. These satellites are in continuous, rapid motion with respect to the surface. But if there are enough of them, they can act like repeaters in a cell phone network, and maintain reliable communications between any two points on the surface at all times. Directional antennas are not necessary in these systems, which are called low earth orbit (LEO) satellite networks. Fiber Optics Beams of IR or visible light can be modulated, just as can RF carriers. The frequency of an IR or visible light beam is higher than the frequency of any RF signal, allowing modulation by data at rates higher than anything possible with radio. Fiber-optic technology offers several advantages over wire cables (which are sometimes called copper because the conductors are usually made of that metallic element). A fiber-optic cable is cheap. It is light in weight. It is immune to interference from outside EM fields. A fiber-optic cable does not corrode as metallic wires do. Fiber-optic cables are inexpensive to maintain and easy to repair. An optical fiber can carry far more signals than a cable, because the frequency bands are far wider in terms of megahertz or gigahertz. The whole RF spectrum, from VLF through EHF, can (at least in theory) be imprinted on a single beam of light and sent through an optical fiber no thicker than a strand of hair!
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