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26-13 An FSTV signal. This shows one line of data.
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The electromagnetic field 491
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26-14 An electromagnetic field consists of electric and magnetic flux lines at right angles.
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have exactly the same form. The difference is in their frequency. The frequency of an electromagnetic wave is directly related to the wavelength in space.
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Frequency versus wavelength
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All electromagnetic fields have frequencies and wavelengths that are inversely related. If fMHz is the frequency of a wave in megahertz, and Lft is the wavelength in feet, then Lft = 984/fMHz for waves in outer space or in the atmosphere of the earth. If the wavelength is given as Lm in meters, then Lm = 300/fMHz The inverses of these formulas, for finding the frequency if the wavelength is known, are fMHZ = 984/Lft
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492 Data transmission and fMHz = 300/Lm
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The electromagnetic spectrum
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The whole range of electromagnetic frequencies or wavelengths is called the electromagnetic spectrum. Theoretically there is no limit to how low or high the frequency can be, nor, correspondingly, to how long or short the wavelength can be. The most common electromagnetic wavelengths range from about 106 m, or 1000 km, to around 10 12 m, or a trillionth of a meter. Scientists use a logarithmic scale to depict the electromagnetic spectrum. A simplified rendition is shown in Fig. 26-15, labeled for wavelength in meters. To find the frequencies in megahertz, divide 300 by the wavelength shown. For frequencies in hertz, use 300,000,000 instead of 300. For kilohertz, use 300,000; for gigahertz, use 0.300.
26-15 The electromagnetic spectrum, with the radio portion expanded.
Radio waves fall into a subset of this spectrum, at frequencies between approximately 9 kHz and 1000 GHz. This corresponds to wavelengths of 33 km and 0.3 mm. The radio spectrum, which includes television and microwaves, is blown up in Fig. 26- 15, and is labeled for frequency. To find wavelengths, use the conversion formulas. Just be sure you have the decimal point in the right place with respect to the 3.
Transmission media 493
Transmission media
Data can be transmitted over various different media. The most common are cable, radio, satellite links, and fiberoptics. Cable, radio/TV, and satellite communications use the radio-frequency spectrum. Fiberoptics uses infrared or visible light energy.
Cable
The earliest cables were wires that carried dc. Nowadays, data-transmission cables more often carry ac at radio frequencies. One advantage of using RF is 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 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 is used. This has a center conductor surrounded by a cylindrical shield. The shield is grounded, and the center conductor carries the signals (Fig. 26-16). The center conductor is kept in place by an insulating dielectric, usually made of polyethylene. The shield keeps signals confined to the cable, and also keeps external electromagnetic fields from interfering with the signals.
26-16 A coaxial cable has a center conductor surrounded by a cylindrical shield.
Cable signals can be modulated using any of the techniques outlined earlier in this chapter. The most familiar example is cable television.
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 PA output to the antenna itself. Most 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.
494 Data transmission 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 degrees out of phase, so that their electromagnetic 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 radio frequency bands are generally categorized from very low frequency (VLF) through microwaves, according to the breakdown in Table 26-1. These waves propagate through the atmosphere, or through space, in different ways depending on their frequency. Radio signal propagation is discussed in the next chapter.
Table 26-1.
Classification Very Low Frequency Low Frequency (Longwave) Medium Frequency High Frequency (Shortwave) Very High Frequency Ultra High Frequency Microwaves
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