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| Line Communication - 2 - Wire Lines, Cables |
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| Wave propagation in unbounded media |
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| (media of infinite extent) |
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| This type of wave propagation is said to be unguided. The uniform plane wave exists throughout all space. The electromagnetic energy associated with the wave spreads over a wide area. Wave propagation in unbounded media is used in radio or TV broadcasting where the information is meant for everyone who may be interested. Such means of wave propagation will not help in a situation like telephone conversation where the information is received privately by one person. |
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| Guided structures |
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| Another means of transmitting power or information is by guided structures. Guided structures serve to guide (or direct) the propagation of energy from the source to the load. Typical examples of such structures are transmission lines and wave guides. The discussion of waveguides is beyond the scope of the present text. |
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| Transmission lines |
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| These are commonly used in power distribution (at low frequencies) and in communications (at high frequencies). |
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| Biodiversity is most simply defined as the variety of life and its processes. |
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| A transmission line may be defined as a device for transmitting or guiding energy from one point to another. The energy may be for lighting, heating, or performing work, or it may be in the form of signal information (speech, pictures, data, music). |
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| Basically a transmission line has two terminals into which power (or information) is fed and- two terminals from which power (or information) is received. Thus, a transmission line may be regarded as a four-terminal device for connecting any and all electrical devices. |
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| A transmission line consists of two or more parallel conductors used to connect a source to a load. The source may be hydroelectric generator, transmitter or an oscillator. The load may be a factory, an antenna or an oscilloscope. |
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| The power cord on a lamp or appliance is a transmission line and so are the wires from a generating station to a factory or home. Telephone and telegraph wires; audio, video, and radio cables; and the myriad nerve fibers in our bodies are all transmission lines. The interconnections of all electric circuits are transmission lines, and in a broad sense waveguides and optical fibers, and even radio links maybe regarded as transmission lines. A few examples are shown in the below figure. |
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| A few examples of Transmission Lines |
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| Transmission lines are everywhere and are of infinite variety but regardless of type, length, or construction, all operate according to the same basic principles. |
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| Typical transmission lines include coaxial cable, a two-wire line, a parallel-plate or planar line, a wire above the conducting plane and a micro strip line. Note that each of these lines consist of two conductors in parallel. |
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| Coaxial cables are routinely used in electrical laboratories. These are also used in connecting TV sets to TV antennas. |
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| Micro strip lines are particularly important in integrated circuits where metallic strips connecting electronic elements are deposited on dielectric substrates. |
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| It is convenient to classify transmission lines into three main groups: |
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| (i) those with transverse electromagnetic (TEM) modes |
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| (ii) those with higher-order modes and |
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| (iii) those with transverse electromagnetic space waves (as in a radio link). |
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| In a TEM mode both the electric and magnetic fields are entirely transverse to the direction of propagation. There is no component of either E or H in the direction of transmission. Higher-order modes, on the other hand, always have at least one field component in the direction of transmission. All two-conductor lines such as coaxial or two-wire transmission lines are examples of TEM-mode types, while hollow single-conductor waveguides or dielectric rods are examples of higher mode types. |
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| To summaries, transmission lines may be classified as follows: |
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| (i) TEM-mode type: E and H entirely transverse. All two-conductor types, including coaxial lines. Power flows along and between conductors. |
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| (ii) Higher-mode type: E and H or both have components in the direction of transmission. |
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| Hollow single-conductor waveguides, dielectric rods, and optical fibres. Power flows in space inside hollow conductor waveguides or in or close to dielectric rod or fiber. |
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| (iii) TEM space waves between antennas of a radio link. Power radiates through space. Transmission line problems are usually solved using electromagnetic field theory and electric circuit theory. These are the two major theories on which electrical engineering is based. The discussion of these theories is beyond the scope of the present text. |
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| A coaxial cable consists of an inner conductor and an outer conductor, separated by a dielectric insulating material. The inner conductor is made of a copper wire encased inside the dielectric material. As for the outer conductor, it is made of copper, tinned copper, or copper-coated steel. Typically, a coaxial cable has a characteristic impedance of 50 or 75 ohm. Compared to a twisted-pair cable, a coaxial cable offers a greater degree of immunity to EMI. Moreover, because of their much higher bandwidth, coaxial cables can support the transmission of digital data at much higher bit rates than twisted pairs. Rates up to 20 Mb/s are feasible using coaxial cables, with 10 Mb/s being the standard. |
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| Whereas the use of a twisted pair has been confined mainly to point-to-point service, a coaxial cable can operate as a multiple-access medium by using high-impedance taps. A common application of coaxial cables is as the transmission, medium for local area networks in an office environment. |
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| Another common application of coaxial cables is in cable-television systems, also known as community-antenna television (CATV) systems. In this application coaxial cables are used to distribute television, audio, and data signals from the head end to the subscribers. The head end is tile central originating unit of the CATV system, where all signals are carried and processed. |
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