|
The primary line constants are parameters that describe the characteristics of conductive transmission lines, such as pairs of copper wires, in terms of the physical electrical properties of the line. The primary line constants are only relevant to such lines and are to be contrasted with the secondary line constants, which can be derived from them, and are more generally applicable. The secondary line constants can be used, for instance, to compare the characteristics of a waveguide to a copper line, whereas the primary constants have no meaning for a waveguide. The constants are conductor resistance and inductance, and insulator capacitance and conductance, which are by convention given the symbols ''R'', ''L'', ''C'', and ''G'' respectively. The constants are enumerated in terms of per unit length. The circuit representation of these elements requires a distributed element model and consequently calculus must be used to analyse the circuit. The secondary constants of characteristic impedance and propagation constant can be derived in this way. A number of special cases have particularly simple solutions and important practical applications. Low loss cable requires only ''L'' and ''C'' to be included in the analysis, useful for short lengths of cable. Low frequency applications, such as twisted pair telephone lines, are dominated by ''R'' and ''C'' only. High frequency applications, such as RF co-axial cable, are dominated by ''L'' and ''C''. Lines loaded to prevent distortion need all four elements in the analysis, but have a simple, elegant solution. ==The constants== There are four primary line constants, but in some circumstances some of them are small enough to be ignored and the analysis can be simplified. These four, and their symbols and units are as follows: ''R'' and ''L'' are elements in series with the line (because they are properties of the conductor) and ''C'' and ''G'' are elements shunting the line (because they are properties of the dielectric material between the conductors). ''G'' represents leakage current through the dielectric and in most cables is very small. The word loop is used to emphasise that the resistance and inductance of both conductors must be taken into account. For instance, if a line consists of two identical wires that have a resistance of 25 mΩ/m each, the ''loop'' resistance is double that, 50 mΩ/m. Because the values of the constants are quite small, it is common for manufacturers to quote them per kilometre rather than per metre; in the English-speaking world "per mile" can also be used.〔Connor, p.8.〕〔Bird, pp.604-605.〕 The word "constant" can be misleading since there is some variation with frequency. In particular, ''R'' is heavily influenced by the skin effect. Furthermore, while ''G'' has virtually no effect at audio frequency, it can cause noticeable losses at high frequency with many of the dielectric materials used in cables due to a high loss tangent. Avoiding the losses caused by ''G'' is the reason many cables designed for use at UHF are air-insulated or foam-insulated (which makes them virtually air-insulated).〔Porges, pp.223-224.〕 The actual meaning of constant in this context is that the parameter is constant with ''distance''. That is the line is assumed to be homogenous lengthwise. This condition is true for the vast majority of transmission lines in use today.〔Bird, pp.502-503, 519.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Primary line constants」の詳細全文を読む スポンサード リンク
|