Jump to navigation Jump to search This article แดนซ์ 2018 about impedance in electrical circuits. For impedance of electromagnetic waves, see Wave impedance.
A transmission line drawn as two black wires. The characteristic impedance of a lossless transmission line is purely real, with no reactive component. Energy supplied by a source at one end of such a line is transmitted through the line without being dissipated in the line itself. Schematic representation of an elemental length of a transmission line.
0, and subsists for finite transmission lines until the wave reaches the end of the line. In this case, there will be in general a reflected wave which travels back along the line in the opposite direction. When this wave reaches the source, it adds to the transmitted wave and the ratio of the voltage and current at the input to the line will no longer be the characteristic impedance. Although an infinite line is assumed, since all quantities are per unit length, the characteristic impedance is independent of the length of the transmission line. A surge of energy on a finite transmission line will see an impedance of Z0 prior to any reflections arriving, hence surge impedance is an alternative name for characteristic impedance. Consider one section of the transmission line for the derivation of the characteristic impedance. This figure is to be used for both the derivation methods.
The differential equations describing the dependence of the voltage and current on time and space are linear, so that a linear combination of solutions is again a solution. The positive directions of V and I are in a loop of clockwise direction. Both V and I satisfy the same equation. Since ZY is independent of z and t, it can be represented by a constant -k2. If lumped circuit analysis has to be valid at all frequencies, the length of the sub section must tend to Zero.