Transmission Line

A guided wave structure that transmits electrical energy and/or electrical signals in the form of transverse electromagnetic (TEM) modes. The characteristic of transmission line is that its transverse dimension is much smaller than the working wavelength. The main structural types are parallel double-wire, parallel multi-wire, coaxial line, stripline, and microstrip line working in quasi-TEM mode (Fig. 1)(see telecommunication cable) , which can be analyzed by simple double-wire model. The waveguides of TE mode, TM mode, or their hybrid modes can be considered as generalized transmission lines. The distribution of the electromagnetic field along the Propagation Direction in the waveguide is similar to that of the voltage and current on the transmission line.

The transmission line equation, also known as the Telegraph equation, is a set of differential equation that describes the relationship between voltage U and current I on a transmission line. From the point of view of distributed parameter circuit, a short transmission line is equivalent to a t-shaped network of lumped elements (for lossless lines, R1 = G1 = 0) consisting of distributed resistors r 1(o/m) , distributed inductors l 1(H/M) , distributed conductance g 1(W/M) , and distributed capacitance c 1(F/M) , the actual transmission line is represented as a cascade of equivalent networks.