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They were developed by Oliver Heaviside who created the transmission line model, and are based on Maxwell's equations. Schematic representation of the elementary component of a transmission line. The transmission line model is an example of the distributed-element model. It represents the transmission line as an infinite series of two-port ...
Applying the transmission line model based on the telegrapher's equations as derived below, [1] [2] the general expression for the characteristic impedance of a transmission line is: = + + where R {\displaystyle R} is the resistance per unit length, considering the two conductors to be in series ,
Heaviside's model of a transmission line. A transmission line can be represented as a distributed-element model of its primary line constants as shown in the figure. The primary constants are the electrical properties of the cable per unit length and are: capacitance C (in farads per meter), inductance L (in henries per meter), series resistance R (in ohms per meter), and shunt conductance G ...
Approximated model for Short Transmission Line Phasor diagram of short transmission line. The transmission lines which have a length less than 60 km are generally referred to as short transmission lines. For its short length, parameters like electrical resistance, impedance and inductance of these short lines are assumed to be lumped.
Unlike the transmission line example, the need to apply the distributed-element model arises from the geometry of the setup, and not from any wave propagation considerations. [3] The model used here needs to be truly 3-dimensional (transmission line models are usually described by elements of a one-dimensional line).
"Black box" model for transmission line. The terminal characteristics of the transmission line are the voltage and current at the sending (S) and receiving (R) ends. The transmission line can be modeled as a black box and a 2 by 2 transmission matrix is used to model its behavior, as follows:
Looking towards a load through a length l of lossless transmission line, the normalized impedance changes as l increases, following the blue circle. At l=λ/4, the normalized impedance is reflected about the centre of the chart. Standing waves on a transmission line with an open-circuit load (top), and a short-circuit load (bottom).
This pulse will be partially reflected and transmitted according to the transmission-line theory. If we assume that each line has a characteristic impedance , then the incident pulse sees effectively three transmission lines in parallel with a total impedance of /. The reflection coefficient and the transmission coefficient are given by