Transmission Line Circuit

Transmission lines, used for wired communication services, can be analyzed as a distributed circuit. A distributed circuit is an infinite chain of identical circuits that are interconnected so as to manifest electrical characteristics which will represent a particular electrical device. There are five postulates of distributed circuit analysis.

Postulate 1 The uniform system or line consists of two straight parallel conductors.

Postulate 2 The currents in the line conductors flow only in the direction of the length of the line.

Postulate 3 at the intersection of any transverse plane with the conductors of a transmission line, the instantaneous total currents in the two conductors are equal in magnitude and flow in opposite direction.

Postulate 4 At the intersection of any transverse plane with the line conductors there is a unique value of potential difference between the conductors at any instant which is equal to the line integral of the electrical field along all paths in the transverse plane, between any point on the periphery of one of the conductors and any point of the periphery of the other.

Postulate 5 The electrical behavior of the line is completely described by four distributed electric circuit coefficients, whose values per unit length are constant everywhere on the line.

These distributed circuit coefficients are: series resistance, series inductance, shunt conductance and shunt capacitance. Series resistance represents losses from thermal dissipation along the line. Series inductance represents the introduction of the magnetic flux into the line due to the current that flows through the conductors. Shunt conductance refers to any lack to any leaks in the insulator separating the conductors which results to current division. Shunt capacitance is the representation of the inherent impedance presented by the capacitance resulting from two conductors and a dielectric in between. Furthermore, a transmission line can also be described by a single value to represent the four electrical coefficients. This is called the characteristic impedance. This value in general terms is the square root of the ratio of the series impedance to the shunt admittance. At RF range, the expression loses its real components due to small value approximation.

Posted 2010-12-14 and updated on Dec 14, 2010 4:11pm by crisd

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