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Updating Circuit Theory: Modelling Radiated Interference

Series

By Ian Darney

A circuit model is derived which can simulate the differential-mode response of a signal link when it is exposed to electromagnetic interference.

The concept of the radiation resistance is developed to relate the current in an isolated conductor to that of the current in a dipole transmitting antenna. By invoking worst-case conditions, a simple equation can be used to define the maximum H-field of the transmitted wave in terms of the current in the conductor. The E-field can then be determined by multiplying the H-field by the intrinsic impedance of free space. When the E-field interacts with a conductor in the far distance, a voltage is induced in that conductor. A simple equation is derived which relates that voltage to the magnitude of the E-field. The power received is then calculated by dividing the square of the voltage by the radiation resistor.

Representing a twin-conductor cable as two transmission lines in series allows a circuit model to be created which simulates the differential-mode current induced in the signal link.


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