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34 Common Diagnostic Techniques www.ets-lindgren.com
Radiation is caused by an instantaneous change in current flow, causing a
magnetic field, or by an instantaneous change of a potential difference, causing
an electric field. Experience has shown a high degree of correlation between
magnetic fields with differential mode current flow. Although a change in voltage
will cause a change in current and vice versa, one of these vectors will
predominate. The impedance of the radiating source will determine whether a
predominately magnetic or predominately electric field is produced.
Typically, magnetic fields are produced by local current loops within a unit. These
loops may be analyzed as differential mode. Electric fields require
high-impedance sources. Because the changing potential is isolated by
substantial impedance on all lines into the circuit, all lines will carry just the
forward current.
The impedance in this context is the total impedance at the radiating
frequency. Often what appears as low-impedance connections are
actually high-impedance due to the inductance in the physical circuit.
A common way for all lines in a circuit to become high-impedance lines is for the
ground servicing that circuit to contain a significant inductance. At some
frequency, this ground inductance becomes a high-impedance. Because the
entire circuit references ground, this impedance in the ground path effectively is
in series with every line in the circuit. The return flow in this situation is developed
by capacitive coupling to conductors external to the unit or to fortuitous
conductors within the unit.