The Excitation of Balanced Transmission Line Modes in Bulk Current Injection Measurements

Representative bulk current injection probes are experimentally characterized showing that a significant amount of differential mode current can be excited in a two-wire transmission line, even within the published operating frequency ranges of the probes. The experiment was carried out using a specialized test fixture, which allows precise orientation of the two-wire transmission line with respect to the current probe. It was found that maximum coupling to the balanced transmission line mode occurs when the plane of the primary winding in the probe is parallel to the plane of the two-wire transmission line. The excitation of the balanced mode is tied to an azimuthal variation of the electromagnetic field of the probe. Somewhat unexpectedly, this azimuthal variation has been found to be due at least in part to loss in the magnetic material. This behaviour is qualitatively verified using an electromagnetic simulation in which the dispersion of the ferrite is included.