Theoretical assessment of bulk current injection versus radiation

Field excitation and bulk current injection (BCI) on a wire bundle are analytically compared. The equivalence of the two excitation techniques is investigated in order to ascertain the validity of using current injection testing rather than radiated immunity testing for an equipment connected at one end of the bundle. The wire bundle is modeled as a multiconductor transmission line which, throughout the analysis, is treated as a multiport device. The equivalence is discussed by comparing the effects that the sources produce at the terminal ports, with the aim to control the clamp voltage in order to match the incident field characteristics. This approach yields general results, which are not affected by any assumptions on the terminal networks. It is shown that one injection probe is not sufficient to assure the equivalence in the general case, but it is possible, although practically difficult, to achieve this result by employing two injection probes. In the simpler but more specific case of linear terminations, a test procedure is proposed which allows equivalence by using a single injection current probe. Since the ideal BCI procedure must represent a worst-case stress analysis for the equipment under test, the possibility of adopting BCI tests to reproduce the upper envelope of the radiation-induced currents is also investigated. In this case practical difficulties are evidenced and arise from the need of feeding the injection probes with a complicated frequency-dependent power distribution.