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The common-mode characteristic impedance of a typical cable bundle has been suggested to be approximately 150 Ohms. However, standard test fixtures for BCI probes employ 50 Ohm coaxial ports. Here a representative injection probe (transformer) intended for bulk current injection applications is characterized experimentally in the frequency domain using a 2-port vector network analyzer and a standard (50 Ohm) test fixture as described in the IEC 61000-4-6 standard. The full 3-port scattering representation of the probe in the test fixture is then de-embedded and renormalized such that the coaxial ports of the test fixture are changed to 150 Ohms. The time-domain step response of the representative transformer is then computed from renormalized frequency-domain scattering matrix. It is seen that changing the impedance level from 50 Ohms to 150 Ohms significantly alters both the frequency and time domain performance of the transformer. Thus, the standard test fixture does not accurately predict the frequency- and time-domain performance of a typical BCI transformer. It is shown that the dependence of performance on port impedance can, to some extent, be anticipated by examining the equivalent network for the transformer.