Differential E-Field Coupling to Shielded H-Field Probe in Near-Field Measurements and a Suppression Approach

In near-field scanning using H-field probes, E-field coupling is a major concern. E-field suppression performance must be characterized before an H-field probe can be used for near-field scanning. Common method of measurement involves measuring the E-field coupling in the same location where the strongest H-field coupling occurs. In microstrip line traces, it is assumed that this occurs right above the center of the trace and less coupling at all other locations across the microstrip line. In this paper, we show that the maximum E-field coupling occurs at a location slightly offset from the trace center. The E-field coupling to a shielded H-field probe at such a position leads to differential-mode coupling, which the standard shield of an H-field probe is unable to suppress. The coupling mechanism is investigated and a differential E-field coupling suppression approach is proposed. For the H-field probe used in this paper, a proposed floating plate is shown to improve the measured E-field suppression ratio by a factor of 18 dB compared to a similar probe without this modification.