A Three-Axis, Dual-Feed, Shielded Loop Probe Incorporating a Re-Entrant Infinite Balun

The realization of a 3-axis magnetic-field probe system comprising three orthogonal, co-located, shielded loops presents some difficulties; in particular, the preservation of balance and isolation. Lack of balance results in an undesirable electric field response as well as degradation of the intrinsic isolation. This has been shown to be because the common-mode (CM) electric field responses of the loops are significantly coupled, even when all three loops are perfectly orthogonal to one another. Exploiting two diametrically located feeds in each loop can more effectively enforce balance but then requires an internal balancing mechanism. Previous work has employed internal Guanella current baluns with magnetic cores and reasonably good performance was obtained. However, limitations in isolation were noticed and found to be due to the internal baluns. Here, an internal, re-entrant infinite balun realized with a triaxial cross section is exploited to obtain improved balance and isolation.