Mechanisms for Unwanted Magnetic Field Coupling to a Shielded Magnetic Near-Field Probe

Unwanted field coupling is a major concern when using magnetic near-field probes for high-frequency scanning. The unwanted coupling of the tangential electric field to a tangential magnetic field probe has been studied extensively. This article shows a second unwanted coupling mechanism: coupling from a vertical magnetic field. The standing wave of a shorted pure TEM microstrip in air is utilized to separate the magnetic and electric field coupling. Simulations show that the eddy currents induced within the probe by the vertical magnetic field couple inductively to the pickup loop if the layer stack up is not symmetric. The unwanted magnetic field coupling mechanism is validated through both simulation and measurement. Two improved designs are proposed to mitigate the impact.

I. Introduction

Near-field scanning is often utilized to investigate electromagnetic compatibility (EMC) issues [1], [2], [3]. Many magnetic near-field probes have been designed in recent years. Most of the designs are manufactured using a printed circuit board (PCB) and consist of an outer “shield” (on the top and bottom layers) and an inner trace (on the middle layer). A via fence or edge plating is usually used to achieve better shielding to suppress unwanted electric field (E-field) coupling, as shown in Fig. 1. Probes are built to optimize different properties, such as narrowband resonant probes [4], [5], broadband passive and active probes [6], [7], [8], [9], [10], [11], [12], [13], probes with high E-field suppression [14], [15], [16], probes that use spatial resolution improvement techniques [17], [18], and multicomponent magnetic field probes [19]. A practical and effective probe calibration technique was presented in [20]. Fig. 1.

Two common shield designs to suppress unwanted E-field coupling for the H-field probe. (a) Edge plating technique. (b) Via fence.