Broadband Low-Cost Normal Magnetic Field Probe for PCB Near-Field Measurement

This paper presents a broadband near-field probe designed for measuring the normal magnetic field (Hz) in radio frequency (RF) circuits operating within a frequency range of 2-8 GHz. The proposed probe uses a cost-effective 4-layer printed circuit board (PCB) structure made with an FR-4 substrate. The probe primarily consists of an Hz detection unit, a broadband microstrip balun, and a coaxial-like output. The broadband balun facilitates the conversion from differential to single-ended signals, thereby enhancing the probe's common-mode rejection capability. This design ensures that the probe achieves both cost efficiency and high broadband measurement performance. Additionally, this work investigates the feasibility of employing microstrip lines as calibration standards for the Hz probe. The probe's structural parameters and magnetic field response were initially determined through simulations, and the calibration factor was subsequently verified by calibration experiments. In practical measurements, the field distributions above a microstrip line and a low-noise amplifier (LNA) were captured. The measured field distribution of the microstrip line was compared with simulation results to verify the probe's performance. Meanwhile, the measured field distribution of the LNA was utilized to identify the radiating components within the amplifier.