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

## TL;DR

A low-cost, broadband magnetic field probe is developed for measuring RF circuits from 2 to 8 GHz using a printed circuit board structure.

## Contribution

The novel probe design combines a 4-layer PCB with a broadband balun to enable cost-effective and accurate normal magnetic field measurements.

## Key findings

- The probe achieves high broadband performance and cost efficiency using an FR-4 substrate and microstrip balun.
- Calibration experiments confirmed the probe's accuracy when measuring field distributions on a microstrip line and LNA.
- The probe successfully identified radiating components in a low-noise amplifier through field measurements.

## Abstract

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.