# Switchable Broadband-to-Tunable Narrowband Magnetic Probe for Near-Field Measurements
###### Abstract
The electromagnetic emission from the ground plane was effectively suppressed by adopting a tapered transition between the rectangular loop and the coplanar waveguide 50 \(\Omega\) transmission line. Consequently, this transition eliminated the capacitive effect and enhanced the operation bandwidth compared to the conventional probe with the same covering band. A printed novel circular broadband (up to 9 GHz) magnetic probe was proposed to serve as a sensor for near-field measurement [21]. This probe was based on the fiberglass microwave substrate FR-4 with relative permittivity \(\varepsilon_{r}\) = 4.4 and thickness h = 0.8 mm. A group of quasi-periodic notches was engraved on the feeding line to work as a band-stop  
Figure 1: Block diagram presenting the EMC test with and without the pre-compliance test. filter to eliminate the probe's self-resonance. Moreover, the proposed structure accurately detected the magnetic field with high isolation by suppressing the unwanted electrical field. A magnetic probe with four ultra-band layers up to 20 GHz was proposed in [22]. The via fence technique was employed to suppress the resonance at the operating frequency, making the S21 parameter curve more smooth. Despite utilizing the via fence technique, the resulting probe does not have a flat transition for the whole frequency band. For instance, the sensitivity in terms of S21 parameters was about \(-40\) dB at 2 GHz, then reached \(-30\) dB at 12 GHz, and went back down again to \(-40\) dB at 20 GHz. Ref. [23] presented an ultra-wideband miniature magnetic probe with a high spatial resolution. The operating frequency was extended up to 30 GHz with a high sensitivity of \(-30\) dB by reducing the parasitic inductance and parasitic capacitance using small loops and tapered transition techniques. The high spatial resolution of this probe achieved with a 250 \(\upmu\)m \(\times\) 250 \(\upmu\)m aperture slot enabled it to detect the magnetic field in small devices. However, this probe suffered from high fluctuations in sensitivity across the whole operating band up to 30 GHz. A small broadband shielded magnetic sensor for near-field measurement was presented in [24]. The structure was based on an FR-4 substrate and consisted of four stick-up layers. Furthermore, the relative sensitivity was enhanced by about 5 dB for the covering band from 100 MHz to 6 GHz using an inverted G-shape inductive line. Via fence and metal canned techniques were utilized simultaneously to suppress unwanted electrical fields and achieve high isolation. Narrowband probes usually have high sensitivity at the specific working frequency. A highly sensitive resonance magnetic probe for measuring radio-frequency interference was demonstrated in [25]. Incorporating the LC resonance circuit in differential mode with Marchand balun exhibited a sensitivity improvement of about 8.63 dB at 1.57 GHz. Moreover, this probe showed its validity when it was used to detect the radiation emitted from PCB traces and cell phones in the real world. The authors of Ref. [26] proposed design procedures for an electric probe working at 1.57 GHz. This structure consisted of LC resonators followed by quarter-wavelength transformers to achieve optimal power transformation at the resonance frequency. This probe was dedicated to determining the changing voltage source of the device under test. The sensitivity enhancement was 6.6 dB compared with the result of the broadband equivalent size probe. Ref. [27] presented modeling and investigation of a high-sensitivity narrowband magnetic probe's design. The use of an LC resonator afforded a sensitivity of about 20 dB compared to the conventional equivalent magnetic sensor. The source noise might spread among multi-frequency bands over the device under test (DUT). The electromagnetic interference at multiple frequency bands can be estimated using low-sensitivity broadband probes or multi-narrowband high-sensitivity magnetic probes. A tunable resonance magnetic probe was designed, fabricated, and investigated in [28].