# Switchable Broadband-to-Tunable Narrowband Magnetic Probe for Near-Field Measurements
###### Abstract  
This paper presents a printed magnetic probe that can switch from broadband to tunable narrowband for near-field measurement. In the early design stage, we created a printed loop gap resonator as a magnetic reference sensor for the pre-compliance test in a band up to 6 GHz. Consequently, the results showed a good response in terms of the S11 and S21 parameters of the proposed probe compared with the commercial magnetic sensor XF-R 3-1. The source noise might spread among different frequency bands, making the broadband magnetic probe the closest choice for estimating the magnetic field in the near-field region. Unfortunately, broadband magnetic probes have lower sensitivity than narrowband ones. One of the solutions to get high sensitivity is to connect the LNA to the output of the passive magnetic sensor. This work proposes a novel method to solve this issue using a PIN diode to change the broadband status into a high sensitivity narrowband status and then tune this narrowband across the most critical applications such as 3.5 GHz, 3.75 GHz, 4.8 GHz, and 5.2 GHz with the help of a varactor diode. Compared to the broadband status, an improvement of more than 10 dB has been obtained across all these wireless bands. Furthermore, the proposed structure's isolation between the electrical and magnetic fields is about 13 dB. B 2022222227601  
The United Kingdom (UK) government adopted laws in 1992 to force the manufacturers of all electronic devices or appliances to submit their products to the EMC test before releasing them to the markets. One of the mandatory parts of the EMC test is the electromagnetic sensors (reference antenna and electric/magnetic probe) [8; 9; 10]. The EMC measurement consists of two methods; the far-field method uses the reference antenna to estimate/radiate the electrical field strength (E\({}_{\text{max}}\)) and it is recommended when the level of source interference is significantly high [11]. In contrast, when the source interference is relatively small, the best choice is to utilize magnetic or electric probes in the proximity area above the Equipment Under Test (EUT) [12]. The electric probes suffer from the disturbance of the EUT radiation emitted by the sensor itself when its response approaches the resonance frequency and becomes frequency-dependent [13]. The classical magnetic probe can be easily formed by hand into different shapes like circular and square loops, but it suffers from low accuracy due to its relatively large size [14]. On the other hand, printed magnetic probes have been grown using printed circuit board technology (PCB) in the last two decades, with many advantages like compact size, high accuracy, multilayers, and fabricating ease [15]. The four critical parameters to design any magnetic probe are bandwidth, spatial resolution, sensitivity, and isolation [16]. Several printed magnetic probe structures have been proposed in the last two decades to fulfill these design parameters. The sensitivity measures the strength of the induced magnetic field of the designed probe above the reference plane. Typically, there is a 2 mm reference height between the designed probe and the reference board when measuring the sensitivity. This factor can be estimated using two methods; first, it represents the mutual coupling between the designed magnetic probe and the reference board (S21 parameter). Second, it can be estimated by measuring the two minimum points of the magnetic field distribution, which indicate the detection accuracy. The broadband magnetic probe covers a wide bandwidth, making it an excellent choice for estimating the magnetic field surrounding electronic circuits. Unfortunately, it suffers from low sensitivity compared to narrowband magnetic probes [17; 18]. Connecting a low noise amplifier (LNA) at the output of the passive magnetic probe to enhance the sensitivity is one of the solutions to overcome the broadband probe issue [19]. A novel rectangular broadband (up to 7 GHz) magnetic probe was presented in Ref. [20].