# Switchable Broadband-to-Tunable Narrowband Magnetic Probe for Near-Field Measurements
## 2 Magnetic Probe Design
Moreover, the achieved results have been validated by comparing them to the results obtained with the commercial broadband magnetic probe XF-R 3-1 that is offered for sale [32], as shown in Figure 4.1 \begin{table}
\begin{tabular}{c c c c c c c} \hline
**NO.** & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline PARAMETER & \(L_{s}\) & \(L_{p}\) & \(L_{a}\) & \(r\) & \(r_{vial1}\) & \(r_{vial2}\) \\ VALUE/MM & 50 & 10 & 48.32 & 1.5 & 0.5 & 0.4 \\ \hline PARAMETER & \(W_{s}\) & \(W_{p}\) & \(W_{a}\) & \(t\) & \(S\) & \(d_{vial1}\) \\ VALUE/MM & 10 & 5 & 0.3 & 0.8 & 1.5 & 2 \\ \hline \end{tabular}
\end{table} 
Table 1: The overall dimensions of the structure. Figure 6: The surface current distribution of the broadband magnetic probe: (**a**) top view; (**b**) perspective view. Figure 8: Prototype of the magnetic probe: (**a**) dimensions of the probe; (**b**) S21-parameter measurement setup. Figure 7: S-parameters of the broadband magnetic probe versus frequency. Figure 9: S-parameters of the proposed magnetic probe compared with the commercial XF-R 3-1: (**a**) S11 parameter; (**b**) S21 parameter. The proposed probe performs better than the XF-R 3-1 magnetic probe in terms of sensitivity using S11 and S21 parameters. For instance, the sensitivity of the proposed magnetic sensor is better than the commercial probe, which makes sense because the spatial resolution of the proposed magnetic sensor is 1.6 mm compared to 1 mm for the XF-R 3-1 probe. The 3.5-6 GHz band covers many radio frequency applications like WiMAX at 3.5 GHz, mid-bandwidth 3.6-3.8 GHz for 5G applications, PAN at 4.8 GHz, and WLAN at 5.2 GHz. Therefore, one of the benefits of increasing the broadband magnetic probe's sensitivity is accurately estimating the magnetic field interference with these applications. The switchable magnetic probe can solve this issue using the different switching techniques presented in [33; 34; 35]. A PIN diode type SMP1320-079 LF has been utilized to switch the resonance behavior from broadband to high-sensitivity narrowband. Simultaneously, a varactor diode is employed to tune the achieved narrowband resonance frequency by changing the parasitic capacitance value. The PIN diode is inserted between the beginning point of the loop and the via-2. Thus, it connects the other side of the circular arc with the ground layers, as shown in Figure 10.  
Using the PIN diode will create another surface current direction; the first was from the port toward the loop and through the via-1 toward the via-2, as shown by the yellow arrows in Figure 11. At the same time, the second direction will start from the microstrip line through the via-2 toward the via-1 on the other side of the loop, as is denoted by black arrows, hence forming a circle of surface currents, as shown in Figure 11. As the via connect the top and bottom layers, there will be two slotted patch gap resonators, one at the top and the other at the bottom. After adding a PIN diode, the LGR was changed to two patch-slotted gap resonators (one on the top and the other on the bottom layers). Figure 11b depicts only the bottom layer to provide a clear view. Figure 10: Introducing the PIN diode in the broadband magnetic probe. Moreover, the resonance frequency for this new structure can be calculated using the previous Equations (1-4). The calculated resonance frequency is 13 GHz, and since there are two resonators of the slotted patch gap, the probe shows another frequency band at 5.8 GHz, as shown by the red dashed curve in Figure 12. This narrowband magnetic probe has a high sensitivity enhancement (S21 parameter) of about 6 dB compared to a broadband magnetic probe. The slotted circular gap radius \(r\) controls the resonance frequency location. For instance, increasing the radius \(r\) will shift the resonance frequency toward the lower bands, as shown in Figure 6:0 Moreover, the probe's slotted gap width \(t\) significantly affected the resonance frequency. Figure 14 shows the reflection coefficient versus frequency for different values of the slotted gap width.