## 2 Proposed UWB Antenna with Quadruple Band Rejections
### Performance Analysis and Current Distribution  
As shown in Figure 9a, compared with the C-shaped slot without branches, the slot with branches has a lower frequency, narrower notch bandwidth characteristics, and higher VSWR, which helps filter narrow-band signals. In Figure 9b, the ground plane loaded with single C-shaped slots, nested C-shaped slots, and interdigital inductance slots is compared when those slots occupy the same area. Two narrow-band WLAN frequencies can be accurately filtered out by interdigital inductance slots and retain the valuable frequencies, and it is also more compact. To further study the feature of the band-notched antenna, several parameters of the band-stop structures are investigated in Figure 10. When the L\({}_{8}\) is changed from 4.2 mm to 4.8 mm, the band-stop frequency will shift to the low frequency due to the longer electrical length. When W\({}_{7}\) changes from 7.5 mm to 9.5 mm, the rising edge of VSWR hardly changes, and the falling edge shifts, which leads to the notch bandwidth increasing from 0.38 GHz to 0.56 GHz. The distance of the curled C-shaped slot's tail branches controls the band-stop's width but has little effect on the rising edge of VSWR. From the analysis in Figure 10c,d, and Section 2.3, the essence of increasing L\({}_{8}\) is to increase the equivalent capacitance of curled C-shaped slot. The essence of increasing L\({}_{7}\) is to reduce the structure's equivalent inductance and increase the structure's equivalent capacitance. By choosing the length and distance of tail branches of the slot correctly, the band-notched feature can be controlled efficiently. Figure 10: _Cont._  
Figure 9: Simulated VSWR of the proposed antenna under different structures. (a) Curled C-shaped slots. (b) Interdigital inductance slot. The length and width of the interdigital inductance slots at the interdigital position are analyzed in Figure 10. When the interdigital width L\({}_{b1}\) ranges from 0.1 to 0.4 mm, the center frequency of the band-stop and the band-stop width has significantly changed. When the interdigital length W\({}_{b1}\) is from 2.6 to 2.9 mm, the band-stop frequency and band-stop width also change, but the change is small. From the analysis in Figure 11c,d, and Section 2.3, increasing L\({}_{b1}\) reduces the equivalent capacitance of the interdigital inductance slot. The essence of increasing W\({}_{b1}\) is to reduce the equivalent capacitance of the structure. The band-stop parameter can be controlled by modifying these two parameters simultaneously, or the two parameters can be combined to form a stepped interdigital inductance slot, such as the structure in Figure 4d. Using interdigital inductance slots and the added tail branch in the general C-shaped structure makes it possible to filter out the interference frequency band more accurately and achieve high selectivity of the notch. Figure 11: Schematic diagram of interdigital inductance slots. (a) VSWR under different L\({}_{b1}\). (b) VSWR under different W\({}_{b1}\). (c) S11 under different L\({}_{b1}\). (d) S11 under different W\({}_{b1}\). Figure 10: Simulated of the proposed antenna under different values of tail branches of the curled C-shaped slot. (a) VSWR under different Length L\({}_{8}\). (b) VSWR under different Distance W\({}_{7}\). (c) S11 under different Length L\({}_{8}\). (d) S11 under different Distance W\({}_{7}\). The final antenna size is: L=35 mm, W = 30 mm, L1 = 6 mm, W1 = 3.7 mm, L2 = 2.6 mm, W2 = 0.4 mm, L3 = 7 mm, W3 = 10 mm, L4 = 7.5 mm, W4 = 16 mm, L5 = 14 mm, W5 = 3.5 mm, L6 = 30.9 mm, W6 = 0.4 mm, L7 = 6 mm, W7=8.6 mm, L8=4.8 mm, W8=2.7 mm, L9 = 6.9 mm, W9 = 4.7 mm, R = 8 mm, L4 = 4.5 mm, Wa = 4 mm, L1 = 0.35 mm, Wa1 = 2.9 mm, La2 = 1.8 mm, La3 = 0.15 mm, Wa3 = 0.6 mm, Lb = 4.97 mm, Wb=4 mm, Lb1 = 0.3 mm, Wb1 = 2.9 mm, Lb2 = 2.57 mm, Wb2 = 0.6 mm, t = 1.575 mm. Figure 12b,c show the antenna's surface current distribution for the two frequency bands used by WLAN.