## 3 Results and Discussion
Figure 9b describes the proposed filter performance changing with different W\({}_{6}\) of the T-shaped resonator when \(L_{7}\) = 11 mm. It can be clearly seen that when \(W_{6}\) increases from 0.4 to 0.6 mm, \(f_{01}\) increases from 3.4 to 3.6 GHz, while \(f_{02}\) is 7.5 GHz. The FBW of the first notched band decreases from 16 to 14.8% and the FBW of the second notched band increases from 11.24% to 10.9%. Figure 10 respectively illustrates that the performance of the second notched band is adjusted by changing the relevant parameters (\(L_{7}\), \(L_{8}\), \(W_{7}\)) of the L-shaped defect microstrip structure. The total  
Figure 8: (a) The examination photo by Vector Network Analyzer Plannar 804/1 (Copper Mountain Technologies, Indianapolis, IN, USA); (b) comparison of the simulation and measurement results of the proposed filter. Figure 9: (a) Simulation results with different \(L_{6}\) (\(L_{6}\) = 9.4 mm, \(f_{01}\) = 4 GHz, \(L_{6}\) =1.0 2 mm, \(f_{01}\) = 3.6 GHz, \(L_{6}\) = 10.6 mm, \(f_{01}\) = 3.5 GHz); (b) Simulation results with different \(W_{6}\) (\(W_{6}\) = 0.4 mm, \(f_{01}\) = 3.4 GHz, \(W_{6}\) = 0.5 mm, \(f_{01}\) = 3.5 GHz, \(W_{6}\) = 0.6 mm, \(f_{01}\) = 3.6 GHz). length of the defect microstrip structure is equal to \(L_{7}+L_{8}\) and the width is \(W_{7}\). Figure 10a describes the proposed filter performance with changing \(L_{7}\) of the L-shaped defect microstrip structure. When \(L_{7}\) increases from 7.5 to 9.3 mm, \(f_{02}\) decreases from 7.7 to 6.2 GHz, while \(f_{01}\) is 3.5 GHz. Figure 10b shows the simulated insertion loss only with different \(L_{8}\). When \(L_{8}\) increased from 0.1 to 0.3 mm,\(f_{01}\) is 3.5 GHz but\(f_{02}\) decreases from 7.2 to 7 GHz. It can be derived from the equivalent equation of resonant elements that the \(C_{2}\) of the equivalent circuit increases with the increasing of the total length of the L-shaped defect microstrip structure (\(L_{7}+L_{8}\)). Figure 10c illustrates the proposed filter performance with different \(W_{7}\) of the L-shaped defect microstrip structure. When \(W_{7}\) increases from 0.2 to 0.4 mm, \(f_{01}\) and \(f_{02}\) are maintained at 3.5 GHz and 7.2 GHz. It can be clearly seen that when \(W_{7}\) increases from 0.2 to 0.4 mm, the FBW of the second notched band increases from 15.71% to 22.68%. The resonant frequency \(f\) of the L-shaped defect microstrip structure decreases with the increasing of the total length of the L-shaped defect microstrip structure (\(L_{7}+L_{8}\)) as shown in Equation (10). According to the above analysis, the frequency of the second notched band decreases by increasing the total length of the L-Shaped defect microstrip structure (\(L_{7}+L_{8}\)) and the fractional bandwidths of the second notched band increases with the increasing of \(W_{7}\), while the frequency of the first notched band is still at 3.5 GHz. Finally, a comparison of characteristics between the proposed filter and some other filters is presented in Table 5, in which insertion loss and return loss are over the whole pass-bands, while \(f_{\rm L}\) and \(f_{\rm H}\) means the central frequency of lower and higher frequency notched bands. It can be seen that the proposed filter has dual notched bands at 3.5 and 7.5 GHz with the rejection level of 25.2 dB and 17.3 dB and an FBW 123.9% higher than the other 10 works [22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. The measured results illustrate that the dual notched bands are centered at 3.5 and 7.5 GHz with an FBW of 15.91% and 9.63%.