S21 is Ϫ2.64 dB. In the 5.8 GHz passband, S11 is Ϫ12.49 dB and S21 is Ϫ3.8 dB. Five transmission zeros are generated as shown in Figure 4(b). The insertion loss of the transmission zeros are Ϫ73.2 dB at 0.62 GHz, Ϫ37.38 dB at 2.15 GHz, Ϫ39.42 dB at 3.21 GHz, Ϫ16.48 dB at 4.97 GHz, and Ϫ27.21 dB at 7.13 GHz.The proposed TLTB-BPF is fabricated using two FR4 boards with 0.4 mm thickness of each board. The total thickness is 0.8 mm which is composed of copper layers on both the top and the bottom layers with the loss tangent of 0.0245. The input and output feed lines are placed on the top layer. Figures 5(a) and 5(b) show the substance photograph of the proposed TLTB-BPF. The etched area is 36.9 mm ϫ 11.5 mm.A vector network analyzer is used to measure the return loss and the insertion loss of the filter. S11 and S21 simulation and measurement results of the proposed TLTB-BPF are shown in Figures 6 and 7, respectively. As shown in Figure 7, the first resonant frequency is 1.75 GHz, the second one is 3.48 GHz, and the third one is 5.81 GHz. For 1.75 GHz, the measured S11 and S21 are Ϫ13.2 dB and Ϫ0.97 dB, respectively. For 3.48 GHz, the measured S11 and S21 are Ϫ17 dB and Ϫ2.8 dB, respectively. For 5.81 GHz, the measured S11 and S21 are Ϫ11.1 dB and Ϫ3.8 dB, respectively. The specifications of both simulation and measurement results of TLTB-BPF are listed in Table 1.Although the measured results show well agreement with the simulation ones, the slight difference between the simulated and measured results might be due to the losses from the connection between the board and the vector network analyzer or the tangent loss of the FR4 board. The problems of the insertion loss and the frequency shifting can be improved by more accuracy fabricated technology. Also, the substrate material can be selected with lower tangent loss to have a better performance. Concerning with the frequency shifting problem, the pattern size can be slightly modified to tune the desired passband frequency. Table 2 lists the performance of the proposed TLTB-BPF and other triband filters in the previous design. The size of the proposed TLTB-BPF is appeared to be smaller than most of other triband BPFs except that proposed in [7]. However, the triband BPF in [7] using combined quarter-wavelength SIRs was fabricated by the substrate material with high dielectric constant and less tangent loss. Table 2 shows that TLTB-BPF has the advantage of miniaturization.
CONCLUSIONSA novel trilayer triband microstrip bandpass filter has been proposed to provide three passbands centered at 1.8 GHz GSM channel, 3.5 GHz WiMAX channel, and 5.8 GHz WLAN band. Using microstrip line (top layer), open loop structure (middle layer), and the ground (bottom layer) to implement three different passbands results in a compact topology. A good agreement between the simulation and the measurement results is obtained. All passbands perform low insertion loss as well as low return loss. The center frequency, bandwidth, return loss, and insertion loss can all be adjusted by changi...
