In the current C-band satellite communication applications, usually added one band pass filter with wally stopband attenuation characteristic between the satellite receiving antenna and LNB (low-noise down converter) to suppress the microwave interference and pulse interference at the receiving end, band-pass filter's performance direct impact on the overall performance of the receiver. Meanwhile, in order to match the system installation, the filter length are subject to limitations. If we don't add the transmission zero to the filter, we can only design the filter in the traditional way with over ten cavities and large size, which can not be used in the satellite communication. So we use the miniaturization technology to solve the problem. And in order to get high inhibition out of the pass-band and small size, we should add several transmission zeros to the filter. In a result, a capacity loaded dual-mode filter is designed in this paper. We can get S11 is less than -19dB in the pass band and S21 is less than -40dB in the suppression frequency band of 3.4GHz to 3.6GHz and less than -60dB from 4.27GHz to 5GHz. In a word, this kind of the filter has a higher suppression out of the pass band.
Multi-mode filter is advantageous to the miniaturization of device. Because of symmetry, triangular structure has dual-mode characteristics. Some people have made a dual-mode filter with a triangular support structure. Through simulating triangle resonator, We found that the first two-mode distributed on the border of resonator and the third mode is located in the center of the triangle. We can load in the center of the disc to adjust the frequency of the third mode. When the adjustment discs, the first two degenerate mode frequency substantially was unaffected. Thus, we can make a triangle having three degenerate resonator modes. By adding the appropriate coupling and perturbation, we can make a triplemode filter. This structure is much smaller than the existing structure and easy processing, and the structure is more stable.
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