Cross-Coupling Hairpin Bandpass Filter with Periodic Grooves

This paper introduces a miniaturized bandpass filter with high out-of-band rejection for 5G communication systems. The proposed method includes directly connecting the non-adjacent resonant units of the filter with a section of 1/2 wavelength transmission line to form a special cross-coupling structure, thereby introducing a pair of transmission zeros, and the out-of-band suppression is below -30dB, which improves the performance and the out-of-band rejection capability of the filter. At the same time, periodic square grooves are used on the outer edge of the coupled resonator to achieve phase velocity compensation between odd-mode and even-mode to suppress parasitic passband. The 1dB bandwidth of this filter is 4.8GHz ~ 5GHz, the center frequency is 4.9GHz, and the bandwidth is 4%. The minimum insertion loss in the passband is 1.6dB and the insertion loss in the band is less than 3dB, and the return loss in the band is less than -11dB. Rejection level is - 32.7dB around $2f_{0}$ and -34.2dB around $3f_{0}$, and more than 20dB rejection level is achieved in the out-of-band 5.1GHz ~J25GHz frequency range.