Shielded Stripline Bandpass Filters with Low Insertion Loss for 5G Applications

A novel method to design stripline filters with shielding structure is presented in this paper. By applying a shielded substrate integrated cavity a fully closed, 3rd-order filter with low loss is proposed for 5G N79 band. The shielding vias and copper layers form an enclosed cavity for the stripline filter composed of two gradient impedance resonators (GIR) and one stepped impedance resonator (SIR), which removes the radiation loss and improves the selectivity performance. A transmission zero is obtained by equating the gradient and low impedance parts of the GIR to an open-ended quarter wavelength stub. On the basis of this filter, a 2nd filter is designed by introducing a defected ground structure (DGS) and quarter wavelength stubs, which expands the passband and stopband. Both filters are fabricated and measured. The measured results are in good agreement with simulation, which validate the proposed design methodology. The minimum insertion loss is 0.46 dB and 0.25 dB for these two filters, respectively. The two filters are good candidates for 5G N79 applications.