Design of a compact quint-band bandpass filter using a symmetric dual-mode λ/4 resonator and a pair of inverted F-shaped resonators

In this paper, a compact quint-band microstrip bandpass filter (BPF) based on multiple mode resonating technology is proposed. It is composed of a symmetric dual-mode λ/4 resonator loaded with a shorted stub, two inverted F-shaped resonators, and two pairs of coupling configurations. To miniaturize the filter, the three resonators are bent and cross coupled together. Considering that the BPF has a symmetric structure, the even-odd mode analysis method is employed. On the basis of the simulation, the presented filter was fabricated by using a conventional PCB process with the substrate of Rogers RT/Duroid 5880. The measurement results are consistent well with the simulated data. The measured operating bands are centered at 1.8, 3.5, 4.5, 5.8 and 6.8 GHz, respectively, which are suitable for GSM, WiMAX, 5G, RFID and WiFi6 applications simultaneously. The measured in-band minimal return losses within five passbands are better than 15 dB. Furthermore, the filter exhibits a high selectivity with steep skirt property and the four band-to-band isolations among the five passbands are higher than 27 dB, and the out-of-band rejection is better than 25 dB. The filter’s total dimension occupies only 0.18λg × 0.15λg, where λg represents the free space wavelength corresponding to the center frequency of the lowest passband.