An ultra-small wideband diplexer with triangular-shaped and rectangular-shaped resonators for mobile base stations

This paper details the novel design and evaluation of an ultra-small low-pass/band-pass (LP/BP) diplexer with promising applications in modern mobile base stations. Employing a novel approach that utilizes symmetric coupling lines alongside a dual coupling structure for the band-pass filter (BPF) and a combination of triangular-shaped and rectangular-shaped resonators for the low-pass filter (LPF). Even and odd mode analyses have been used to analyze the BPF and the LC equivalent circuit method has been used to analyze the LPF. Also, the proposed diplexer demonstrates excellent performance within a notably small footprint (179.49 mm² or 0.0019 λ²_g). The performance measurements show that the BPF works with a bandwidth of 0.8 GHz centered at 8.8 GHz, and the LPF has a cutoff frequency of − 3 dB at 0.76 GHz and a strong suppression level of − 20 dB across a wide stopband from 0.96 to 10.97 GHz. This study confirms the efficacy of the filter design through measured results, highlighting the diplexer’s integration of 6 transmission zeros (TZs) in the LPF’s response and 4 in the BPF’s, resulting in improved signal separation for high- and low-frequency bands.