Compact BPF and diplexer using capacitively loaded λ/4 shorted meander line resonator

This paper presents a novel capacitively loaded λ/4 shorted meander line resonator (SMLR). The grounded microstrip lines inserted inside the spaces of SMLR can construct the grounded inter-digital capacitors, which leads to a lower fundamental resonance frequency of capacitively loaded λ/4 SMLR than the λ/4 SMLR. Based on the proposed resonator, two compact fourth order Chebychev response bandpass filters (BPFs) centered at 0.9/1.57 GHz with 3 dB fractional bandwidth (FBW) of 12.5%/15% is designed and fabricated firstly. Then, on the basis of these two BPFs, a BPF-based diplexer operating at 0.9/1.57 GHz with 3 dB FBW of 12.5%/15% is exploited. The designed diplexer employs a novel hybrid microstrip line and phase shifter common T-junction. The phase shifter is implemented by quasi-lumped elements. The designed diplexer exhibits a very high isolation up to 73/68 dB at two channels, respectively. The fabricated diplexer also has a compact size of 0.239λg × 0.06λg. Good agreement can be observed between the simulations and the measurements.