Miniaturized and Low Insertion Loss Diplexer Using Novel Inter-Digital Capacitors and Microstrip Section Inductors

A miniaturized and low insertion loss diplexer is proposed for Wifi applications (2.4 GHz-2.5 GHz and 5.15 GHz-5.85 GHz) in this brief. In each channel, the impedance transformations of ${K}$ -inverters and the couplings of lumped elements in the circuit are employed to create transmission zeros (TZs) in the stopband to ensure high selectivity and out-of-band rejection. Besides, the diplexer composed of two bandpass filters (BPFs) connected by T-junction achieves excellent impedance matching and high isolation. To alleviate difficulties of the design, the equivalent circuit is used for quantitative analysis. For a demonstration, the diplexer based on single-layer printed circuit board (PCB) technology using novel inter-digital capacitors and microstrip section inductors has been fabricated with a compact size of 14.6 mm ${\times }\,\,10.48$ mm. The measured insertion losses are 1.67 dB for the first channel and 1.58 dB for the second channel, and the obtained isolation between the output channels is greater than 25.8 dB.