Compact Diplexer with High Isolation based on Novel Symmetric Double Spiral Resonators

In this paper, the electromagnetic characteristics of the reported miniaturized bandpass filter (BPF) in reference are investigated deeply, and then a planar compact diplexer centred at 1.86/2.16 GHz is designed by using a T-junction to connect the reported BPFs. For verification, the designed diplexer are fabricated and measured. The measured and simulated results of the diplexer are in good agreement with each other, showing that the measured 3-dB bandwidth for the two passbands are 6.45 % (1.79–1.91 GHz) at 1.86 GHz and 6.94 % (2.08–2.23 GHz) at 2.16 GHz with return losses better than 20 dB. Moreover, the insertion losses of the two passbands are less than 0.7 and 0.9 dB, and the isolations are higher than 48 and 36 dB, respectively. Besides, compared with the diplexers reported in recent references, this one has more superior performances.