Compact Balanced-to-Balanced Microstrip Diplexer With High Isolation and Common-Mode Suppression

In this letter, a compact balanced-to-balanced microstrip diplexer with high isolation and common-mode (CM) suppression is proposed for wireless local area networks (WLAN) and worldwide interoperability for microwave access (WiMAX) applications simultaneously. The proposed diplexer is designed via the combination of two balanced bandpass filters (BPF) without extra junction matching network. Both BPFs are composed of uniform impedance resonators (UIR) and short- ended microstrip parallel-coupling feedlines. Finally, the proposed diplexer is fabricated and measured. The measured differential-mode (DM) isolation in the two passbands is more than 33 dB. While for the CM signal, the insertion loss is below 30 dB in the two operation bands. The simulated and measured results exhibit a satisfactory agreement to validate the proposed configuration. The diplexer size is only 0.26λ_g ×0.38λ_g, where λ_g is the guided wavelength at 2.45 GHz.