Novel Four-Port Diplexer–Antenna System with Simultaneous Isolation Enhancement and Impedance Control

—This paper presents the design and simulation-based performance evaluation of a novel four-port diplexer–antenna system that simultaneously achieves high transmit/receive isolation and robust impedance control. Two compact parallel-coupled microstrip bandpass filters operating at 2.0 GHz (Rx) and 2.4 GHz (Tx) were synthesized using Chebyshev low-pass prototypes and umerically modeled on a Rogers substrate RO3006 ( ϵ r = 6.5, h = 1.27 mm, tan δ = 0.002). The filters were integrated into a four-port diplexer topology employing a 180 o phase-inversion path to exploit amplitude-and-phase cancellation, thereby reducing filter order and insertion loss while enhancing port-to-port isolation. Full-wave simulations and parametric analyses reveal that the proposed design yields an isolation of 50 dB at 2 GHz and 78.5 dB at 2.4 GHz, representing a substantial improvement over conventional single-filter configurations. Furthermore, the system’s isolation performance was evaluated under realistic antenna impedance variations; results show the design retains high isolation despite shifts in both the real and imaginary components of the antenna impedance, with the Tx port demonstrating particularly strong resilience near the matched condition. These findings verify that the co-optimized diplexer–antenna approach effectively mitigates impedance mismatch effects, offering a compact and low-loss solution for modern multi-band wireless front ends requiring stringent Tx/Rx isolation.