This paper proposes a harmonic-aware and mode-allocation-based design methodology for a compact dual-band microstrip diplexer targeting 2.4- and 5-GHz wireless local area network (WLAN) applications. The proposed methodology systematically identifies the higher-order resonant modes of each channel and assigns them according to their desired filtering functions. In the 2.4-GHz channel, an undesired higher-order resonance is deliberately controlled and relocated away from the 5-GHz operating region using a stepped-impedance resonator (SIR), thereby reducing potential interference with the upper channel. In contrast, the additional resonant modes of the 5-GHz channel are deliberately exploited using stub-loaded resonators (SLRs): one mode is positioned near the upper passband edge to enhance the passband bandwidth, while two other modes are shifted into the upper stopband to generate transmission zeros and improve selectivity. This complementary treatment of unwanted and useful resonant modes enables the two channels to be tailored according to their distinct frequency-selective requirements. The resulting channel filters are co-designed and integrated through a common T-junction to realize the proposed diplexer. The diplexer has compact dimensions of 0.47λg×0.28λg at 2.4 GHz. Simulation results demonstrate the effectiveness of the proposed methodology, with passband insertion losses (ILs) ranging from 0.42 to 0.54 dB and from 0.47 to 1.0 dB for the 2.4- and 5-GHz channels, respectively. The minimum passband return losses (RLs) are better than 24.7 and 17 dB, respectively, while the inter-channel isolation remains above 38 dB. These results demonstrate the effectiveness of systematic resonant-mode control in realizing a compact, low-loss, well-matched, and highly isolated dual-band diplexer.

WLAN, Dual-Band Diplexer, SIR, SLR, Harmonic Control, Mode Allocation.