Compact differential diplex filtenna with common-mode suppression for highly integrated radio frequency front-ends

A compact, differential diplex filtenna (DDF) with common-mode (CM) suppression is presented. A rectangular dual-mode patch is developed that operates in both its TM10 mode and a reshaped TM30 mode to realize two passbands. Two capacitively loaded loop (CLL) resonators that work at different frequencies are combined with the dual-mode patch to enhance its operational bandwidth and to provide a second-order Chebyshev filter response for both passbands. A slot-coupled differential feed is adopted to suppress the harmonic and CM noise and to obtain symmetrical radiation patterns with low cross-polarization levels. The design process, which is based on filter synthesis theory, is detailed. A prototype operating over the 2.4G and 5.8G WLAN bands was fabricated, assembled, and tested. The measured results, in good agreement with their simulated values, confirm that it attains maximum broadside realized gains of 5.45 and 8.5 dBi and -10 dB fractional impedance bandwidths of 2.4% (2.42-2.48 GHz) and 5.2% (5.65-5.95 GHz) for the lower and upper passbands, respectively. Moreover, it exhibits good out-of-band rejection levels of 18 and 29 dB for the lower and upper stopbands, respectively.