28-GHz High-Isolation SIW Balanced Diplexer with Highly Controllable Transmission Zeros

This paper proposes a 28-GHz, balanced, and high in-band isolation diplexer based on the substrate-integrated waveguide (SIW) structure. To achieve high isolation and high roll-off rate, a novel coupling scheme by controlling the sign and magnitude of two cross-coupling coefficients is proposed to generate two highly controllable, symmetric or asymmetric transmission zeros (TZs) on both sides of each passband. A threepole, balanced, and T-junction based diplexer was prototyped with closely spaced channel frequencies at 26.9/28.9 GHz and fractional bandwidths of 3%. The measured differential-mode insertion loss, in-band isolation and common-mode suppression are 3.1/3 dB, greater than 51.2/51.9 dB and greater than 46.4/43.7 dB, respectively. Moreover, in the frequency range of 20-45 GHz, the wideband isolation and common-mode suppression are greater than 37 dB and greater than 34 dB, respectively.