High-Isolation and Wide-Stopband SIW Diplexer Using Mixed Electric and Magnetic Coupling

In this brief, a high-isolation and wide-stopband diplexer with controllable transmission zero (TZ) nearby the passband, is implemented using substrate integrated waveguide elements. The TZ is generated by the mixed electric and magnetic coupling scheme of $ {TE}_{ {101}}$ mode. It can be flexibly controlled by changing the length of rectangular apertures while keeping good wide-stopband performance unchanged. An ${X}$ -band diplexer with lower and upper channels centered at 9.5 GHz and 10.5 GHz was designed, fabricated, and measured to verify the proposed concept. Good agreement between simulation and measurement can be observed. The designed diplexer exhibits a high isolation level larger than 45 dB over the whole band and a wide stopband up to 20 GHz.