Single-Layer High-Directivity Tight-Coupling Directional Coupler Based on Postwall CPW

A novel tight-coupling structure based on the postwall coplanar waveguide (PWCPW) is presented for the high-directivity coupler. PWCPW is a type of single-layer transmission line consisting of double-side CPW connected by a series of metalized vias. The large parallel-plate coupling capacitance gives the coupled PWCPW the characteristic of tight coupling, which is essential for couplers with high coupling coefficient. The small difference of the even- and odd-mode phase velocities in coupled PWCPW, resulting in efficient signal cancellation at the isolation port, makes the coupled PWCPW suitable for the high-directivity coupler. For verification purposes, a 3-dB high-directivity coupler is designed and fabricated based on PWCPW. The experimental results show the coupler operating in 0.54–1.44 GHz (a center frequency of 0.99 GHz and a bandwidth of 90%), with the return loss better than 25 dB. The peak directivity reaches 55 dB, with a 25-dB directivity bandwidth of 83%.