Microstrip branch-line coupler with optimized spurious suppression based on cascaded PI-type equivalent transmission lines

A microstrip branch-line coupler (BLC) has been design and implemented with optimized spurious suppression. Taking advantage of the PI-type network composed of a uniform transmission line (TL) loaded by two shunt open-ended stubs, a nature transmission line can be equivalent to the cascade of N sections of PI-type networks. A convenient design chart is provided to facilitate the TL equivalent. The coupler constructed by replacing the nature TLs with the equivalent ones possesses multiple structure-inherent transmission zeros which significantly improve the out-of-band rejection. Specifically, a coupler with center frequency f₀ at 1.53GHz has its stopband extended up to 8.33 f₀ with rejection level better than 20dB.