Tri-band/broadband matching techniques for 3-dB branch-line couplers

This paper presents design methods of 3-dB branch- line couplers operating at two arbitrary frequencies and the centre of them. After a brief survey of the equivalent admittance approach, two types of external matching networks for tri-band operation are proposed. One consists of a two-section impedance transformer and a shunt stub; the other two series stubs and an impedance transformer. Then, two tri-band/broadband couplers are designed using these matching networks; the former is constructed in microstrip line and the latter in CPW. The design techniques are verified by em-simulations and experiments.