High directivity microstrip couplers for impedance transforming

The paper describes the design method and simulations results of high directivity planar directional couplers with impedance transformation between the main and the coupled ports. Presented couplers are realized in microstrip line technology. The basic idea is to allow for coupling between two different impedance transmission lines. The coupling is realized by lumped capacitors soldered at two ends of coupled lines section. The proposed directional couplers are narrowband and high directivity. Wide range of coupling factors is easy to achieve. The example structures presented in the paper have a few levels of coupling factors and have been designed for operating frequencies 1.3 GHz and 3.0 GHz. The directivity of the couplers is better than 30 dB in the narrow bandwidth and the impedance matching is better than 25 dB at any port.