Ultra-wideband loose coupling directional couplers with high directivity

This paper presents the development of ultra-wideband directional couplers having weak coupling and high directivity; couplers composed of 50Ω transmission lines and capacitors are cascaded. An even-odd-analysis of the single elements is carried out and design equations are given for the whole structures. In order to overcome the increasing coupling with frequency due to the capacitors an approach using delay lines is investigated. This leads to a flatter coupling characteristic. Three different prototypes have been built and measurement results are shown. The bandwidth and the directivity achieved are in excess of 50% and 28dB, respectively.