In this paper, a novel microstrip directional coupler with high directivity and coupling is proposed. It is based on the introduction of an electromagnetic bandgap (EBG) structure on the coupled lines to strongly enhance the desired backward coupling at the design frequency. The use of microstrip technology implies that the even and odd modes propagate at different speeds. This property causes a frequency dependent interchange of the energy between the direct and isolated ports. By adjusting the total length of the device in order to achieve a null of energy at the isolated port at the design frequency, a very high directivity and coupling (51.2 dB and 1.36 dB, respectively, at 10 GHz in the example presented) is achieved in the novel directional coupler proposed.
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