This letter shows the procedure of design of a hybrid directional coupler based on Empty Substrate Integrated Waveguide (ESIW). Once proved in previous works the feasibility of ESIW and the good performance indexes this technology can achieve, the following step is designing useful devices to be integrated in planar circuits. The proposed hybrid coupler, which is the first ESIW 4-port available circuit, is designed at 15 GHz with 3-dB coupling factor and a minimum of 20 dB of isolation over 30%

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... bandwidth. After manufacturing the circuit, simulated and measured results are presented and discussed. Index Terms-Hybrid directional coupler, empty substrate integrated waveguide (ESIW), millimeter-wave devices. This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication. The final version of record is available at http://dx.