Based on the theory of multi-hole waveguide directional couplers, an X-band multi-hole directional coupler with a folded substrate-integrated waveguide has been designed and fabricated. Simulation results show that the proposed coupler has isolation greater than 19 dB and a return loss better than 18.5 dB over a 50% fractional bandwidth at 10 GHz. The proposed folded substrate-integrated waveguide coupler has been fabricated, and measured data are almost consistent with simulation results. Isolation greater than 17 dB and return loss better than 13 dB are experimentally validated

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