In this article, an unequally spaced resonant slotted‐waveguide antenna array with low side lobe level (SLL) is presented. In the conventional resonant arrays with longitudinal slots on the waveguide broad wall, slots must be separated one half guided wavelength (λg/2) from each other and be on opposite sides of the center line to have an in‐phase excitation. The open‐ended substrate integrated waveguide (SIW) resonator can support a resonance mode with the infinite wavelength. Hence using this kind of resonator, distances between slots in the resonant array are free parameters and can be optimized to achieve a low SLL. Based on the mentioned resonator, a 10‐element resonant slotted‐waveguide antenna array at 10 GHz with 10° beam‐width is designed and simulated. The distances between slots are determined by using genetic algorithm (GA). This antenna has been fabricated and measured. The measurement results show that the SLL of the presented antenna is 16 dB.

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