Slot array antenna

A slot array antenna includes a waveguide and a horn. The waveguide has a first surface including multiple slots arranged along a longitudinal direction of the first surface to radiate radio waves. The width of the first surface in the lateral direction is greater than half of a wavelength of the radio waves. The horn, fixedly attached to the waveguide, enlarges towards a radiation direction of the radio waves. The horn has upper and lower portions bent inwards twice with respect to a lateral direction of the first surface to form first and second narrow openings. An opening width of the first narrow opening is less than the width of the first surface. An opening width of the second narrow opening is less than or equal to two fifth of the wavelength. Thus, a side lobe generated due to horizontal polarization of the radio waves is suppressed.