A planar Vivaldi antenna array, and method of forming such an array, the array comprising: a plurality of slots at an end of the antenna array, the slots extending through the whole thickness of the planar structure of the antenna array; and a plurality of grooves extending from the slots; wherein: the grooves do not extend through the whole thickness of the planar structure of the antenna array; and the cross-sectional shape of the grooves is complementary to the cross-sectional shape of the slots.