A directional slot antenna comprises a radiating component coupled to a reflector. A reflector spacing gap or cavity between the radiating component and the reflector has a height which is less than a predetermined height of a free-space reflector spacing cavity associated with desired gains for frequencies of interest. A dielectric material insert is positioned within the reflector spacing cavity and fills or partially fills the cavity. The reduced-height cavity including the dielectric material insert provides an increased electrical separation between the radiating component and the reflector that corresponds to the predetermined height of the free-space reflector spacing cavity.