A directional slot antenna, comprising: a radiating component, including a conductive layer located on a first surface of a substrate, configured to operate as a slot antenna and to radiate electromagnetic energy in both directions along an antenna axis; a transmission line, located on a second surface of the substrate, configured to be electromagnetically coupled to the radiating component such that the directional slot antenna operates to transmit signals and receive signals, the transmission line being a spiral transmission line; a reflector electrically separated from the radiating component, the reflector emplaced in opposed parallel relationship to a back surface of the radiating component, the reflector providing directionality by reflecting the radiated electromagnetic energy emitted in the direction of the reflector; a reflector spacing cavity between the radiating component and the reflector, the reflector spacing cavity having a height g'; a dielectric material insert positioned within the reflector spacing cavity, the dielectric material insert being placed on top of the reflector and partially filling the vertical dimension of the reflector spacing cavity such that an unfilled vertical portion of the reflector spacing cavity remains, and the dielectric material insert being shaped as a disk or ring, with the placement of the dielectric material corresponding to a location of an active radiating portion of the radiating component, and with a center hole corresponding to a location of the transmission line; and the reflector spacing cavity including the dielectric material insert providing an increased electrical separation between the radiating component and the reflector and the reflector spacing cavity including the dielectric material producing desired gains at frequencies of interest that are the same as the desired gains at the same frequencies produced by a free-space reflector spacing cavity having a height g>g'. a radiating component, including a conductive layer located on a first surface of a substrate, configured to operate as a slot antenna and to radiate electromagnetic energy in both directions along an antenna axis; a transmission line, located on a second surface of the substrate, configured to be electromagnetically coupled to the radiating component such that the directional slot antenna operates to transmit signals and receive signals, the transmission line being a spiral transmission line; a reflector electrically separated from the radiating component, the reflector emplaced in opposed parallel relationship to a back surface of the radiating component, the reflector providing directionality by reflecting the radiated electromagnetic energy emitted in the direction of the reflector; a reflector spacing cavity between the radiating component and the reflector, the reflector spacing cavity having a height g'; a dielectric material insert positioned within the reflector spacing cavity, the dielectric material insert being placed on top of the reflector and partially filling the vertical dimension of the reflector spacing cavity such that an unfilled vertical portion of the reflector spacing cavity remains, and the dielectric material insert being shaped as a disk or ring, with the placement of the dielectric material corresponding to a location of an active radiating portion of the radiating component, and with a center hole corresponding to a location of the transmission line; and the reflector spacing cavity including the dielectric material insert providing an increased electrical separation between the radiating component and the reflector and the reflector spacing cavity including the dielectric material producing desired gains at frequencies of interest that are the same as the desired gains at the same frequencies produced by a free-space reflector spacing cavity having a height g>g'.