A slot antenna has an array of slot pairs where the E-plane beamwidth of the transmitted energy can be controlled. The antenna includes at least one pair of slots which are fed by a microstrip, and electric field barriers positioned between and parallel to the slots. The electric field barriers extend between the front and rear panels of the slot antenna. The distance between the electric field barriers is used to adjust or tune the antenna to a particular transmit or receive frequency, and the distance between the slots is used to control the E-plane beamwidth of the transmitted energy. When the slots are placed closer together, the beamwidth becomes wider, and when the slots are moved further apart, the beamwidth becomes narrower. In one embodiment, the electric field barrier is a series of closely spaced conductors, and in another embodiment, the electric field barrier is a conductive strip.