An antenna capable of being joined to an antenna feed perpendicular to a ground plane includes a conductive radiator and a cylindrical shell. The conductive radiator is tubular and has a longitudinal slot along the entire length thereof. The slot is parallel to the radiator's axis. The antenna feed can be connected across the slot. A cylindrical shell of a uniaxial dielectric material is provided outside and spaced apart from the conductive radiator and extends beyond ends of the conductive radiator. The cylindrical shell electrically contacts the ground plane. The shell is made from a material having a dielectric tensor with high impedance in the axial direction. This antenna gives enhanced bandwidth over ordinary slotted antennas. The shell can be applied to preexisting antennas.