An antenna having a longitudinal axis, the antenna comprising: a planar ground plane, wherein the planar ground plane is disposed orthogonal to the longitudinal axis; a planar exciter comprising: four slots, wherein the four slots comprise a first slot, a second slot, a third slot, and a fourth slot symmetrically disposed azimuthally about the longitudinal axis; and four excitation pins, wherein the four excitation pins comprise: a first excitation pin electrically connected across the first slot; a second excitation pin electrically connected across the second slot; a third excitation pin electrically connected across the third slot; and a fourth excitation pin electrically connected across the fourth slot, wherein: the planar exciter is disposed orthogonal to the longitudinal axis; the planar exciter is spaced apart from the ground plane; the planar exciter is configured to excite right-hand circularly-polarized electromagnetic radiation; the planar exciter is configured to excite first currents orthogonal to the longitudinal axis; and the planar exciter is configured to excite substantially no current parallel to the longitudinal axis; and a plurality of passive elements, wherein: the plurality of passive elements is symmetrically disposed azimuthally about the longitudinal axis; the plurality of passive elements is spaced apart from the planar exciter; the plurality of passive elements is electromagnetically coupled to the planar exciter; and the plurality of passive elements is configured to excite second currents parallel to the longitudinal axis and third currents orthogonal to the longitudinal axis. a planar ground plane, wherein the planar ground plane is disposed orthogonal to the longitudinal axis; a planar exciter comprising: four slots, wherein the four slots comprise a first slot, a second slot, a third slot, and a fourth slot symmetrically disposed azimuthally about the longitudinal axis; and four excitation pins, wherein the four excitation pins comprise: a first excitation pin electrically connected across the first slot; a second excitation pin electrically connected across the second slot; a third excitation pin electrically connected across the third slot; and a fourth excitation pin electrically connected across the fourth slot, wherein: the planar exciter is disposed orthogonal to the longitudinal axis; the planar exciter is spaced apart from the ground plane; the planar exciter is configured to excite right-hand circularly-polarized electromagnetic radiation; the planar exciter is configured to excite first currents orthogonal to the longitudinal axis; and the planar exciter is configured to excite substantially no current parallel to the longitudinal axis; and four slots, wherein the four slots comprise a first slot, a second slot, a third slot, and a fourth slot symmetrically disposed azimuthally about the longitudinal axis; and four excitation pins, wherein the four excitation pins comprise: a first excitation pin electrically connected across the first slot; a second excitation pin electrically connected across the second slot; a third excitation pin electrically connected across the third slot; and a fourth excitation pin electrically connected across the fourth slot, wherein: the planar exciter is disposed orthogonal to the longitudinal axis; the planar exciter is spaced apart from the ground plane; the planar exciter is configured to excite right-hand circularly-polarized electromagnetic radiation; the planar exciter is configured to excite first currents orthogonal to the longitudinal axis; and the planar exciter is configured to excite substantially no current parallel to the longitudinal axis; and a first excitation pin electrically connected across the first slot; a second excitation pin electrically connected across the second slot; a third excitation pin electrically connected across the third slot; and a fourth excitation pin electrically connected across the fourth slot, wherein: the planar exciter is disposed orthogonal to the longitudinal axis; the planar exciter is spaced apart from the ground plane; the planar exciter is configured to excite right-hand circularly-polarized electromagnetic radiation; the planar exciter is configured to excite first currents orthogonal to the longitudinal axis; and the planar exciter is configured to excite substantially no current parallel to the longitudinal axis; and the planar exciter is disposed orthogonal to the longitudinal axis; the planar exciter is spaced apart from the ground plane; the planar exciter is configured to excite right-hand circularly-polarized electromagnetic radiation; the planar exciter is configured to excite first currents orthogonal to the longitudinal axis; and the planar exciter is configured to excite substantially no current parallel to the longitudinal axis; and a plurality of passive elements, wherein: the plurality of passive elements is symmetrically disposed azimuthally about the longitudinal axis; the plurality of passive elements is spaced apart from the planar exciter; the plurality of passive elements is electromagnetically coupled to the planar exciter; and the plurality of passive elements is symmetrically disposed azimuthally about the longitudinal axis; the plurality of passive elements is spaced apart from the planar exciter; the plurality of passive elements is electromagnetically coupled to the planar exciter; and