Achieving Broadband Near-Field Directionality with a 3D Active Janus Antenna

Directional sources like the Huygens source enable electromagnetic wavefront forming/shaping with great flexibility and have made impacts in photonics, applied physics, metasurfaces, and antenna engineering. The related Janus source features a strongly directional near-field and a quasi-isotropic far-field, which gives it promising application potentials distinct from, and complementary to, the Huygens source. Nevertheless, most existing Janus sources face strong limitations in efficiency and/or 3D operation, hindering their practical application. This paper introduces a 3D active Janus source that achieves near-field directionality over a wide bandwidth and a power efficiency much improved over existing passive Janus sources. It is shown that a class of quasi-isotropic antennas are actually active Janus sources (which is referred to as the Janus antenna). A well-designed Janus antenna is demonstrated which (a) exhibits near-field directionality over a broad bandwidth of 28.9%,and (b) in a practical usage environment, achieves a power efficiency ≈60times higher than a passive Janus source. This work elucidates the connection between the “Janus dipole” concept in physics and the “quasi-isotropic antenna” concept in antenna design, and paves the way for the design of future directional devices with much improved bandwidth and efficiency. © 2024 The Author(s).