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

Abstract 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 ≈60 times 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.