Achieving Broadband Near-field Directionality with a Three-Dimensional Active Janus Source

The Janus source - comprising a pair of co-located, orthogonally oriented electric and magnetic currents with a 90 degrees phase shift - exhibits unique near-field selective coupling. However, the existing Janus sources are limited to ideal current sources or passive particles, hindering their efficient use in near-field directional systems. In parallel, antenna research has focused on the far-field characteristics of a configuration similar to the Janus dipole, named the "quasi-isotropic antenna" due to its far-field characteristic. In this work we show that an electrically small quasi-isotropic antenna is also a 3D active Janus source (we hence term it a Janus antenna). We investigate its near-field directionality and show it achieves near-field directionality over a broad bandwidth of 27.8%. We also demonstrate that, by using the Janus antenna, directional power coupling can be demonstrated with a power efficiency more than 100 times higher than using a passive Janus dipole. Achieving broadband near-field directionality with high power efficiency can find attractive applications in compact and multifunctional directional devices.