A circularly polarized hemispherical antenna for millimeter wave applications

A hemispherical helix antenna with air core for sensing application is proposed. The antenna is a specific form of resonant multi-arm helix, which operates at the center frequency of 30 GHz. Circularly polarized radiation is obtained by exciting TE₁₁ and TM₁₁ spherical modes. To achieve nearly perfect circularly polarized radiation over the whole upper half space in the far field region, limited ground plane with specific dimension is introduced. This provides the necessary spherical TM radiation in the far field, while making negligible changes in the near field stored energies, and without affecting the resonant characteristics of antenna.