Slow-Wave Dielectric Helical Structure and Its Applications in Compact Omnidirectional Circularly Polarized Antennas

This letter presents a new approach to designing omnidirectional circularly polarized (OCP) antennas with simple and compact structures using full-dielectric helical resonators. First, the properties of an infinite-long small-diameter dielectric helical structure are studied to have found that it can support a slow wave propagating along the longitudinal axis of the structure with a proper structure size, achieving a phase constant much smaller than that of an infinite-long full-metal helix, which means the waves along the axis is much faster than that of a metallic helix but still slower than plane-waves in free space. The relationship between the propagation constant and the structure parameters such as permittivity, diameter of helix, thickness of the arms, and screw pitch are established to show the operating principles. Second, two OCP antennas using ceramic (ϵ_r = 32) and pure water (ϵ_r = 78.3) both working as a λ/2 resonator by cutting off the infinite-long helix, are designed, respectively, to verify the advantage of a λ/2 dielectric helix that is having a much larger aperture than that of a λ/2 metallic helix, so as an enhanced radiation and bandwidth. Measured results show an overlapped CP bandwidth of 19.7% for the ceramic helix and 6.6% for the water helix are achieved, both of which are significantly wider than a typical bandwidth of 0.9% for the λ/2 OCP full-metal helix.