The rectangular dielectric resonator antenna (DRA) centrally fed by a probe is investigated. Its operating mode is analogous to the TM 011 mode of a cylindrical DRA. The DRA radiates like an electric monopole, generating omnidirectional linearly polarized (LP) fields. Based on this LP design, a novel omnidirectional circularly polarized (CP) DRA is studied for the first time. Slots are introduced to the sidewalls of the DRA, exciting a degenerate mode for the generation of CP fields. To demonstrate the idea, an omnidirectional CP DRA was designed for WLAN (2.4-2.48 GHz) applications. The reflection coefficient, axial ratio (AR), radiation pattern, and antenna gain are studied, and reasonable agreement between the measured and simulated results is observed. © 2011 IEEE.