A Compact Wideband Omnidirectional Circularly Polarized Antenna Using TM01 Mode With Capacitive Feeding

An omnidirectional circularly polarized antenna with compact size and wideband characteristic is presented and thoroughly investigated. The antenna consists of a shorted monopolar patch proximity-fed by a disk-loaded coaxial cable and two sets of curved branches sequentially located along the radiating edges. To achieve the compact size, the patch is excited to operate in the fundamental TM01 mode and the curved branches are designed with meander-line configurations. For a wideband operation, a capacitive feeding technique is employed with an additional resonance generated by the curved branches. By properly choosing the position of the stubs, a wide low-axial-ratio performance in the azimuth plane can be realized. To verify the design concept, an antenna prototype with overall size of 0.50λL × 0.50λL × 0.08λL, where λL is the free-space wavelength at minimum operating frequency, has been fabricated and measured. The measurement shows that the antenna exhibits a wide operation bandwidth of 30% with a symmetric dipole-like pattern. The realized gain in the azimuth plane is around 0.5 dBic across this band.