A dual-band omnidirectional circularly polarized
antenna is proposed. The antenna comprises back-to-back
microstrip patches fed by a coplanar waveguide. A very low
frequency ratio of 1.182 has been achieved, which can be easily
tuned by adjusting four lumped capacitors incorporated into the
antenna. An analysis of the omnidirectional circular polarization
mechanism as well the dual band operation is provided and
confirmed by numerical and experimental data. Key parameters
to tune the resonant frequencies and the axial ratio have been
identified. The prototype antenna provides omnidirectional
circular polarization in one plane with cross polar isolation better
than 12 dB for both frequency bands.
