Design of A Compact Tri-band Omnidirectional Circularly Polarized Antenna

A compact tri-band omnidirectional circularly polarized (CP) antenna using the zeroth- and the first-order resonance modes of epsilon-negative transmission lines is proposed and designed. The antenna structure is based on a circular mushroom patch with two-group unequal length curved branches with low profile and small size. A left-hand CP (LHCP) at the zeroth-order resonance (ZOR) and two right-hand CP (RHCP) at the first-order resonance (FOR) are obtained, and the omnidirectional axial ratios at the resonance modes in the azimuthal plane are less than 1.44 dB, 1.45 dB and 2.41dB., respectively.