In this correspondence, an ultra-wideband omnidirectional antenna with stable radiation patterns is proposed. The antenna consists of a pair of symmetrical planar dipoles, PCB feeding circuits and a radome. By conducting characteristic mode analysis(CMA) on planar dipoles, two resonant modes can be excited with wideband operation. By utilizing a pair of symmetrical dipoles, gain variation is further reduced. The proposed antenna features simple structure and compact size of 0.25 × 0.25 × 0.34 λ₀, which make it suitable for array application. For demonstration, the antenna element and 2-element sub-array are implemented. The relative operating bandwidth (S₁₁ ≤ −15 dB) is 99%(1.7–5 GHz), which can cover most licensed LTE and 5G bands. Stable in-band omnidirectional radiation patterns are realized with gain variation lower than 2.5 dB. These features make it suitable for vehicular wireless applications to support both 4G and 5G communications.