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A broadband circularly polarized (CP) antenna is developed with an omnidirectional radiation pattern in the horizontal plane. Four broadband CP rectangular loop elements are employed for broadband omnidirectional CP radiation. The four rectangular loop elements are first printed on a flexible thin dielectric substrate and then rolled into a hollow cylinder. A conducting cylinder is introduced inside the hollow cylinder for achieving desired omnidirectional CP performance. A feeding network consisting of four broadband baluns and an impedance matching circuit is designed to feed the four rectangular loop elements. The omnidirectional CP antenna has a circular cross section with diameter of 0.38 λ0. Experimental results show that the omnidirectional CP antenna has bandwidths of 41% (1.65-2.5 GHz) for axial ratio <; 3 dB and 45% (1.58-2.5 GHz) for return loss > 10 dB. The gain variation in the omnidirectional plane is less than 1 dB for the frequency range from 1.65 to 2.5 GHz. Good agreement is obtained between simulated and measured results.