This letter discusses the development of a dual-band omnidirectional CP antenna using the zeroth- and the first-order resonance modes of epsilon-negative (ENG) transmission lines (TLs). The antenna is based on a circular mushroom structure with curved branches and designed without additional feeding data.parts for 90 phase difference between two orthogonal modes as well as additional radiators for dual-band operations. Also, this antenna is planar despite having an omnidirectional CP at
both modes of the zeroth- and the first-order resonance frequencies.To confirm the characteristics of the dual-band CP antenna,the antenna designed at 1.8 and 2.8 GHz was fabricated and
measured.