Low-profile folded-monopole antenna with unbalanced composite right-/left-handed transmission line

A low-profile folded-monopole antenna with high radiation efficiency is presented, consisting of only two unbalanced composite right-/left-handed transmission line unit cells. An even-odd mode analysis shows that the even mode necessary to realise the folded-monopole effect prevails at the zeroth-order frequency, determined by the shunt resonance of the unbalanced unit cell. Loading a spiral inductor in each unit cell´s top patch can keep the antenna in a low profile at a relatively low self-resonant frequency, and a substantial increase in the radiation resistance is generated according to the multiple folded monopole theory, enabling the antenna to be well matched to 50 Ω without adopting any external matching network. A prototype antenna of λ/10 × λ/20 × λ/10, operative at 3.0 GHz, has been fabricated and measured, which can achieve radiation efficiency of above 90% and fully omnidirectional monopole-type radiation pattern.