Novel zeroth-order folded-monopole antenna in low profile

Abstract

A novel low-profile folded-monopole antenna is proposed in this paper, which is composed of two metamaterial unit cells coupled with microstrip gap. It works under the zeroth-order resonance mode, and its vertical radiation vias can be fed in phase without loading any lumped surface mounted elements or inserting any auxiliary phase-shifting lines compared to previously reported designs. An equivalent circuit model for the novel antenna is presented, based on which an even-odd mode analysis is carried out, testifying that the even-mode current is predominant on the radiation vias at the zeroth-order frequency. According to the multiple folded monopole theory, there is a substantial increase in the radiation resistance, enabling the antenna to be well matched without adopting any external matching network. In order to verify the theoretical analyses, a prototype antenna of λ/10 × λ/20 × λ/10, operative at 3.0 GHz, are fabricated and measured, which can achieve radiation efficiency of above 90% and fully omnidirectional monopole-type radiation pattern.