A Small Monopole Antenna Based on Composite Right/Left-Handed Transmission Line by Coaxial Structure

Metamaterials are artificially constructed to have special electromagnetic properties not found in nature. In recent years, metamaterial is noted as a new kind of device, and the composite right/left-handed transmission lines have been proposed, which are characterized by antiparallel phase and group velocities and negative refractive index. The materials were termed as left-handed materials due to the relationship between electric/magnetic field and wave vectors which have the sign of the effective material parameters opposite to that of ordinal right-handed materials. The left-handed material has the possibility of applications to the microwave antenna, the circuit device, and etc. In order to realize the LH metamaterial artificially, a very fine structure should be used. From the viewpoint of realizing the metamaterial practically, the printed structures were mainly examined. Due to their complex structures, its radiation efficiency is degraded by the losses of the dielectric substrate. To confirm if the LH material can be used for the small antenna or not, it is not enough only to employ a LH material by the printed one, but also to examine a closed metal structure using coaxial structure. Thus we have presented the CRLH transmission line by the coaxial structure. This paper presents the Composite Right/Left-Handed (CRLH) transmission line by coaxial structure and the property of high relative permittivity in negative region to obtain short monopole antenna. A four-unit cell resonant antenna is designed and tested at 475 MHz. The height of the proposed antenna is 1/25lambda₀. Using the same method, a five-unit cell resonant antenna is also designed. When each via position of the proposed five-unit cell antenna is arranged, the resonant frequency is shifted to lower ranges with its radiation figure-of-eight of pattern.