ANISOTROPIC ZERO INDEX MATERIAL: A METHOD OF REDUCING THE FOOTPRINT OF VIVALDI ANTENNAS IN THE UHF RANGE

Abstract

In this work, an anisotropic zero index material is designed for use in Vivaldi antennas. The metasurface structures are placed within the aperture of a Vivaldi antenna to improve the directivity and gain of the emitted radiation. The range of operation is in the ultrahigh frequency (UHF) range, between 300 MHz and 3 GHz. Two approaches are presented: a type of resonant metallic metamaterial that belongs to the larger class of anisotropic zero index metamaterials and a non-resonant material. A technique for lowering the dimensions of the resonant metamaterial unit cell is presented and applied. The work presented consists of simulation results obtained with HFSS modelling software from ANSYS. Metamaterials (1) are man-made materials, engineered such that they produce specific, well-defined interactions with the electromagnetic fields. While there exists a certain amount of ambiguity when defining metamaterials, for example when it comes to including photonic crystals in the metamaterials category, the particular class of materials used in this work is represented by a periodic array composed of the so-called meta-atoms. The meta-atoms are within the periodic unit cells of the material and are the substitute for atoms and molecules within natural materials. The physical size of the meta-atoms is much smaller than the wavelength of the electromagnetic wave that excites the material. The general consensus is that the unit cell should be approximately λ/10, where λ is the wavelength of the field. Just like natural materials are characterized by macroscopic material properties, metamaterials are characterized by effective material properties. In electromagnetics, the pertinent material properties are the dielectric permittivity and the magnetic permeability, therefore, when characterizing an electromagnetic metamaterial, one has to derive its effective dielectric permittivity and magnetic permeability. Zero index materials (ZIM) (2) are a class of materials for which one or both of the effective material parameters, dielectric permittivity and magnetic permeability can be zero. In certain applications, for instance antenna research, even though the effects of such ZIM materials are desired, they produce an unwanted impedance mismatch in the system. Another class of materials has evolved in order to address this problem: anisotropic zero index materials (AZIM). In this work, a design of such an AZIM is presented, as well as its application in antenna design. The application is a Vivaldi antenna with a coplanar design. Vivaldi antennas have a number of advantages that make them very attractive for many RF applications: they can be easily fabricated with PCB fabrication methods, and they have a high directivity and hence gain. These characteristics, along with their compact design makes them particularly suitable for antenna arrays (3). This antenna, also called notch antenna, was introduced by Gibson in 1979 (4).