Applications of Metamaterial Inspired Structures in Design of Circularly Polarized Antennas

Abstract

This research is devoted to the potential properties of metamaterial (MTM) inspired structures in the design of novel circularly polarized (CP) antennas. Metamaterials(MTMs) are the artificially designed structures that have generated significant research interest through their unusual properties to achieve the demands of the modern era. Modern wireless communication like satellite, radar and mobile communication demands single feed, low profile, compact and low-cost antennas with multiple functionalities. To achieve these requirements, different metamaterial inspired structures are analyzed and utilized to design CP antennas with wideband, multi-band, high gain and RCS reduction properties. In addition, characteristic mode analysis technique is utilized to properly analyze the modes of an antenna, which contributes to radiation performance of the antenna. This thesis focuses on metasurface(MTS) based antenna design, which gives multiple functions with CP characteristics. MTS is designed and analyzed using the theory of characteristic modes to achieve CP. Parameters like eigenvalues, modal significance, and characteristic angle help to analyze modes, which contribute to produce CP waves. Later, the absorptivity property of MTS is investigated to obtain its RCS reduction capability. The other design presents a slot antenna with the concept of an artificial magnetic conductor (AMC). The proposed AMC acts as a polarization-dependent frequency selective surface(FSS), which produces a zero degree reflection phase at the required resonance frequency. This reflective property helps to design wideband CP antenna with good gain. Also, the concept of zero refractive index is introduced, which is used to enhance the gain of a primary antenna with simultaneous polarization conversion capability. MTS cover atop of LP patch antenna increases gain and converts LP into CP waves. Transmission and reflection property of the designed unit-cell is studied and its constitutive ivparameters are extracted to justify its functionality. Zero-refractive index based MTS has no phase variation at its interface, which helps to focus energy towards the intended direction, thus enhancing the gain. Multiband CP antennas with independent control of resonance frequency bands are much needed for wireless technologies to operate in a mobile environment. A dualband CP antenna with wideband characteristics is designed using a slot and a pair of copper strips. Triband CP antenna is designed using the MTM structure called splitring resonator(SRR) to obtain triple bands and control each resonance frequency bands independently. Also, polarization sense in each band is controlled trough structural modification of the antenna design. Proposed designs are fabricated using LPKF protomat machine S103 and experimentally verified using radiation pattern measurement set-up. Impedance bandwidth, peak gain, axial ratio bandwidth, radiation efficiency, radar cross-section and crosspolarization level are various metrics considered for the performance measures of the proposed designs with earlier work presented in the literature.