INVESTIGATION ON DIFFERENT METASURFACES FOR ANTENNA DESIGN

Abstract

The thesis investigated the use of metamaterials, electronic band-gaps (EBG), high impedance surfaces (HIS), ferrites, artificial magnetic conductors (AMC) in microstrip antennas design. Metamaterials are typically engineered by arranging a set of small scatterers or apertures. The surface version of metamaterial is called metasurface or metafilm. EBG structure is one of the metamaterials with the property to suppress electromagnetic wave propagation in a particular frequency band. Over the last decade, EBG have moved from being simply theoretical concept to a field with developed and marketed applications. Reflective metasurface structures, which have numerous potential applications such as mantle cloaking, polarization dependent surfaces, controllable surface, absorbers and so on, have increasingly attracted attentions in recent years. Most of the reflective metasurfaces are isotropic with the same responses for both TE and TM polarized waves. However, the anisotropic reflective metasurface provides a method to manipulate the TE polarized and TM polarized waves separately, which has much stronger ability to control electromagnetic waves. Different methods are employed for designing anisotropic metasurfaces like incorporating diode, inductive grid and ferrite/magnetic loading. Advancement in technologies has provided huge challenges to RF engineers for realizing miniaturized, large band-width, more efficient and cheap antennas. The reconfigurable, tunable and switchable antennas are excellent solutions to these problems. As integrated and evaluated with other components, antennas have been more opposing towards miniaturization with degradation of antenna performance. Novel methods are required to improve the performance and size reduction of antennas as well as increasing bandwidth, efficiency with multi frequency tunability and multiband operations. Here, for pattern reconfigurable antennas, different pattern for same frequency prevails. A combination of frequency and polarization reconfigurable antennas as well as frequency and pattern reconfigurable antennas also prevail. Pattern and polarization reconfiguration together is another aspect. Different materials like ferrite, crystals etc., electrical devices, different feeding methods are employed for realizing aforementioned reconfigurabilities The dissertation studied ferrite based tunable and reconfigurable metasurface antennas. It presented a novel design of polarization dependent EBG to get tunability for different polarizations. The designed tunable ferrite based PDEBG was used to obtain circular polarization based on phases of TM/TE polarized waves for WiMAX application. Other frequency and polarization reconfigurable antennas using metasurfaces were also discussed. Modulated metasurface was used with a Fabry Perot cavity antenna to design a frequency reconfigurable antenna. The reconfigurable antenna was designed for its application in WiMAX and WLAN. The thesis also designed a dual-polarized tunable circular patch antenna using HIS structure wherein varactor diodes were used to tune the reflection phase of the dual polarizations independently. Owing to the electromagnetic invisibility concealing or cloaking features of metamaterials, the thesis also discussed the use of modulated metasurfaces in patch antenna Radar cross section (RCS) reduction. A coaxial patch antenna was designed for reduction in RCS by mounting the antenna on a modulated artificial magnetic conductor configured using the concepts of modulated structure. The design consisted of two unit cells, undergoing perfect phase difference of 180° for the reflected waves in the frequency of operation which led to the destructive interference of the reflected waves for 6 GHz to 14 GHz. Remarkable RCS reduction was achieved by this fabricated modulated AMC. A CPW fed dual band bow tie slotted patch antenna was also discussed for WiMAX application using HIS. Right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) pattern was observed at two frequencies. Further, using different parameters, three different cases of high impedance surface were realized. Desirable performance of antenna in terms of good gain, return loss, satisfactory radiation pattern at the overall WiMAX band for 3.5 GHz frequency band and size reduction of patch was achieved. The antenna designs were simulated using computer simulation technique (CST) and measured using vector network analyzer (VNA). The radiation patterns were characterized in anechoic chamber.