DESIGN OF MILLIMETER MICROWAVE DEVICES USING METAMATERIALS AND ELECTROMAGNETIC BANDGAP STRUCTURES

Abstract

Microstrip patch antennas are planar in nature with a radiating patch printed on the top of the substrate and the whole structure is mounted on the ground plane. These antennas can be easily integrated with very large scale integrated (VLSI) circuits. The compactness of these antennas can be achieved by using techniques such as varying substrate thickness, stubs, slots, shorting pins, Defective Ground Structures (DGS), Electromagnetic Bandgap Structures (EBG) and metamaterials thereby making them suitable candidates for High Frequency (30MHz 300MHz) communications. But these antennas are prone not only to ohmic and radiation losses but also suffer from unstable gains and radiation patterns. A large co-polarization radiation component is desired in conventional patch antennas especially in beam-forming and beam switching applications. But unstable radiation patterns and gains in patch antennas decrease the co-polarization levels and increase cross-polarization levels. Cross-polarization levels depend on the surface current distribution of higher frequency modes. These modes are responsible for generation of surface waves in the antennas and thus deviating radiation from antenna boresight. Cross-polarization levels are usually reduced by thinning the substrate thickness and/or by connecting shorting pins to the antenna. But these techniques offer limited solution to reduce cross-polarization level in antennas. Using different techniques mentioned in the first paragraph, the cross-polarization levels can be reduced. The antennas for different applications such as C-band, WLAN, WMAX, X-band, K-band, Ku band applications etc are designed in this thesis. In the first chapter, various types of fundamental antennas like rectangular, semi-circular, square, oval and circular are studied analysing different performance parameters. Due to the planar nature, these antennas radiate in z-direction due to fringing fields when they are placed in x-y plane. The analysis of near and far fields using cavity model is discussed. Cavity model is used to analyse the electric and magnetic fields in the fabricated antenna discussed in the further chapters. The structures of the antenna are further modified by the above-mentioned techniques and their performance parameters are studied in the further chapters. In second chapter, different types of stubs loaded small antennas are designed and discussed. These stubs loaded antennas act as solution to the problems encountered by the antennas in the first chapter. In this chapter, a ‘L’ shaped stub loaded serpentine shaped patch antenna and a ‘L’-shaped stub and meander shaped stub loaded oval shaped patch antenna are discussed. To x miniaturize the antennas and make them operate at lower giga-hertz frequencies, the antennas are attached with ‘L’-shaped stubs and folded meander lines. The effect of stubs on the antenna’s performance parameters is studied. Folded stub line loaded serpentine shaped patch antenna possesses the simulated bandwidths of 662MHz, 1.6GHz and measured bandwidths of 660MHz and 1.52GHz. Measured gains of 3.2dBi, 5.42dBiand 5.56dBi along with the efficiencies of 82.1%, 86.3% and 88.1% are obtained at 6.8GHz, 9.51GHz and 9.89GHz respectively. The radiation patterns of the fabricated prototype are unidirectional E- and H planes at 6.8GHz. At 9.51GHz, the radiation patterns are omnidirectional in H-plane and unidirectional in E-plane. The radiation patterns are unidirectional in E- and H-planes at 9.89GHz. Stub loaded oval ring patch antenna resonates at 6.21GHz, 9.4GHz, 16.78GHz and 25GHz. The simulated and measured values of performance parameters of the proposed antenna are obtained. The measured gain values are observed to be 1.01dBi at 6.21 GHz, 1.2dBi at 9.4 GHz and 5.6dBic at 16.78GHz. At 25GHz, there is a gain of 8.1dBi. The efficiencies of 60.14% is obtained at 6.21GHz, 74.3% at 9.4GHz and 78.6% at 16.78GHz. Similarly, an efficiency of 83.64% is obtained at 25GHz. The radiation patterns of the proposed patch are unidirectional in all the planes at 6.21GHz, 9.4GHz, 16.78GHz and 25GHz.The proposed prototype is circularly polarized at 16.78GHz and linearly polarized at other frequencies of operation. Antennas discussed in the second chapter suffer from unstable gain and higher order modes. To overcome these problems, antennas designed in the third chapter act as a solution. In this chapter, a study on dumbbell shaped High Impedance Surfaces (HIS) is carried out. Stub loaded oval-shaped patch antenna is mounted on dumbbell-shaped metamaterials (MTM), electromagnetic bandgap structures (EBG), and defective ground structures and the effect of the latter on the earlier is discussed. Effect of the ground plane on the metamaterial is discussed. The selection of particular HIS based on the application’s requirement is discussed. These structures are used along with the stub loaded oval-shaped patch antenna to further enhance the performance parameters. Metamaterial antenna possesses high bandwidths in the frequency ranges 12.72GHz – 14.23GHz (1.51GHz) and 16.52GHz – 18.71GHz (2.19GHz). EBG mounted patch antenna is used to achieve further miniaturization and DGS mounted patch antenna is used to achieve high gain at all its resonant frequencies. Though HIS loaded antennas show superior performance, these antennas show degradation in the performance when more high impedance cells are loaded in the antenna structure. The xi antenna designed in the fourth chapter discusses the holistic approach to achieve the improved performance with reasonable gain, efficiency and bandwidth. This chapter discusses the design and analysis of ultra-wideband, dual polarized and highly efficient dual feed dumbbell shaped patch antenna. The design of splattered ring EBG is discussed. The effects of using EBG on the antenna and the variation in the performance parameters are described. Total dimensions of the proposed antenna are 25 x 30 x 1.524mm3 . The axial ratio of the antenna at 11.78GHz is 2.1dB and 4dB at remaining frequencies of operation. A wide-bandwidth of 20.2GHz (171.4%) along with a varying gain in the range of 6.21dB to 9.03dB in the range of frequencies 10.8GHz to 31GHz is achieved by the proposed structure. A wide range of efficiencies varying from 88.34% to 91.06% is achieved. Another antenna discussed in this chapter is swastika EBG mounted slotted antenna. This antenna is designed to radiate at 21.29GHz. The proposed antenna is simulated using Advanced Design System-2016. Gain and bandwidth of the proposed antenna are observed to be 11.92dB and 3.2GHz respectively. The purpose of achieving the miniaturization at lower gigahertz range is served by the antenna discussed in the fifth chapter. The antenna is designed by placing circular patches of different radii along the circumference of a central circular patch and thereby forming the proposed flower-shaped patch antenna. The design of proposed antenna is inspired from kissing circles theorem in geometry. The proposed structure is mounted on and loaded with Double Negative (DNG) triple Complementary Split Ring Resonator cells. The overall dimensions of the patch are 23.5 × 16 x 1.6 mm3 . This antenna operates at 5.2GHz as well as 8.25GHz. This also owns a wider bandwidth of 1.2 GHz (24.1%) in the ranges of 4.95 to 6.15 GHz and also a wider bandwidth of 2.2 GHz (26.5%) in the ranges of 7.1 to 9.3 GHz. There is a gain of 3.93 dBi and 5.02 dBi observed in the frequency ranges of operation. The radiating electric, magnetic fields and the green’s function are analysed using cavity model. The designed structure is useful for several applications, such as WLAN, WiMAX, and ISM band. Important parameters in modern day wireless communications are beam switching and low cross-polarization levels. This feature cannot be achieved with the antenna discussed in the previous chapter. Hence, chapter five provides the solution to the above requirements. This chapter throws the light on design and analysis of a meta post loaded slotted waveguide antenna array. The resonant frequencies of the proposed structure are 3.1GHz, 3.3GHz, and 4.1GHz. The simulated gains of 11.3dBi, 14.1dBi and 12.6dBi are observed at these frequencies. The proposed antenna operates in the frequency ranges of 3.01GHz to 3.12GHz, 3.2GHz to xii 3.44GHz and 3.91GHz to 4.45GHz. Also, the meta post loaded slotted waveguide antenna array exhibits beam switching feature in E-plane with the side lobe level with low cross polarization levels. Meta post loaded slotted array waveguide antenna is an excellent candidate for high gain applications. But complex and bulky nature of this antenna makes it prone to high ohmic losses and radiation deflections. Hence, stable radiation patterns cannot be achieved with these types of structures. The solution to these problems is to design concatenated antennas as discussed in the seventh chapter. The patch antenna presented in the second chapter is loaded on a serpentine shaped EBG ground is described in this chapter. Here, the radiation element is an oval shaped patch antenna and serpentine shaped patch slots are etched on the ground. Overall dimensions of the proposed patch antenna are 18 x 18 x 0.8mm3 . The structure is miniaturized by 89.1%. The resonant frequencies of the proposed patch are 4.28GHz, 6.15GHz, 6.78GHz and 12.15GHz. The peak gains at the frequency of operations are 1.1dBi, 1.3dBi, 2.8dBi and 2dBi. The radiation patterns are seen to be stable at the resonant frequencies. In miniaturized antennas, it is difficult to maintain high gain up to higher frequencies and at the same time it is difficult to maintain stable radiation patterns. Artificial intelligence devices must be used to direct the radiation leakages towards the antennas. These artificial devices direct the current on the patch with improved power gain and reduce the eddy current losses. Further, many techniques can be developed to improve the quality of communication from one place to another by introducing MIMO, fractal antennas, superstrates, Substrate Integrated Waveguide etc.