Study Of Near-zero Index Metamaterial Superstrates For High-gain Antennas

Near-zero index metamaterial superstrates has a very broad application prospects in the field of wireless communications, because of the propagation of electromagnetic waves with phases wavefront shaping and energy tunneling effects. However,conventional superstrate antenna makes the profile of the antenna substantially increased, which greatly limits the applications. The nearly zero refractive index of the metamaterial can result in vertically emitting electromagnetic waves. In this way, the antenna gain increase can be achieved without significantly increasing the profile of the origin antenna. This dissertation will give some theory guidance for low profile design and optimum loading scheme from these aspects.This thesis first describes the principle of near-zero index metamaterials and its application in the high-gain antenna. A brief introduction of the research background, current situation and future research directions is given. Two important characteristics of the near-zero index metamaterials regulating electromagnetic wave propagation are systematically introduced, namely tunneling effect and wavefront shaping effect. By using two-dimensional and three-dimensional simulations, we have verified the electromagnetic energy tunneling effect when the effective permittivity is nearly zero. When the relative permittivity and relative permeability become nearly zero at the same time, the electromangetic wave can propagate through the medium without reflection. According to Snell’s law, the near-zero refractive index of metamaterials can make the propagation of electromagnetic wave perpendicular to the surface of materials, which have been confirmed by simulations, which is the theoretical foundation of high-gain antenna design.Then, by using the Matlab-HFSS API, a Matlab program is built to control HFSS software to setup a large number of fishnet models and calculate their S-parameters under different sizes. And substituting the S-parameter into the retrieval program, we can find the working frequency when the relative permittivity or permeability is near zero. A fitting formula is given by using the frequency and structure parameters. The effectiveness of the fitting formula is verified by comparing the results obtained from the formula and from full-wave simulation.Furthermore, the ideal Epsilon near-zero(ENZ), Mu near-zero(MNZ), and Double near-zero(DNZ) index metamaterials are implemented on a standard X-band horn antenna, respectively. The results show that when the horn antenna integrates with ENZ metamaterial superstrate, the radiation pattern in E-plane of the antenna will be compressed. When the horn integrates with MNZ metamaterial superstrate, the radiation pattern in H-plane of the antenna is compressed. When the horn integrates with DNZ metamaterial superstrate, both the E-plane and H-plane pattern of the antenna are compressed. The three kinds of near-zero index metamaterials are demonstrated to be effective on improving the gain of the horn antenna. But when the height between the horn antenna and the superstrates is determined, the gain at some frequencies would drop dramatically. In the form of an equivalent circuit, the relationship of the excitation source impedance with frequency is obtained. After examining the distance between the superstrates and excitation source, we can decide that microwave resonances are happened at these frequencies in the system. The equivalent circuit of the excitation source and the horn waveguide has been developed. Finally, the observation of the field diagram explains the reason of the depression point decline from microwave perspective. The shaping effect of ENZ metamaterial on electromagnetic wave has been verified. Beside the center of the horn antenna exists two relatively strong field distribution, which resulting in the gain pattern with two strong side lobes and therefore the gain decreases.Finally, three near-zero refractive index metamaterial are realized with actual topology structures. The measured results agree well with the simulated results.