Study On High-Gain And Directional Antennas Based On Metamaterials

Electromagnetic metamaterial is an important discovery in physics and electromagnetics in recent years. Composed of an array of periodic or non-periodic artificial unit cells whose scales are much smaller than free-space wavelength, metamaterials can be treated as actual electromagnetic materials at macro level. As the permittivity and permeability can be controlled arbitrarily, metamaterials gain extensive attention of international and domestic academics. By controlling the electromagnetic properties of metamaterials, one can manipulate the transmission of electromagnetic waves more freely. Metamaterials can be applied widely in enhancing the directivity, improving the gain, and controlling the beams of an antenna, the introduction of which provides a new means for designing high-performance antennas. The dissertation mainly discussed on the retrieval method of the effective electromagnetic parameters for a bianisotropic metamaterial, and the applications of metamaterials on high-gain and directional antennas with many novel and promising antennas proposed. The main contents and contributions of the dissertation are listed as follows:1. For bianisotropic metamaterials, an effective parameter retrieval method is proposed, which is derived from the parameter retrieval theory for inhomogeneous metamaterials. With this method, one can determine the effective permittivity, permeability, and magnetoelectric coupling coefficient from the scattering (S) parameters of a metamaterial. For the metamaterial composed of split ring resonators (SRR), bianisotropy will present as the structures of the elements are asymmetric in the direction of waves transmission. By comparing the results of effective electromagnetic parameters from retrieval and analytic methods, the validity of the retrieval method is verified. Furthermore, as the elements of the metamaterials are SRR and wire which are printed on two sides of a dielectric substrate, different electromagnetic parameters are obtained using the proposed retrieval method as the waves are incident from two directions.2. It is proposed that by placing the meander line based metamaterial in front of the periodic end-fire (PEF) antenna, the gain of the PEF antenna can be enhanced over a broad bandwidth. Simulated results show that the gain of the PEF antenna operating within the frequency band of 5.95-11.65 GHz can be enhanced by the meander line based metamaterial, and that the gain can be enhanced further by increasing the sizes of the metamaterial with certain limits. Moreover, by comparision it can be see that meander line based metamaterial presents a wider operating bandwidth than the metamaterial based on SRR or I-shaped structures. For PEF antennas with 2 and 4 rows of meander line structures, fabrication and measurements are performed with the results showing that the gain of the original PEF antenna is enhanced by 0.6-3.6 dB and 1.2-5.7 dB, respectively, which indicates the proposed metamaterial can improve the gain of the PEF antenna effectively.3. A parallel line based broadband metamaterial is proposed. Using the retrieval method of effective parameter, we obtain that the real part of the refractive index of the material is greater than 1 in the non-resonant region. The metamaterial presents a good transmission feature, and behaves as a homogenous dielectric lens, which can be used to improve the gain of a printed directional antenna. Two kinds of antipodal tapered slot (ATS) antennas are designed, which operate in the frequency bands of 6-18 GHz and 5.5-15 GHz, respectively. With different parallel line based metamaterials placed in front of the two antennas, a novel kind of high-gain ATS antenna is obtained. Simulated and measured results of the two antennas show that the proposed metamaterial can enhance the gain of the ATS antenna greatly in the operating frequency band, making it more directional.4. A multi-layered metamaterial lens based on closed square ring structures is proposed. The retrieval results show that the real part of the refractive index of the lens is greater than 1 in the frequency band of 8-12 GHz. The lens is placed above a patch antenna and the proper lens dimensions and the distance between the lens and antenna are optimized by simulations. Fabrication and measurements are performed for the lens loaded patch antenna with the measured results showing that the gain of the reference patch antenna is enhanced by 6.5 dB at 10.25 GHz. In addition, an improved design of the lens for horn antennas is proposed. It is implemented by printing the metal mesh and an array of closed square rings in different areas of the dielectric substrates to realize a gradient refraction, and by placing the lens at the mouth of a pyramidal horn antenna which operates within 5.5-7 GHz, the gain of the horn antenna can be enhanced greatly.