Artificial Anisotropic Materials for Antenna Designs and Measurements

人工各向異性材料在天線設計及測量中的應用

Student thesis: Doctoral Thesis

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Award date30 Oct 2020

Abstract

This thesis presents a series of novel circularly polarized (CP) antennas and new gain measurement techniques using artificial anisotropic materials. The artificial anisotropic material, initially used in the waveguide, has a wide bandwidth, low reflection and high flexibility in manipulating the polarization of electromagnetic (EM) waves. With the newest fabrication technology such as 3D printing, the artificial anisotropic material shows great potential in building the polarizers for CP antennas and the antenna probes of the measurement systems.

First, miniaturization of the polarizer using the conventional artificial anisotropic material is studied. An analytical solution for the optimal dielectric ratio is provided for obtaining minimal thickness of the polarizer. For verification, a novel wideband high-gain CP antenna is designed for the 60-GHz high-throughput communication and the 77 GHz automobile radar system. The proposed design has a relatively high-gain and wideband performance with a minimal thickness. The antenna exhibits a wide impedance bandwidth of more than 39% (VSWR < 2) for 55-82 GHz and a wide axial-ratio bandwidth of more than 32% (axial ratio < 3 dB) for 56-77 GHz. The average gain is 17 dBic. Good unidirectional radiation characteristics are achieved with low fabrication cost.

Second, the thickness of the planar dielectric polarizer is further reduced using high-dielectric-constant materials. Two anisotropic anti-reflection layers are used on both sides to provide impedance transformation and anisotropy simultaneously. The unit cells of the middle layer and the anti-reflection layer use separate periods to better adapt to the fabrication limitation. A polarizer prototype centered at 24 GHz is designed and fabricated using a material with a dielectric constant of 10. Experiments show that the prototype has a 25% operating bandwidth with a thickness of 0.52λ00 is the wavelength in the free space). The designing methodology widens the choice for material selection and miniaturizes the dimensions of dielectric polarizers.

Third, a wideband omnidirectional CP antenna using a conformal artificial anisotropic material is proposed for millimeter-wave applications. The antenna consists of three parts: an omnidirectional LP radiator, an annular hyperbolic lens, and a conformal anisotropic polarizer. The annular hyperbolic lens regulates the spherical wavefront from the LP radiator into a cylindrical shape prior to propagating into the conformal artificial anisotropic polarizer. The conformal polarizer is the three-dimensionalization of the two-dimensional planar artificial anisotropic material. The methodology for designing the polarizer is the first of its kind to generate wideband CP wave omnidirectionally. Measurement results show that the antenna has a wide operating bandwidth of 45% (24-38 GHz), which completely covers potential 5G 28-GHz spectrums.

Fourth, the major axis of the elliptically polarized (EP) wave generated by an LP radiating source and artificial anisotropic material is analyzed. Both theoretical and experimental results show that the major axis of the EP wave generated by such a combination has a major-axis angle of 0 or 90 degrees to the E-plane of the LP radiator. The angle between the major axis and the E-plane depends on the phase difference between the two orthogonal electric components after propagating through the polarizer. Measurement is conducted for verification in the entire V-band (50 – 75 GHz). The result provides instructions for the usage of artificial anisotropic materials in detection and measurement, such as the calculation of the polarization efficiency during transmission.

Fifth, a novel rotation-free phaseless far-field gain measurement method for antennas with any polarization characteristics is proposed. The method requires a pair of identical polarizers made of artificial anisotropic material to generate EP wave with a pair of LP radiators. Only amplitudes of the transmission coefficients are needed in the estimation of the antenna under test (AUT) gains. No probe rotation or phase information is required in the process of the measurement. The measurement method is tested in the entire V-band (50 – 75 GHz). Three sets of CP standard-gain horns (SGHs) and one LP SGH orientated at different angles are measured as AUTs. Experiments show that the measured gain errors are within 1 dB and measured major-axis errors are within 2 degrees. The proposed gain measurement method has lower requirements in the equipment than conventional methods, showing great advantages in higher frequencies, especially in the millimeter-wave and terahertz range.