Circularly-Polarized Beam-Steering Reflectarray
Date Issued
2015
Date
2015
Author(s)
Hung, Wan-Ting
Abstract
Three reflectarrays, including a near-field focusing reflectarray, a polarization-agile reflectarray, and a circularly-polarized beam-steering reflectarray, are proposed in this dissertation sequentially. In the first design, instead of designing for far-field radiation, which is common in conventional reflectarray design, we modify the methodology of phase compensation of the reflecting elements so that the reflected waves are in phase and focused at a predetermined focal point, enhancing the associated power density effectively. This design is utilized in the microwave virus sanitizer for H3N2 influenza virus. The measured death rate of the virus samples illuminated by the proposed sanitizer is up to 93%. For the second and third reflectarrays, in order to achieve circularly-polarized (CP) radiation, a linearly-polarized (LP) antenna is first adopted as the feed instead of circularly-polarized (CP) antennas. A pair of orthogonal conductor-backed strip dipoles lying respectively on the top and bottom faces of the substrate is then used to manipulate the reflection phases of the two associated LP wave components, respectively. CP reflected waves can thus be obtained in the designed far-field direction by setting the reflection phase difference of the two orthogonal LP wave components to be 90 and making the polarization tilt angle of the LP feeding horn to be 45. As a result, a CP reflectarray can be achieved. To verify the presented method, a polarization-agile reflectarray and a CP beam-steering reflectarray are designed, fabricated, and tested. The unit cell of the polarization-agile design is composed of two orthogonally-polarized conductor-backed strip dipole elements. Each strip dipole is further encircled by a parasitic rectangular loop, resulting in a tuning phase range greater than 400. The 1616 prototype reflectarray designed at 10 GHz is center-fed and illuminated by an LP horn antenna with a 45 tile angle. By adjusting the compensation reflection phases of the two orthogonal strip dipoles to be 90, a CP main beam for the reflectarray can be realized. The measured RHCP peak gain is 22.3 dBic with the 1-dB gain bandwidth of 5.1%, and the measured 3-dB axial ratio bandwidth is 4.7%. The aperture efficiency of the polarization-agile reflectarray is 21.1%. Likewise, the unit cell of the CP beam-steering reflectarray is composed of two orthogonal conductor-backed strip dipoles. Each dipole is loaded with a varactor diode at the center for independent control of the reflection phase of the associated LP wave component via changing the reverse bias voltage of the varactor. To prevent the coupling of the orthogonal dipoles, the parasitic loops are removed in this design; therefore, the tunable phase ranges of the two LP wave components measured at 5.5 GHz are reduced to only 235 and 290, which are lower than the optimal range of 360. In addition, the hexagonal shape of the unit cell is adopted to suppress the specular reflection due to the back conducting plane. The main beam of the CP reflectarray can be electronically steered by properly changing the bias voltages of the varactors, and thus a CP beam-steering reflectarray can be implemented. The fabricated 59-element reflectarray is capable of not only polarization agility but also beam steering for any polarization states (LP or CP). The dc bias circuits are carefully designed to avoid unwanted scattering interferences. The measured beam-scanning range of our proposed CP beam-steering reflectarray is from 20 to 20 with a RHCP peak gain of 13.6 dBic. The measured aperture efficiency is 13.6 %.
Subjects
beam-steering
circular polarization
focusing antennas
frequency-selective surfaces
reflectarrays
Type
thesis