Circularly polarized antennas

In recent years low profile, lightness, compactness, easy manufacturing and integrability with solid-state devices on a circuit board have become a priority in the antenna design. Microstrip antennas are a possible answer to these new requirements in several areas, such as remote sensing, mobile satellite and cellular communications, direct broadcast satellite (DBS) system and global positioning system (GPS). Many of the above mentioned applications make use of circularly polarized antennas. However, such requirement in planar microstrip antenna is not a trivial task since many factors have to be considered including fabrication tolerance and systematic errors in the manufacturing process. In the present thesis we address the task to achieve circular polarization with very compact planar antennas in two ways: 1) a more conventional one dealing with two innovative designs; 2) a less conventional one through of artificial materials, such as High Impedance Surface (HIS). As far as the first point is concerned, two standard designs have been developed for a Dedicated Short Range Communication (DSRC) system. In particular, we have proposed two innovative compact antennas that represent a step forward with respect to other available DSRC solution. The two proposed antenna prototypes have been optimized, fabricated and experimentally tested. They represent an answer to the need of achieving good circular polarization antennas in a small mobile On-Board Unit (OBU) of a DSRC system. As a further study, we have considered to improve performance such as the radiated power, the gain and the directivity, or to reduce the global antenna thickness. For this reason we have developed a full-wave model for periodic surface and considered their employment to achieve circular polarization. As a proof of concept, we have tested a simple arrangement that allows circular polarization of the radiated field. The thesis is organized as follows. In the first part, we briefly introduce some relevant technical features of microstrip antenna s. Then the properties of circular polarized waves are discussed, with particular interest in wireless communications. The circular polarization quality factors and antenna measurement setup are briefly introduced and discussed. In the second part, we address the design and the experimental characterization of two circularly polarized microstrip patch antenna prototypes. In the last part of the thesis, High Impedance Surfaces (HIS) are briefly introduced. Then, a simple FEM-model is presented and compared with the standard FTDT-model used in literature. In this respect, a rectangular-array-patch working as HIS surface is considered and tested to achieve circular polarization with a printed dipole.