الفهرس | Only 14 pages are availabe for public view |
Abstract Mushroom-like electromagnetic band gap materials have numerous applications. One of these applications is its use as a polarization converter. The polarization converter transforms the polarization of an incident wave from any type of polarization to any other required type. In this thesis a simple technique for the design of polarization converters was developed. Given the polarization of the incident wave, the working frequency, and bandwidth, the technique is capable of designing the structure that gives the required polarization. The design technique is based upon the effective medium model. In this model the material is treated as a parallel resonant circuit. The accuracy of the design depends upon the dimensions of the material compared to the wavelength. The smaller the dimensions, the more accurate the results will be. However, a procedure was developed to increase the accuracy of the technique. The developed design technique was used to design materials for the following five cases: (a) Avoidance of blockage due to subreflector in Cassegrain antennas. In this case the polarization converter transforms an incident linearly polarized wave to another linearly polarized wave but the electric field is normal to the incident field. (b) Simplifying the structure of dual polarized satellite communications reflector antennas. The antenna structure reflects two incident waves linearly polarized with their electric fields perpendicular to each other. The designed polarization converter reflects the two incident linearly polarized waves keeping their polarization without any change after reflection. (c ) Simplifying the feed structure of circularly polarized parabolic reflector antennas. The designed polarization converter transforms the polarization of the incident wave from linear to circular. Thus avoiding the use of complicated wave guide sections used for that purpose. (d) Enhancement of radar cross section of trihedral corner reflectors used for civil maritime radar. Circular polarization is used in such cases to overcome clutter due to rain. But right handed circular polarization is reflected fro trihedral corner reflector to be left handed circular. Thus it can not be received by the radar, effectively reducing the radar cross section of the trihedral corner reflector. The designed polarization converter covers one face of the reflector and transforms the circularly polarized incident wave to a linearly polarized one. Thus enabling the radar to receive the backscattered signal, effectively increasing the radar cross section of the trihedral corner reflector. (e) Calibration of polarimetry radar. Polarimetry radar uses two linearly polarized waves with their electric fields normal to each other for the study of weather conditions. The radar sends the two waves ( H and V) and receives four ( HH, HV, VV, VH). Calibration of the radar needs several targets. One of these targets must return strong cross polar field. The thesis suggest the use of a trihedral corner reflector with one face covered by a polarization converter. The converter transforms the incident linearly polarized wave to a linearly polarized wave with its electric field normal to the incident electric field. The polarization converter was designed. The HFSS and CST packages were used for the simulations. |