| New approaches for improving the performance of a Rotman lens are presented in this thesis. Invented in 1963 by W. Rotman and R. F. Turner, the Rotman lens has been intensively used to generate multiple beams due to its simple design and broadband characteristics. However, the size of a conventional Rotman lens is inconveniently big at microwave frequencies, and the conductor loss of the lens is significant at millimeter-wave frequencies. In this thesis, several aspects of a Rotman lens for micro/millimeter applications are examined to improve it. First, a new approach to design a Rotman lens with a high dielectric material has been presented. The size of the lens has been determined from the scaling factor that is defined on the basis of the relationship between the path length error and the array size. A new guideline for reducing the size of a Rotman lens is established.; Second, a new form of a Rotman lens has been proposed for use at millimeter-wave frequencies. The proposed lens can be figured as a dielectric slab fed by slot lines. The new form is expected to show lower loss and lower mutual coupling than the conventional Rotman lenses fabricated with conducting plates at millimeter-wave frequency. TE0 mode was chosen to excite a dielectric slab for several reasons, such as high confinement, low dispersion, and appropriate feeding structure. The obtained efficiencies are 34.6% and 14.5% for the dielectric slab lens system and the conducting plate lens system, respectively. Therefore, it can be concluded that the dielectric lens is comparable to the conducting plate lens even though a spillover loss of the dielectric slab is expected; This thesis also studies the aspect of a Rotman lens as a Fourier transformer. For modern communication systems, it is necessary to process signals adaptively to increase S/N or channel capacity after they have been received at an antenna array. This corresponds to the concept of the smart antenna proposed for wireless communications. Usually, the smart antenna system consists of a spatial pre-filter (Fourier transformer) and an adaptive processor. In order to employ a Rotman lens as the spatial filter, the processing times have been calculated and compared for Fourier transformations using a Rotman lens and a computer. In addition, by adding voltage-tunable phase shifters between a Rotman lens and an antenna array, the wavefront can be controlled to increase the resolution. From the theory of Fourier optics, the beam propagation method has been developed to show improvement of the resolution for near-field detection. A discussion of results from investigation of these Rotman lenses and suggestions for future works are presented. |
