Design of a focal plane dielectric rod array for feeding a multiple beams reflector antenna

Abstract

INTRODUCTIONAt the Delft University of Technology, different radar systems have been developed. One of the larger systems is the S-band Delft Atmospheric Research Radar (DARR), which is used for atmospheric profiling. DARR is a permanent system constructed in an urbanized coastal area. As atmospheric conditions depend on the geographical situation, measurement done at different locations could give extra information on the atmosphere. Therefor, At IRCTR a new transportable atmospheric radar system (TARA) [l], having a sophisticated antenna system, is under construction. The antenna system of TARA has multiple beams pointing at different directions (i.e. Oo, 15' in both horizontaland vertical-plane) and switched electronically. This is achieved by means of a parabolic reflector fed by a small phased array located in the focal region. The Focal plane method [2] was used to select the appropriate location and excitations of each individual element of the phased array. It has been found that, in order to form a beam at 0' one element placed at the focus of the parabolic reflector is sufficient. However, for each 15' beam, an array consisting of two elements, separated by 0.771, where 1 stands for the wavelength, is needed to achieve the required performance. The dielectric rod antenna can be used as feed array element because it can produce a relatively high gain with a small cross-section, due to its dielectric filling and extension, so that feed elements can be packed close together. This makes it possible to design arrays with relative spacing of (0.5<dkl.O). However, due to this small spacing, degradation of radiation pattern and mismatches due to mutual coupling can become a serious problem. The design and construction of an array of two dielectric rod antennas with compact size (i.e. 0.55hx1.54h cross dimensions) and very low inter-ports coupling (Slz), even at separation distance 0.77h, is the subject of this paper. All computed secondary patterns were obtained with the commercial code GRASPI. To predict mutual coupling effects (i.e. mismatches and pattern degradations) between two dielectric rod antennas, an approximate technique was used [3]. Related measurement results are provided. FOCAL PLANE ANALYSIS AND ARRAY DESIGNWhen a wide-scan beam is desired, an array of feed elements should be used to yield the required amplitude and phase taper across the reflector aperture which, . 0-7803-5639-X/99/$10.00 Q 1999 IEEE. 1152in turn, produces a well-shaped beam. The first design information needed, is the location and geometry of the array. Methods for optimizing the array feed are very efficient when a fixed array-geometry is utilized and only the feed excitations are optimized. An optimization which, allowed the element type and spacing to vary, would be extremely time consuming. The location of the feed array elements as well as other useful design information, such as the size of the feed array and element spacing, can be obtained from the knowledge of the focal region distribution. The focal region fields may...