VIBRATION ANALYSIS OF REFLECTOR ANTENNA

Abstract

Large deployable reflectors are required for high-gain space antenna systems for land- mobile telecommunications payloads. These reflectors must have low mass and low packaged volume. This requirement has fuelled a resurgence of interest in membrane structures for space application in the last two decades. This renewal is motivated in large part by the great potential for reduced launch mass and stowed volume that membrane structures can afford. The Collapsible Rib-Tensioned Surface (CRTS) reflector is a new concept for multipurpose deployable membrane reflectors. It is based on the inherent geometrical stability of a doubly-curved membrane with high in-plane stiffness, maintained in a state of tension by a series of flexible ribs. This reflector consists of three basic elements: a parabolic membrane, a number of collapsible ribs and an expandable hub. The membrane itself serves both as the structure and as the radio-frequency reflective surface, resulting in a highly versatile lightweight antenna with substantial applications potential. CRTS reflectors are generally lightweight and flexible, and there is the possibility that vibration of the reflector in the deployed configuration, induced by the attitude control system of the spacecraft, may degrade its desired performance. The attitude control subsystem maintains the communications "footprint" in the correct location. The dynamic response of a structure depends on the coupling between dynamic loads exerted on the structure and the dynamic characteristics of the structure itself. If the frequency of the exciter is close to one of the natural frequencies of a structure, the response of the structure will be greatly amplified. Hence, a thorough understanding of the dynamic characteristics of CRTS reflectors is of great practical interest. Therefore, in the present work modal analysis of CRTS reflector has been attempted. The reflector after deployment is pre-stressed by the ribs. Therefore, a suitable method to pre-stress the reflector is found out and the fundamental frequency of the reflector for different pre-stress levels is estimated. Modeling the complete reflector and analyzing it for fundamental frequency involves lot of data input and computational time. Therefore, the fundamental frequency is found out by modeling only one gore of the reflector using CYCLIC symmetry. In addition to the effect of pre-stress, the effect of number of ribs and material properties of the membrane on the natural frequency of the reflector has also been investigated. iii