Analysis Of Antenna Structure

Abstract

Complex real life structural analysis problems can now be handled easily using a galore of powerful and high speed digital computer hardware, packed and zapped with the bewitching computer graphic facilities. The engineering ingenuity helps in actually zeroing down on the keen issues related to design of complex structure with respect to the given design specifications. In addition to that, better insight into the behavior of the structural aspects can be now explored, up to the hilt. Satellite Communication Earth Station Antennas are subjected to wind loads and dead loads and have severe restrictions on their surface RMS depending on their frequency of operation. In order to cover the landmass for different satellite locations the antenna structures are made steerable: 0 to 360 degrees in azimuth and 0 to 90 degrees in elevation. Hence to have a standardized design usually the earth station antennas are steerable as said above and use servo motors. One of the restrictions of the servo motor is the servo loop frequency should be lower than the natural frequency of the antenna. Hence estimation of the natural frequency of the antenna is necessary. Calculation of natural frequencies and mode shapes is also the first step to estimate the dynamic analysis of antenna subjected to earthquake loads. In this piece of work, it is proposed to carryout the static and dynamic analysis of an antenna structure used for the satellite communication. The antenna has to perform successfully for a wind speed of 80 KMPH for different elevation angles between 0 to 90 degrees. As per the electrical specifications, the RMS of the reflector surface has to be within the specified limit. For a wind speed of 200 KMPH, the antenna will be stored and locked in zenith position and the stresses in the structure members should withstand the wind load of 200 KMPH in this position with a factor of safety of 2. For static analysis, the major specifications are: • Survivalibility of structural members for 200 KMPH wind speed in zenith position. The stresses should be within the permissible limit of the maximum allowable stress limit for the aluminum material of 200 N/mm2. • The overall RMS value of deflection for other elevation angles to be less than 1 mm to have satisfactory electrical performance. Wind load on antenna structure is generally not calculated by using any Indian Standard Code. The Wind load on the antenna structure is calculated based on the paper entitled ”CALCULATION OF WIND FORCES AND PRESSURES ON ANTENNAS” by Edward Cohen and Joseph Vellozzi presented at ANNALS OF THE NEW YORK ACADEMY OF SCIENCES on June 26, 1964. The paper is the work based on several wind tunnel tests made on different diameter antennas and results are presented for pressure distribution as a % of chord length for various angles of attack. The pressure distribution on the reflector for the various angles of attack is taken from Fig.25 of the paper. In this piece of work, wind force calculations, for operational wind speed condition of 80 KMPH, are carried out for the different elevation angles ranging from 0 to 90 degrees for an 11 meter diameter antenna. The azimuth angle is kept constant as 0 degrees. The RMS values are found out and a comparison is done to find the critical position of the antenna structure. Wind force calculations, for survival wind speed condition of 200 KMPH, are carried out for the zenith position only and stresses are checked to fall under the allowable stresses of the material. Different components of the whole antenna structure like hub, yoke, elevation module, azimuth module and pedestal are modeled. Linear Static Analysis as per IS 1893 (Part-1): 2002 and Time History Analysis is done for different components along with the whole antenna structure. For Time History Analysis, BHUJ Earthquake Response Spectrum is used for time Vs acceleration response details. Frequencies and time periods are found out for the same. Also, the stresses are checked for being less than the permissible limits. STAAD Pro software is used for the modeling and analysis of whole antenna structure.