Impact of optical baffle on antenna pattern

One of the major concerns of antenna design for spacecraft applications is the effect of surrounding structures which can reflect and diffract the antenna´s radiated energy and cause degradation in the antenna directivity, beam shape, and sidelobe levels. A case in point is the NASA Earth Observing System (EOS) spacecraft which has a large number of instruments on board. Among these is the Multiple Imaging Spectral Radiometer (MISR). The downlink antenna system (DAS) for the spacecraft is a very broad beam antenna and as such irradiates much of the spacecraft leading to the possibility of distortion of its pattern. The MISR, on the other hand, must be protected from optical glint resulting from the structures onboard the spacecraft and to this end the instrument has been fitted with a baffle. The optimal position for this baffle (from the point of view of the MISR instrument) places it in the near-field of the downlink antenna. The baffle material must be chosen so as to minimize the effect on the antenna pattern while adequately protecting the instrument. In an effort to select an optimum material, several optically black materials were considered, all of which were RF transmissive in varying degrees. The RF reflection coefficient of samples of these materials was found to be less than 0.03 as measured in a waveguide. The results were then used in an analytical assessment of the antenna pattern perturbation. Details of the modeling are described.