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Analysis of segmented reflector antenna for a large millimeter wave radio telescope

German Cortes-Medellin, University of Massachusetts - Amherst

Abstract

We have developed a computational tool which serves to characterize the performance of large segmented reflector antennas under different sets of conditions. We have applied this tool to the characterization of a large millimeter telescope. A 50 meter diameter instrument of this type specified to operate to wavelengths as short as 1 mm is being design with an actively controlled main surface consisting of 126 hexagonal segments.^ To simulate the effect of the necessarily imperfect control system, we generate samples of tilt and piston errors for the segments from which the antenna radiation patterns and aperture efficiencies are calculated. We make a comparison of these results with models of antenna tolerance theory developed by Ruze, which relates the aperture efficiency to the rms phase error. We find that Ruze's formula have a different range of validity when the aperture rms phase error, rather than the rms surface error, is used as a parameter. When appreciable tilt errors are present in large segmented antennas, the aperture rms phase error tends to a constant value, independent of the aperture illumination and of the shape of the segments. We conclude that the antenna rms surface error is a better tracer of the aperture efficiency than is the aperture rms phase error when Ruze's formula is used. We find that this well-known expression stands as a lower limit to the performance of large segmented reflector antennas.^ We have analyzed the effect that gaps between the segments of the active surface of this antenna as well as the imperfect positioning of the subreflector surface have on the aperture efficiency, antenna gain and radiation pattern of this antenna. We have found that the gaps produce a series of grating lobes distributed in a regular pattern in the far field of this antenna, whose relative position is correlated with the size and shape of the segments. We have found that the large millimeter telescope is very sensitive to axial subreflector positioning errors, requiring that the subreflector actuators be able to maintain is optimum position within a small fraction of a wavelength.^ With the interest to use a focal plane array in the LMT, we have made a comparative study of the imaging properties of the LMT with that of two aplanatic Cassegrain designs, namely, the Schwarzschild and the Ritchey-Chretien telescope. We found that operating at millimeter wavelengths the three Cassegrain systems have an equivalent performance. This study also revealed the potential benefits of an aplanatic configuration at shorter wavelengths or smaller system focal ratios. ^

Subject Area

Engineering, Electronics and Electrical|Physics, Astronomy and Astrophysics

Recommended Citation

German Cortes-Medellin, "Analysis of segmented reflector antenna for a large millimeter wave radio telescope" (January 1, 1993). Doctoral Dissertations Available from Proquest. Paper AAI9329588.
http://scholarworks.umass.edu/dissertations/AAI9329588

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