  In the construction of a space radio telescope, it is essential to use
materials with a low noise factor and high mechanical robustness for the
antenna surface. We present the results of measurements of the reflection
performance of two candidates for antenna surface materials for use in a radio
telescope installed in a new millimeter-wave astronomical satellite, ASTRO-G.
To estimate the amount of degradation caused by fluctuations in the thermal
environment in the projected orbit of the satellite, a thermal cycle test was
carried out for two candidates, namely, copper foil carbon fiber reinforced
plastic (CFRP) and aluminum-coated CFRP. At certain points during the thermal
cycle test, the reflection loss of the surfaces was measured precisely by using
a radiometer in the 41-45 GHz band. In both candidates, cracks appeared on the
surface after the thermal cycle test, where the number density of the cracks
increased as the thermal cycle progressed. The reflection loss also increased
in proportion to the number density of the cracks. Nevertheless, the loss of
the copper foil surface met the requirements of ASTRO-G at the end of the
equivalent life, whereas that of the aluminum-coated surface exceeded the
maximal value in the requirement even before the end of the cycle.
