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This paper describes measurements of the characteristics of a large deployable antenna using the tensioned cable truss scheme in orbit. From observations of celestial radio sources, the antenna gain degradation after launch was shown to be acceptable at L- and C-bands. At Ka-band, however, serious degradation of 24 dB was measured. From observations of the earth and moon, the degradation at Ka-band was shown to be attributed to the transmission line loss. The systematic and random errors of the main reflector surface are separated on the basis of these measurement results and theoretical analysis. Accordingly, the reflector status in orbit is estimated.