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We report our recent progress toward the development of microwave brightness-temperature (BT) standards. As one of the crucial parameters, the target illumination efficiency (IE) was traditionally determined from the relative antenna pattern. We propose a measurement technique to extract the target IE solely by the use of passive radiometric measurements for characterizing the BT of the blackbody radiator. Such a technique allows us to skip the complexities that are often encountered during the measurement and calculation of the antenna pattern. Taking advantage of the variable heating capability available on most blackbody targets, we varied the temperature of a heated blackbody target and ran a series of radiometric measurements when the target was separated at different distances away from the the antenna with the radiometer operating at a few frequencies. Our experimental results show excellent measurement accuracy on the IE, with uncertainty of about 1% at close separation distance between the antenna and the target. We further measured and computed the BT of the blackbody target at the locations where we had measured the extracted IE. The BT was slightly lower than the physical temperature of the target and exhibited 0.7 K to about 1 K uncertainty when the target was located no more than 1 m away from the antenna. A measurement uncertainty of 1 K already meets the accuracy requirements of some climate variables, and such results reflect a significant step toward the establishment of BT standards at microwave frequencies.