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A fully automatic method of calibrating precision rotary-vane attenuators has been devised and evaluated at X-band. The rotary-vane attenuators have been fitted with servo drive units, optoelectronic homing systems, and digital angular measuring systems (with a resolution of 0.0001°). Our X-band modulated subcarrier system has been automated by replacing the manual inductive-voltage divider with a programmable one and adding to it a timing unit, a controller, a frequency stabilizer, a data logger, and servo drive units for the rotary phase shifter and set-zero potentiometer. The results achieved without and with automation are in very good agreement. With automatic operation, the random component of the uncertainty is typically ±0.0002 dB (for 10 measurements and a confidence level of 99 percent). After calibration, these automated rotary attenuators are used in self-balancing radiometers.