An analysis of the measured errors of a large number of rotary-vane attenuators has shown that the systematic error present is correlated with vane misalignment. This paper shows that a study of the gear mechanism used to drive the rotating vane has revealed an additional systematic error for which corrections can be made. This error is the angular displacement caused by eccentricity. Graphs are used to illustrate the readout scale error in decibels at various magnitudes of the drive-gear eccentricity and driven gear pitch diameter. An electrical alignment technique utilizing a correction equal to the "average vane-angle error," which is determined from calibration data, is used to align the rotating vane to an accuracy of ±0.0012°. Vane alignment by this technique can reduce the systematic error of vane misalignment by an order of magnitude and decrease the deviation in attenuation at high values of attenuation as much as two orders of magnitude.