Analysis of Rotation Errors of a Waveguide Rotary-Vane Attenuator

In the Microwave Calibration Services Section of the NBS Radio Standards Laboratory, Boulder, Colo., and analysis of calibration data obtained from measurements made on a rotary-vane type attenuator reveals that the vane alignment produces an inherent vane-angle error. It is found that the standard deviation of the measured values of attenuation is correlated with the vane-angle error. Approximately two-thirds of the attenuators calibrated were in error by less than 50 per cent of the manufacturer's stated specifications of accuracy. The treatment of the errors of a rotary vane attenuator is extended to cover the vane-angle error, resettability error, and mismatch error. The method of analysis is applicable to all attenuation values and vane-angle errors of this sample. Graphs are used to estimate the limits of these errors. Several representative cases are given using the deviation from the "estimated vane-angle error" to establish limits in the correction of vane-angle error. A modified rotary-vane attenuator with a resolution equal to 30 times the resolution of the attenuators comprising the sample was used to verify calculated vane-angle error at various values of attenuation. Inherent vane-angle error can be reduced by increasing the dial resolution and by a technique of precise vane alignment at zero dial setting.