Techniques for Reducing the Effect of Measurement Errors in Near-Field Antenna Measurements

NIST 18 term error analysis has been used for some time to estimate the uncertainty in the far-field antenna parameters determined from near-field measurements. Each of the error terms is evaluated separately to estimate the uncertainty it produces in parameters such as gain, directivity, side lobe level, cross polarization level and beam pointing angle. This identification and evaluation of uncertainties has led to the development of procedures that can be used to reduce the effect of individual error sources and therefore improve the reliability of the results. Automated, real time systems have been added to the measurement hardware and electronics that can reduce the effect of such things as probe position errors and cable flexing. Measurement and special computer processing techniques have also been developed to self-calibrate and correct for transmission path differences of dual mode probes. More recently, a number of techniques have been developed that provide a means to reduce the effect of measurement errors without the need of special hardware or additional measurements. These procedures often involve additional data processing steps to identify and reduce the presence of the error in the measured data, but the processing time is small and the improvement in some parameters can be very significant. In some cases, the error signal level can be reduced by 10 to 20 dB. Such techniques have been developed for errors due to bias error leakage in the receivers, non-ideal rotary joints, spherical rotator misalignment, and room scattering. Further improvements can be realized by making additional measurements to reduce multiple reflection effects, position errors and room scattering in spherical systems. Examples of these techniques will be presented to illustrate the methods and demonstrate typical improvement.