Millimeter-Wave Near-Field Measurements Using Coordinated Robotics

The National Institute of Standards and Technology (NIST) recently developed a new robotic scanning system for performing near-field measurements at millimeter-wave (mm-wave) frequencies above 100 GHz, the configurable robotic millimeterwave antenna (CROMMA) facility. This cost-effective system is designed for high-frequency applications, is capable of scanning in multiple configurations, and is able to track measurement geometry at every point in a scan. The CROMMA combines realtime six-degree-of-freedom optical spatial metrology and robotic motion to achieve antenna positioning to within 25 μm rms. A unified coordinated metrology approach is used to track all positional aspects of scanning. A vector network analyzer is used to capture amplitude and phase. We present spherical near-field measurements of the forward hemisphere of a 24-dBi standard gain horn at 183 GHz. Using the configurable scanning ability, two different scanning radii were used. Near-field data were taken at a 100-mm radius. Direct far-field measurements were also taken at 1000-mm radius. The E- and H-plane patterns are determined from the measurements and compared to theoretical patterns. We describe the system components of the CROMMA and the coordinated metrology approach used. An analysis of the positional repeatability and accuracy achievable is also presented.