NEAR FIELD SCANNING ON AN EXTERNAL SPHERE

NEAR-FIELD ANTENNA MEASUREMENT SYSTEMS PROVIDE A POWERFUL, AND OFTEN THE ONLY FEASIBLE, METHOD OF DETERMINING THE FAR FIELD OF VERY LARGE APERTURE ANTENNAS OR OF SPACEBORNE ANTENNAS. EXISTING NEAR-FIELD MEASUREMENT FACILITIES REQUIRE PRECISE POSITIONING SYSTEMS FOR THE PROBE ANTENNA AND AS A CONSEQUENCE ARE EXPENSIVE AND RISKY. A POTENTIAL IMPROVEMENT TO NEAR-FIELD MEASUREMENT IMPLEMENTATION IS A SYSTEM THAT USES A FOUCAULT PENDULUM AS THE SCANNER WITH A PROBE AT THE BOTTOM OF THE PENDULUM'S ARM SWINGING ABOVE THE ANTENNA UNDER TEST (AUT). WE PROPOSE TO DESIGN A NEAR-FIELD MEASUREMENT SYSTEM BUILT AROUND THIS CONCEPT, AND THIS PROPOSAL CONTAINS OUR APPROACH TO SUCH A DESIGN. WE PRESENT THE THEORY OF SCANNING OVER ARBITRARY EXTERNAL SPHERES AS WELL AS SAMPLING CRITERIA, FILTERING, AND OTHER NECESSARY SIGNAL PROCESSING TECHNIQUES APPLIED TO THE MEASURED NEARFIELD DATA. WE ALSO CONSIDER THE SOURCES OF ERRORS PRODUCED IN SUCH A MEASUREMENT SYSTEM AS WELL AS PRESENT THE EQUATIONS DESCRIBING THEIR FUNCTIONAL BEHAVIOR. ADDITIONALLY, WE ADDRESS SUCH ISSUES AS BANDWIDTH, DYNAMIC RANGE, AND A/D CONVERSION SPEED. IN ORDER TO TEST THE PHYSICAL CREDIBILITY OF THE PENDULUM-SUPPORTED PROBE MEASUREMENT SYSTEM, WE HAVE MADE SOME PRELIMINARY ESTIMATES OF PENDULUM SIZE FOR LARGE ANTENNAS OPERATING AT X-BAND. FINALLY, WE PRESENT A BLOCK DIAGRAM FOR A PROPOSED NEAR-FIELD MEASUREMENT SYSTEM.