Radar echoes from metal spheres large and small

The radar echoes from a metal sphere depend critically on the precision of its fabrication whether large or small. Based on RCS measurements of more than 3 spheres in a set, it is possible to select the good ones for calibration. From the frequency sweep on a sphere, the creeping wave contribution may usually be resolved in a 1-D DFT on the data. Below the resolution limit, however, an anomalous dip is observed for the ball bearing of 0.50″ dia. A possible correlation is that both the Mie-series and the measured RCS for the 0.50″-dia. ball bearing exhibit a minimum in the middle of the band from 8 to 18 GHz. Use of the DFT window function is discussed.