Direct Experimental Verification of Creeping Waves

Abstract

The Watson transformation has been developed for diffraction problems in the frequency region where the numerical values can be obtained more easily than by the use of the exact normal-mode series. A physical interpretation of this mathematical technique was given by Franz as creeping waves that travel around the surface at a velocity less than that in free space and that are attenuated by radiation. Thus far, only indirect measurements have been made experimentally. Herein is presented the report of a series of experiments that give direct verification of these waves for scattering by rigid spheres and cylinders in air. Radiation of creeping waves circumnavigating the obstacle was verified by interference measurements in the backscattered field with continuous waves. To detect the “isolated” creeping wave as it circumnavigated the scatterers, incident wave pulses in conjunction with high-speed digital logic techniques were used. Coincidence techniques were used for precise measurement of velocity, and amplitude decay was determined by sampling and comparing CW pulses at two points in space and time by Lissajous pattern techniques.