Radiation Characteristics of Ultrasonic Focusing Radiators

Abstract

An experimental study has been made of the complete radiation pattern of a 5 mc, spherical, concave, focusing quartz radiator. This is the same radiator as elsewhere described with regard to its quality of focusing and power of concentrating ultrasonic energy. Because of the curvature of the radiator, the radiator thickness direction varies relative to the crystallographic axes from truly x cut at the center of the radiator to about 13° off x axis at the edge of the radiation region (18° off at the mounting boundary). Hence the effective electromechanical coupling and frequency constant, relative to the respective thickness directions, differ over the radiator surface. Their difference from that at the center varies differently in different directions, but in general, increases on receding from the center. The result is that the radiation efficiency also decreases on receding from the center. Radiation patterns showing these effects in different planes through the axis of the radiator will be shown. Since the elastic and electrical constants of quartz are well known, it was possible to calculate the variation of electromechanical coupling, frequency constant, and radiation efficiency variations over the surface of the radiator. The experimental results above check the theory excellently. There is a plane, parallel to the x axis and inclined about −22° to the z axis (toward parallelism with the major cap face) in which the frequency constant is nearly constant with recession from the x axis and the electromechanical coupling nearly as good as in the best plane. Therefore a preferred orientation for an x-cut, cylindrical focusing radiator is that in which the cylindrical axis of the radiator is normal to this plane. Further, even with spherical focusing radiators the off-resonance effects, due to frequency constant variations, may be compensated for by varying the thickness proportionally to the frequency constant. The result of such thickness shaping is to produce a radiator of improved radiation efficiency and greater concentrating power. Theoretical curves will be presented.