Analysis and Design Considerations of Microwave Radial Cavities for Ultrasonic Applications

Abstract

Abstracf-The theoretical and practical aspects involved in the design of radial microwave cavities for ultrasonic applications are considered. A semigraphical approach to the solution of the trans- cendental resonance equation is developed along with a complete set of relations by which the performance and efficiency of the cavity design may be determined. Special application of this analysis is made in the design and construction of S band microwave ultra- sonic cavities for use in viscoelastic relaxation investigations in liquids at 3 GHz. N THE FIELD of microwave ultrmonics and laser communirations it is frequent,ly found t.hat radial microwave cavity resonat.ors provide a convenient, and efficient, means of exciting piezoelectric and electro- opt,ic materials. This paper is intended to present a treatment of the analysis and design considerations in- volved in the development of these radial cavities. The treatment developed here is presented primarily with a view towards the generation and detection of ultrasound for viscoelastic relaxat,ion studies;*') however, the basic cavit,y t,heory is quite general and should in principle be equally valid for elast.ooptic and electrooptic applications. The cavity adopted for analysis is of the symmet,ricttl double-reentrant configurative shown in Fig. 1. For the sake of clarity all tuning, coupling, and associated mech- anisms have been omitt,ed. A complete assembly drawing and description of the 3 GHz cavities construrted for viscoelastic studies is availahlc in the ljt,erat>ure. ('l Normally the material to be excited is placed in t,he gap region between the reentrant cavit.y electrodes. This material may be any of several low loss substances surh as ZnO, CdS, KDP, LiNboa, etc., depending on the :qplication. For the ultrnsonic application considered here the material was a single crystal disk of piezoelectric quartz, which was held in position by a teflon assembly as indicated in Fig. 1. The radial configuration was chosen for two basic reasons, t,hese being high filling factor and relative freedom from nonaxial elect,ric field components. These nonaxial fields can cause considerable trouble by generating un- wanted modes in t,he crystal. In order to provide flLsh over protect,ion under high pulsed power conditions and to insure that the excit.ed region was free of any edge mounting strains, the diameter