Low reflectivity apodized surface acoustic transducer with means to prevent wavefront distortion

Claim 1

In a surface acoustic wave transducer of the type having: a substrate; a pair of arrays of electrically conductive elements on said substrate; means for connecting an electrical circuit across said pair of arrays; said arrays being interdigitated over an active area for coupling between said electrical circuit and an acoustic wave traveling in an acoustic beam direction across the surface of said substrate; said arrays including an element at one end of said transducer; an inactive area of said transducer in which no elements of said arrays are interdigitated; said active area having a first boundary nearest said end element; and a continuous covering of material adapted to alter the surface acoustic velocity of said substrate over only a portion of said inactive area between said first boundary and said end element; said covered area having a second boundary between said first boundary and said end element; said boundary being so shaped that the surface acoustic transit time along a path parallel to said beam direction between said first boundary and said end element is substantially equal across at least the major portion of the aperture of said transducer; the improvement wherein said second boundary is shaped along the major portion of its length so as to substantially satisfy the equation (a+b)/V.sub.T =(a/V.sub.M)+(b/V.sub.F) where a is the distance along a line parallel to said beam direction from any point on said first boundary, b is the distance along said parallel line from said intersection with said second boundary to said end element, V T is the surface acoustic velocity in said active area, V M is the surface acoustic velocity along said distance a, and V F is the surface acoustic velocity along said distance b.