the device is to be designed to operate surface waves having substantially equal magnitude are propagated away from input transducer 15 in two directions as indicated by arrows both surface waves propagate away from input transducer 15 in opposed directions until reaching first and second frequency selective reflective gratings 21 and 23 frequency selective reflective gratings 21 and 23 are spaced equal distances from input transducer 15 both surface waves are reflected at a substantially 90 u00b0 angle from reflective gratings 21 and 23 toward reflective gratings 22 and 24 respectively reflective gratings 22 and 24 again reflect the surface waves at a substantially 90 u00b0 angle toward output transducer 16 spaced equidistantly between reflective gratings 22 and 24 reflective gratings 21 24 are both frequency selective and frequency dispersive that is surface waves having a frequency only within a predetermined bandwidth of frequencies are reflected and surface waves propagating from grating 21 toward grating 22 and from grating 23 toward grating 24 have a spatial frequency dependence for example portions of the surface wave having a frequency in the upper portion of the passband propagate from grating 21 to grating 22 and from grating 23 to grating 24 nearer to the center of the device than surface waves having a lower frequency surface waves having the lowest frequency within the predetermined passband propagate between gratings at the outer edges of the gratings because of the symmetry of the device the two surface waves incident upon output transducer 16 arrive in phase that is the arriving surface waves add in amplitude at output transducer 16.