Side-shifted dual PPM EMATs with multiple rows of magnets and reduced lateral gap by flexible printed circuit board racetrack coils

Shear horizontal (SH) guided waves can be generated by permanent magnet electromagnetic acoustic transducers (PPM EMATs). Conventional PPM EMATs generate waves that propagate both backwards and forwards. Recently, we presented designs of side-shifted PPM EMATs that generate SH waves in a single direction through the interference mechanism of two independent periodic sources of forces. These designs, however, can present unwanted backward side-lobes whose amplitude decreases as the lateral separation between rows of magnets shortens. Originally, the side-shift EMATs were fabricated with two hand-wound multi-lap racetrack coils. This process is time-consuming and hinders the possibility of short lateral separation between magnets' rows. In this paper, we present an alternative fabrication method for this design using flexible printed circuit boards that allow us to shorten the lateral separation, increase repeatability and, consequently, enhance the unidirectional behavior of the side-shifted dual-PPM EMAT.