The high-frequency coil is the most important component of an EMAT. Its geometrical configuration determines the modes of generated and detected elastic waves. The liftoff (or air gap) between the coil and specimen surface must be minimized as much as possible because it exponentially lowers the efficiency of the EMAT as indicated by Eq. (1.29). Moreover, the coil should be flexible for bending and twisting to accommodate curved specimen surfaces. There are two fundamental and useful geometries of EMAT coils: spiral elongated coil and meander-line coil. The former is usually used for bulk-wave EMATs. The latter is for surface SH-wave EMATs, Rayleigh-wave EMATs, Lamb-wave EMATs, etc. Coils for the focusing and chirp EMATs are rather complicated, requiring a high accuracy, and a handmade approach is impracticable.
KeywordsPulse Generator Matching Network Spiral Coil Match Circuit Sintered Magnet
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