In many piezoelectric transducer applications, we call for directional rather than omnidirectional transducers. One way to obtain the desired directional radiation pattern is to deploy multiple transducer elements in a long linear array or large planar array. Another interesting method is to use an acoustic helical antenna which consists of an inactive cylindrical helix made of a brass rod and exciter of the Langevin piezoelectric transducer.
In this paper, the effect of the variation of phase velocity in a metal wire helix of the acoustic helical antenna on the directional pattern is studied, and it is clarified that the directional pattern of endfire radiation occurs for the helix over a wide band of frequencies. In addition, it is suggested that a conical logarithmic-spiral antenna is suitable to reduce greatly side lobes without paying the expense of a wider main lobe.

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