Dual‐type helical acoustic antenna

Abstract

A dual‐type helical acoustic antenna, which operates as an under‐water endfire hydrophone, is described. It consists of two brass‐wire helices wound in a concentric configuration and a PZT piezoelectric transducer of Langevin type. The structure is analogous to a helical antenna operating in the axial mode of radiation at microwave frequencies. The progressive wave along the helix yields the phase shift kbI due to wave propagation corresponding to the length of one turn I, while the phase shift due to the spacing between turns (the pitch) S is ku′S, where kb and kw are the wavenumbers in the brass wire and in water. For the radiation from the corresponding segments of all turns to add in phase in the axial direction, it is necessary for the helices to satisfy the conditions of kbIi = ku′Si and kbIo = ku′So, where the subscripts of i and o represent the inner and outer helices. The relationship between directional patterns and the dual helix parameters is shown in detail.