Acoustic slow-waveguide radiators which work as an underwater end fire source consist of two elements : the guide and the exciter. They are classified into two groups according to the structure of the guide, namely, the slow-rod and the slow-tube radiators. In this paper, simplified expressions for directivities are derived under a few assumptions, and directional patterns are calculated for slow-waveguide radiators corresponding to the following conditions :
(1) slow-rod radiators of various rod-diameters with optimum rod-lengths.
(2) slow-tube radiators of various tube-diameters with optimum tube-lengths.
(3) slow-rod radiators of various rod-lengths with common rod-diameter.
(4) slow-tube radiators of various tube-lengths with common tube-diameter and common wall-thickness.
From directional pattern data presented in this paper, it is clarified that for sufficiently thin-walled slow-tube radiators it is possible to obtain a narrow main lobe with small side lobes as compared with slow-rod radiators.

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