Since higher communication speed than the acoustic wave communication is required in the ocean, the high frequency radio wave communication method is developed. Because the method requires propagation of narrow channels which are made from less attenuation materials, communication characteristics of the method can be different from propagation theories in the air. Therefore, communication characteristics of the method have been researched since the method is developed. In this article, the effect of the propagation channel inflection is experimentally confirmed. Results show there is a threshold value of inflection angle that communication strength is suddenly decreased. Furthermore, smoothing the corner of the propagation channel decreases variability of communication strength.