I. Introduction
Frequency band such as 28 GHz and 40 GHz which has not been allocated for mobile communication have been actively utilized since the widespread of 5G [1]. However, radio waves have a high straight advancing property in the high frequency band, so the coverage holes where user terminal can not link to base station in the communication area exist due to blockage of buildings or trees [2]. To solve this problem, the reflectarray has been gained attention to reduce the coverage holes with low cost. The reflectarray can reflect incident wave in the arbitrary direction. To obtain sufficient power from the reflectarray to the user terminal, large size reflectarray is required. However, the install space is usually limited, thus, dual bands reflectarray which can reflect incident wave in dual bands with one reflectarray is effective [3]. The dual band reflectarray using the unit cell of fractal patch has been proposed, which operates at 28 GHz and 32 GHz [4]. The proposed reflectarray can shape reflection pattern in each operating frequency and each polarization. However, it is not designed by the supercell structure, resulting in complicated structure. In our previous work [5], the reflectarray using cross dipole and four patch structure, operating 28 and 39 GHz has been proposed, which can reflect vertical incident wave for one-dimensional reflection. The reflectarray is designed by the supercell structure for simple structure. To employ for supercell structure, the unit cell size is decided by the operating frequency and reflection angle as described in section II. Thus, appropriate unit cell design is required. To the best of our knowledge, the dual bands reflectarray operating at 28 and 39 GHz for two-dimensional reflection has not been reported. In this study, the design procedure and unit cell design are presented.