This paper presents a study of the monostatic and bistatic radar cross-section (RCS) of three planar two-dimensionally periodic antenna arrays with the same aperture size: arrays of conductors of variable square cross-section, TEM horns, and rectangular metal waveguides. The results of numerical simulation of the arrays using the finite element method showed that, under normal plane-wave incidence, the monostatic RCS of the array of square conductors is smaller by 10…30 dB than that of the waveguide array in the frequency band wider than 10 : 1. For one linear polarization of the incident plane wave, the RCS of the array of TEM horns is close to that of the waveguide array and, for the other, it is close to the RCS of an array of conductors of variable square cross-section. With an increase in the incidence and observation angles, the difference in monostatic RCS decreases. The scattering patterns of the array of conductors of variable square cross-section for various frequencies and angles of plane-wave incidence are presented and compared with those of the waveguide array under normal incidence.