In this paper, the characteristics of the card-sized paging antenna composed of plates and wires are estimated in view of its efficiency. To analyze this type of antennas using the reaction matching technique for the wire antenna, we approximate the plate parts to the wire grids. In the analysis of the antenna with a complex configuration, the numerical input impedance by the usual reaction technique assumed the local symmetry in calculating one mutual impedance, is sometimes incorrect. Therefore, we carry out the reaction matching on the condition that the dipoles are placed at their own positions throughout calculating the mutual impedances. In modeling the plate with a wire-grid, we estimate the loss effect of the plate parts by introducing the equivalent radius. Furthermore, we examine the validity of the wire-grid model for the card-sized antenna. And, we estimate the radiation efficiency as a function of the gap distance of the plates, and discuss the polarization matching and the impedance matching of the thin paging antenna, in view of the total efficiency.