A reflectarray antenna (RA) with a novel sub-wavelength-frequency selective surface (FSS) is introduced as a ground plane. The unit element consists of two concentric circular rings, where the width of the outer ring is constant and the width of the inner ring varies. The proposed multi-resonance sub-wavelength element has a linear reflection phase and improved element loss, which results in increased bandwidth and improved aperture efficiency. The triple-connected C-shaped resonator as FSS is used in each unit element instead of the ground plane, so that it is reflective in the frequency range of 9.42-12.55GHz and is transparent outside the working frequency band of the antenna. The reflective and pattern characteristics of the unit elements with the solid-metal and FSS ground planes are compared with each other. The proposed FSS-backed RA has a peak gain of 26.26dBi at 10GHz with a 42.87% aperture …