A novel X-band single-layer unit-cell structure with enhanced bandwidth for linearly polarized microstrip reflectarray antenna is proposed. The unit-cell structure is composed of two circular rings, each with a pair of gaps that are orthogonally placed, and two identical phase-delay lines attached to the outer circular ring. With this novel structure, a linear phase response ranging about 550 degrees is achieved by varying the length of the phase-delay line. An offset-fed reflectarray antenna composed of 277 elements forming an octagon-shape aperture is designed, fabricated, and measured to verify the broadband characteristic of the proposed unit cell. Measurement results show that 20% 1-dB gain bandwidth is realized. Moreover, the maximum gain at 10 GHz is about 26.38 dBi, which means about 51.3% efficiency is achieved. At the same time, the sidelobe level and cross polarization for E-plane are also measured which are below -17.5 and -26 dB, respectively.
