A new type of X-band (8-12GHz) unit cell structure on a single-layer for broadband reflectarray antennas is presented in this study. To obtain a broadband performance, a unit cell composed of a circular ring and a complementary diagonal patch ring is proposed. This new type unit cell is designed on a 0.183x(0) (air-dielectric) thin dielectric substrate and has a 430(degrees) phasing range with slow phase variation. Based on the wide phase variation of the proposed unit cell, a centre-fed broadband reflectarray antenna is designed with 9x9 unit cells, 81 elements on a square plane geometry. The designed reflectarray has 40% aperture efficiency and 96% antenna efficiency at 10.5GHz. The validity of the reflectarray antenna is verified through measurement and simulation results. The experimental and simulation results are in good harmony. The measured and simulated results show that the maximum gain of the reflectarray antenna is 21.6dBi at 10.5GHz and has 23% 1dB gain bandwidth and 29% 3-dB gain bandwidth. Also, the minimum cross-polarisation discrimination is 31.2dB at 11GHz frequency.