In this paper, a dual-band antenna at 4.5 GHz and 25 GHz is presented. The antenna is a low-frequency phased patch antenna array combined with a high-frequency transmitarray with a fixed beam. The lowfrequency patch antenna array and the high-frequency transmitarray feed share the same aperture area. The high-frequency transmitarray surface only uses a single substrate layer and is electrically transparent to the low-frequency phased array with beamforming. The antenna is measured to achieve an impedance bandwidth of 350 MHz and 3 GHz, and a gain of 15.4 dBi and 23.5 dBi, at S- and K-band respectively. With an impressive frequency-ratio of 5.55 and an aperture area of only 12x12cm the antenna achieves an aperture efficiency of 66% and 15% in the two bands respectively. Additionally, the low-frequency beamforming capabilities (with the existence of transmitarray surface) are measured and the antenna is shown to have a 60-degree scanning range with only a 0.22 dB gain drop-off.