We present two high-efficiency baseband-to-D-band transmitting systems based on a novel channel-aggregation architecture. They comprise a first integrated circuit for channel aggregation at intermediate frequency, a two-channel 45-nm CMOS transmitter flip-chipped on antenna-in-package which illuminates a high-gain flat lens antenna in standard printed circuit board technology. The two signals emitted by the active antenna module, in adjacent bands, are aggregated over the air and collimated by the flat lens. Two transmitters using, respectively, a standard and a folded transmitarray antenna, three times thinner than the former one, are realized and compared. They achieve similar performance in the operating band (139.3-156.6 GHz), with a measured peak effective isotropic radiated power of 30 dBm. A wireless point-to-point link is demonstrated with the proposed transmitting system and a commercial receiver. A data rate of 57.6 Gb/s is measured at 1 m using 8 channels and a 16-QAM scheme, with an energy efficiency of 27.4 pJ/b.