In this article, we present a high-efficiency high-power radiator with on-chip frequency stabilization and quadrature generation capabilities for short-coherence-time imaging and wireless communication applications. To achieve efficient sub-terahertz (sub-THz) signal generation, a systematic design approach using the device-centric gain-plane optimization is employed. In addition, an on-chip frequency stabilization mechanism utilizing the inherent power level variation over the tuning range is proposed. To show the feasibility of this approach, a 170–176-GHz radiator prototype is demonstrated, which has an effective isotropic radiated power (EIRP) of +24.6 dBm and a dc-to-RF efficiency of 9.2%. The prototype is fabricated in a 55-nm SiGe bipolar complementary metal–oxide–semiconductor (BiCMOS) process and achieves one of the highest output power and dc-to-RF efficiency among designs beyond $f_{\text {max}}$ /2. The frequency stabilization mechanism reduces the long-term frequency drifts by 4.3 times.