This paper presents a 268-325 GHz high output power, transformer-based frequency doubler for radar applications achieving 5.2dBm saturated power (Psat) at 304 GHz and 57 GHz Psat 3 dB-bandwidth. We propose a circuit concept based on a modified octagonal transformer device that is used to efficiently extract the second harmonic from a differential power amplifier (PA), operating at half the output frequency. The device exploits the fact that the second harmonic is present in common-mode at the PA output, whereas the fundamental harmonic remains in differential mode. Hence, the transformer device enables the extraction of the second harmonic by the means of mode separation and is applied as output matching network, simultaneously. Driven by a two-stage, cascode-based wideband driving stage, it provides a Psat exceeding 0 dBm over a bandwidth of 85 GHz from 251-336 GHz. A peak conversion gain of 20 dB is achieved at 304 GHz and remains above 10 dB for operation at maximum output power. The chip is fabricated using a 130 nm SiGe BiCMOS technology with ${f_{t}/f_{\max}}$ of 250 GHz / 370 GHz and consumes 505 mW from a 3.65 V supply. To the best of the authors knowledge, the power of 5.2 dBm is the highest reported in that frequency range using SiGe technology.