In this paper, a novel 180°hybrid with different input frequencies is proposed to combine RF and local oscillator (LO) signals with different frequencies in a gate/base-pumped harmonic mixer. The detailed analysis and design procedures are presented in this paper. To further reduce the chip size, the multilayer metallization above the lossy silicon substrate is employed to implement the hybrid. A V-band down-converted 2× harmonic mixer in 90-nm CMOS process and a D-band down-converted 4× harmonic mixer in the 130-nm SiGe process are designed, fabricated, and measured to verify the concept. The 2× harmonic mixer possesses 0-dB conversion gain at 60 GHz with 0-dBm LO power with merely 2.4-mW dc power. The 4× harmonic mixer achieves 0.5-dB conversion gain at 120 GHz with 2-dBm LO power and 27.3-mW dc power. With the proposed reduced-size 180° hybrid, gate/base-pumped harmonic mixers are very attractive in transceivers demanding low LO frequency and power.