A 14-30 GHz sub-harmonic single-balanced gate-pumped mixer with a wideband transformer combiner in 0.18 μm CMOS technology is presented. The transistors of the sub-harmonic mixer core are biased at weak inversion to lower the required local oscillator (LO) driving power and DC power consumption. The wideband combiner including a Marchand balun and a 1:1:1 trifilar transformer was designed to combine the radio frequency (RF) and LO driving signals to produce four states of signals, where the RF frequency is about twice the LO frequency. Two quarter-wavelength transmission lines are adopted to improve the RF-to-intermediate-frequency (IF) and 2LO-to-IF isolations. The measured results show that the maximum conversion gain of -1 dB is at 18 GHz and a 3 dB bandwidth is from 14 to 30 GHz while applying the LO driving power of only 1 dBm. The total power consumption including the output buffer is only 7.8 mW.