The design, fabrication and testing of broadband fixed-tuned 183 GHz and 366 GHz sub-harmonic mixers is discussed. Two mixer circuits using planar Schottky diodes monolithically integrated with RF/LO and IF matching and filtering networks onto a 3.7 um thick GaAs membrane are presented. The mixers were designed and fabricated using a novel approach in post processing of commercial foundry wafers. The double sideband measurements at room temperature on the 183 GHz sub-harmonic mixer yielded conversion loss and noise temperature of 4.9 dB and 608 K, respectively. The measured double sideband conversion loss and noise temperature of the 366 GHz mixer were 6.9 dB and 1220 K, respectively. To the authors' knowledge these are the best published results for sub-harmonic MMIC membrane mixers at these frequencies.