The design and fabrication of a fixed-tuned 320-360 GHz sub-harmonic mixer, featuring a newly developed small anode junction anti-parallel pair of planar Schottky diodes chip, are presented. A traditional E-plane split-block waveguide architecture was adopted in the mixer's design: the diodes chip was flip-chipped onto a quartz-based microstrip circuit and suspendedly glued to the bottom half of an equally split waveguide block with silver epoxy. A method of combination of field and circuit was applied to simulate and optimize the performance of the mixing circuit. Every functioning part of the mixing circuit was calculated with field simulating software to create its own S-parameters package, which was then combined with the diodes' barriers to be used by nonlinear circuit simulating software to simulate and optimize the performance of the mixer. The final test indicates that the ndicates that the mixer's double side band (DSB) conversion losses were lower than 9 dB, over 12% of bandwidth (320-360 GHz), with 4-6 mW of local oscillator (LO) power input; and at room temperature a minimum DSB equivalent noise temperature of 780 K was measured for RF frequency between 310 GHz and 340 GHz.