The beams first pass through a lenslet array which efficiently couples the input beams to the power pattern of each mixer's feedhorns. Diagonal feedhorns were chosen because of their high performance and ease of construction. The mixer has a waveguide transition to half-height, rectangular waveguide. The quartz substrate containing the niobium SIS junction rests across the center of the waveguide. The on-subtrate tuning structure and the suspended stripline rf-choke structure are designed to provide a good rf match from 300-400 GHz. There will be no mechanical tuners. An electromagnet connected to field concentrators embedded in the mixer block provides the quantum of magnetic flux necessary to suppress the Josephson effect. The electromagnet is the same design used in our current SMTO and AST/RO receivers. A 4-6 GHz IF was chosen in order to have a 2 GHz bandwidth over less than an octave of frequency, thereby relaxing design constraints on the IF components. A prototype IF matching network has been designed, built and tested. The IF output of each mixer then passes through an isolator before entering the first amplifier.