The goal of this mixer design was to obtain fixed-tuned operation around 345 GHz (280-370 GHz band). It is a single backshort waveguide mixer. In the mixer block the waveguide height is reduced to a quarter of the standard size. In this mixer two 1--square micron connected seriesly Nb--Al oxide--Nb junctions, with RwC=6, have been used. The individual inductive tuning elements are integrated with the junctions. A new version of a non contacting backshort has been developed for the reduced height waveguide mixer block. A magnetic field is applied to the junctions in order to suppress the Josephson current. Measured mixer DSB noise temperature is 10 K ( level). According to this data the mixer sensitivity may be estimated as one photon. During the test the mixer backshort, once fixed, remains in the optimal position. For this backshort position the receiver exhibits a low noise operation across the band.