Performance of a single-ended fundamental frequency diode mixer, designed for cooled operation in the frequency range 190–260 GHz, is reported. At 230 GHz the best single sideband (SSB) mixer temperature obtained was 300 K, when the mixer was cooled to 20 K, with a corresponding SSB conversion loss of 5.9 dB. The significant improvement compared to previously reported results is attributed to the novel mixer mount design and to the optimum empirical choice of diode electrical parameters.