Sub-harmonically pumped SIS mixers operating at very high LO harmonics have been used to measure beam patterns of SIS mixers in the laboratory, and to measure the line-widths and phase-locking performance of flux-flow oscillators. Using the sub-harmonically pumped mode of operation for beam pattern measurements allows the measurement to be carried with a single phase-locked sub-mm source, without a beamsplitter in front of the mixer feed. This allows the near field amplitude and phase to be measured, and allows the far field pattern to be measured at very high angles which would be blocked by the presence of a beamsplitter. Kittara, Withington and Yassin[1] have recently described a procedure for modelling the non-linear behaviour of very high harmonic SIS mixers. They use a fully non-linear multitonal mixer theory[2] to analyse the behaviour of a SIS mixer pumped by the 20th harmonic of a 13.5 GHz LO signal, with the mixer down-converting both sidebands around 270 GHz to a 1.4 GHz IF. This analysis shows that sub-harmonic mixers can achieve reasonable dynamic range. The pattern of behaviour seen in the simulations are in remarkable agreement with published experimental results[3]. In this work we describe the small-signal behaviour of subharmonic SIS mixers using the CalTech's SuperMix software[4]. This method has the advantage of faster convergence than the non-linear analysis and hence allows the exploration of the complex behaviour of subharmonic SIS mixers. Our analysis is compared Kittara et al's results. We show that SuperMix can accurately calculate the small-signal behaviour of high harmonic SIS mixers, as well as providing predictions of mixer noise performance.