The Atacama Large Millimetre Array will be a ground based millimetre to submillimetre band interferometer. The instrument will be comprised of up to 50 high precision 12m Cassegrain antennas. Each antenna will cover a frequency range from 30 to 950 GHz, which will be split into 10 observing channels/bands. Each frequency channel will have its own specifically designed front end optics to couple radiation from the secondary reflector focal plane to the accompanying receiver. We present a full electromagnetic analysis of the band 5 front end optics system using physical optics, which covers a range from 163 to 211 GHz. This band is being developed by the Group for Advanced Receiver Development (GARD) at Chalmers University, Gothenburg, Sweden. Two software packages are utilised for this analysis; the industry standard reflector antenna software package GRASP9 developed by TICRA  and a new optical software package MODAL [2,3] (Maynooth Optical Design Analysis Laboratory) developedat NUI Maynooth, Ireland. Electromagnetic predictions of beam patterns are presented at the Cassegrain focal plane and at the subreflector vertex. The basis of the analysis is primarily to determine optical performance and efficiency and the effects of beam truncation by the off-axis reflectors of the front end optics. Three levels of beam truncation are modelled varying rim diameter.