The performance of an amplitude sensing monopulse radar is largely affected by the hardware imperfections of the receiver. This paper describes an analysis on the effects of the pre-comparator phase and amplitude unbalance on the radar antenna. In this paper an anlaytical antenna model is used, the radiation patterns of the two squinted beams of the monopulse are represented by the Gaussian function. The sum and difference patterns are formed by the addition and subtraction of the two squinted beams, with the pre-comparator phase and amplitude unbalance included in the model. The discriminator curve obtained from the difference-divided-by-sum will then be an expression of the phase and amplitude unbalance. It is concluded from the analysis that the pre-comparator unbalance is insensitive to the post-comparator magnitude calibration. The effect of the phase unbalance on the boresight error is also summarised and presented in graphical form.