RF sensor technology in mm-wave range has improved significantly in recent years, which led to its widespread use in mission-critical systems, such as automotive radar. With the advent of multi-Antenna systems, cascaded radar designs has gained prominence to increase resolution in terms of distance, speed, and angle. However, the performance of the radar systems can be highly dependent on dynamic conditions and they may require in-field calibration, specifically in terms of magnitude and phase mismatches of gain and input reflection coefficient. While in-field measurement of gain using built-in power sensors is more or less straightforward, measurement of the reflection coefficient requires the implementation of an N-port reflectometer on the chip. In this paper, we present an analytical model for noise and gain imperfections in N-port network analyzers. Based on this model, we propose a methodology for optimizing the design of the N-port reflectometer to obtain the highest accuracy under given realistic constraints, such as coupler gain/loss, splitter loss, power detector non-idealities, and noise.