In a multiple-input-multiple-output (MIMO) communication system, the multipath fading tends to vanish with increasing number of radio links. This well-known channel hardening phenomenon plays a central role in the design of massive MIMO systems. It is quantified by the coefficient of variation of the channel gain. The aim of this paper is to study channel hardening using a physical channel model in which the influences of propagation rays and antenna array topologies are highlighted. Our analyses and closed form results extend the hardening properties beyond the classical Rayleigh fading models and offer further insights on the relationship with channel characteristics.