In this letter, it is demonstrated that the beam sensitivity of an antenna array receiving system can be analyzed by using an equivalent single-channel receiver model. In this model, the antenna array is represented by an equivalent single-port antenna and the multiport active beamforming network is replaced by an equivalent two-port amplifier. Herein, the beam sensitivity is defined at the input of the receiving system and is a function of the equivalent antenna model parameters. Such a simplified representation helps us to identify the predominant factors that affect the receiver sensitivity of complex antenna array systems, without having to consider the entire system in full detail. The receiver noise is computed with the proposed model and compared to the one computed by a direct numerical method to validate its consistency. For this purpose, we consider a four-element actively beamformed dipole array with strongly coupled antenna elements causing significant noise coupling effects.