Abstract: The G/T value of an active phased array antenna is a key figure of merit determining the receiving sensitivity and operational range of radar and communication systems. In engineering practice, there is an urgent need for an accurate noise figure budgeting method for the array front-end under non-ideal operating conditions such as channel failures and amplitude weighting. In this paper, an equivalent noise figure model for the receiving chain of an active phased array is established by combining the scattering parameter characteristics of Wilkinson power combining networks with the mechanism of signal correlation. The degradation laws of the array noise figure caused by inherent losses of unequal power dividers/combiners, active channel failures, and amplitude weighting are systematically analyzed. This model provides theoretical support for the optimized design of the receiving chain and accurate G/T budgeting of phased array antennas. Based on this model, a Ka-band phased array radio frequency front-end is developed, which consists of 1,840 waveguide slot radiating elements and integrates 96 independent active transceiving channels. System-level G/T validation is carried out through darkroom (anechoic chamber) measurements. The measured G/T value deviates from the theoretical budget by less than 0.5 dB, verifying the effectiveness and engineering accuracy of the proposed model. This work provides a reliable basis for G/T budgeting and performance optimization of phased array antennas.