Abstract: To accurately evaluate the independent noise contribution of passive components (radiating elements and feed networks) in active phased array antennas, and to address the "black-box" testing challenge where antenna noise and receiver noise cannot be separated by traditional Y-factor methods, a joint measurement technique combining the solar Y-factor method and the ambient load method is proposed in this paper. First, the total system noise temperature is determined by utilizing the sun as a far-field standard noise source. Subsequently, an ambient load is introduced as a secondary thermal source, and the equivalent noise temperature of the receiver is precisely calibrated. Finally, the antenna noise temperature is derived based on the cascaded noise theoretical model. Field tests on a C-band active phased array antenna are conducted by constructing an experimental system. Under a solar radio flux of 2.27×10^-20 W/(m²·Hz), the total system equivalent noise temperature is measured to be 163.20 K, and the antenna equivalent noise temperature is calculated to be 55.30 K. A good agreement between the measured values and the theoretical design values is observed, with an error of only 2.24 K being obtained. Furthermore, the background noise test results at different elevation angles are shown to be consistent with atmospheric radiation transmission characteristics, by which the sensitivity and consistency of the measurement system for weak signal perception are verified. Through the study in this paper, effective decoupling of the noise contributions between the antenna and the receiver is achieved, and a reliable quantitative analysis method is provided for the loss evaluation and performance diagnosis of high-performance active phased array antennas.