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 traditional Y-factor methods fail to separate antenna noise from receiver noise, a joint measurement technique combining the Solar Y-factor method and the Ambient Load method is proposed. First, the Sun is utilized as a far-field standard noise source to measure the total system noise temperature. Subsequently, an ambient load is introduced as a secondary thermal source to precisely calibrate the equivalent noise temperature of the receiver. Finally, the antenna body noise temperature is derived based on the cascaded noise theoretical model. An experimental system was constructed to verify the method on a C-band active phased array antenna. Under a solar flux condition of 227 SFU, the measured total system noise temperature was 163.2 K. The receiver equivalent noise temperature was separated as 107.9 K, resulting in a calculated antenna body noise temperature of 55.3 K. The measured value (55.3 K) aligns closely with the theoretical design value (57.54 K), with an error of only 2.24 K. Furthermore, background noise tests at different elevation angles were consistent with atmospheric radiation transmission characteristics, verifying the sensitivity and consistency of the measurement system. This study achieves effective decoupling of antenna and receiver noise contributions, providing a reliable quantitative analysis method for loss evaluation and performance diagnosis of high-performance radar antennas.