Phased-array weather radar have potential to replace reflector dish radar in major weather radar networks such as NEXRAD, providing faster update times and greater scan flexibility. However, the use of electronic scanning introduces polarization errors on weather radar measurables, requiring polarimetric bias calibration. The sources of polarimetric bias have been described theoretically, but experimental verification is still limited. Additionally, no standard method of calibration for polarimetric bias exists for phased-arrays. Therefore, the University of Massachusetts Amherst (UMass) presents a fully operational X-Band phased-array weather radar polarimetric testbed. The testbed evaluates the calibration of a planar dual-polarization X-band phased-array radar through simultaneous operation with a co-located mechanically-scanned polarimetric reference radar. A detailed description of both radar systems is provided, as well as radar installation and data collection procedures. In addition, this research proposes a novel method of phased-array polarimetric calibration which improves on prior work by decomposing bias sources into independent linear terms. This modular approach is practical, easy to implement, and adaptable to any radar system. Finally, this research delivers a detailed demonstration of both the calibration method and the weather radar polarimetric testbed. The calibration method is evaluated by direct comparison of weather observations using the testbed. Notable results include: the polarimetric calibration improves the phased-array’s correlation with the dish radar in all cases. However, the linear correction appears insufficient to calibrate all variables over the entire scan range. Increasing the time offset between observations decreases the correlation between radars in every case. Lastly, this evaluation provides insight into the effects of ground clutter, rain attenuation, and noise on a direct comparison between radars.