Variations of the load impedance at the output of a transmitter (TX) can cause a significant performance degradation of the power amplifier (PA) characteristics, such as output power, linearity, and efficiency. This performance can be restored by making a PA actively or passively reconfigurable, which requires a sensor to measure its load impedance. To this end, we propose a complex impedance sensor to measure impedance variations at the output of a PA operating around 28 GHz. As the sensor employs lumped sensing elements, this complex voltage sensing method is compatible with most currently existing PA designs. The sensor is combined with the output balun of a differential PA with only a small impact on the PA performance. Compared to earlier designs, this sensor comes with an improved dynamic range and bandwidth. The prototype is fabricated in a 22-nm FD-SOI CMOS technology; it consumes 8 mW of power and occupies 0.0043 mm2. By sweeping the input from 24 to 31 GHz with an input power from −13 to −6 dBm, this sensor can sense reflection coefficients up to a magnitude of 0.74 with a maximum error of 0.17 and root-mean-square (rms) error lower than 0.042.