A payment reader includes a tuning circuit that provides a tuned transmission source signal to an antenna for transmission. A sense circuit coupled to the antenna provides a measured transmitted signal to a binary phase detection circuit. The binary phase detection circuit filters and processes the signal to provide an analog phase signal that corresponds to a phase difference between the measured transmitted signal and the transmission source signal. A comparison circuit compares the analog phase signal to a reference signal, and a decision circuit adjusts the operation of the transmission circuitry based on the comparison.