An automatic frequency tuning system and method for a transmit power amplifier. The transmit power amplifier has an antenna feed line including a series capacitor and is coupled to an output of an output driver. In one embodiment, the system includes: (1) a shunt capacitor array having a plurality of capacitors selectably couplable to the antenna feed line to apply a programmable shunt capacitance thereto, (2) a peak detector circuit couplable to nodes of the antenna feed line associated with both terminals of the series capacitor and (3) a processor configured to control the peak detector circuit to determine a ratio of voltage levels measured at the nodes at a given power level of the output driver.