The operating environment of mobile phones fluctuates continuously, due to changing handling conditions and nearby objects. The resulting fluctuations in antenna impedance cause both a decrease in link quality and a higher standing wave ratio, that requires more robust and hence less efficient power amplifier implementations. In this paper, an automatic antenna tuner system for handheld applications is presented that uses two series reactances combined with three simple RF peak detectors to sense both reactive and real impedance mismatches. The control loop only requires low-frequency electronics which makes it low cost, low power and relatively easy to integrate. Measurements on a demonstrator system show correct behaviour for voltage standing-wave ratio up to 10.