Electronic devices may be provided that contain wireless communications circuitry. The wireless communications circuitry may include radio-frequency transceiver circuitry coupled to multiple antennas. Signal strength measurements may be gathered using the antennas and corresponding signal strength difference measurements may be produced to reflect which of the antennas is exhibiting superior performing. Information may be gathered relating to the fading environment of the communications circuitry, such as whether the wireless communications circuitry is transitioning between a fast fading environment and a slow fading environment. For example, the wireless communications circuitry may further include a satellite positioning system receiver or an accelerometer, which may be used in gathering the information. The difference measurements may be filtered and compared to antenna switching criteria such as antenna switching thresholds. An antenna switching threshold may be adjusted in real time based at least in part on the gathered information.