An electronic device, comprising: an antenna with which the electronic device transmits radio-frequency signals; phase and magnitude detector circuitry that is coupled to the antenna and that makes radio-frequency signal phase and magnitude measurements; storage and processing circuitry that determines whether an external object is adjacent to the antenna by processing the radio-frequency signal phase and magnitude measurements; wireless circuitry that uses the antenna to transmit the radio-frequency signals at a given transmit power, wherein the storage and processing circuitry is configured to adjust the transmit power based on the radio-frequency signal phase and magnitude measurements; wherein the storage and processing circuitry is further configured to reduce the transmit power in response to determining that the external object is adjacent to the antenna by processing the radio-frequency signal phase and magnitude measurements, and wherein the antenna is included in a plurality of antennas that are operable to transmit the radio-frequency signals, wherein the phase and magnitude detector circuitry is coupled to each of the plurality of antennas, the storage and processing circuitry determines for each antenna whether the external object is adjacent to that antenna based on the radio-frequency signal phase and magnitude measurements, and the storage and processing circuitry is configured to temporarily disable a given antenna of the plurality of antennas in response to determining that the external object is adjacent to the given antenna; the electronic device further comprising a proximity sensor, wherein the storage and processing circuitry is configured to adjust the transmit power based on the radio-frequency signal phase and magnitude measurements in conjunction with data from the proximity sensor; wherein the proximity sensor is selected from the group consisting of: a camera sensor, a thermal sensor, a resistance sensor, and an acoustic sensor. an antenna with which the electronic device transmits radio-frequency signals; phase and magnitude detector circuitry that is coupled to the antenna and that makes radio-frequency signal phase and magnitude measurements; storage and processing circuitry that determines whether an external object is adjacent to the antenna by processing the radio-frequency signal phase and magnitude measurements; wireless circuitry that uses the antenna to transmit the radio-frequency signals at a given transmit power, wherein the storage and processing circuitry is configured to adjust the transmit power based on the radio-frequency signal phase and magnitude measurements; wherein the storage and processing circuitry is further configured to reduce the transmit power in response to determining that the external object is adjacent to the antenna by processing the radio-frequency signal phase and magnitude measurements, and wherein the antenna is included in a plurality of antennas that are operable to transmit the radio-frequency signals, wherein the phase and magnitude detector circuitry is coupled to each of the plurality of antennas, the storage and processing circuitry determines for each antenna whether the external object is adjacent to that antenna based on the radio-frequency signal phase and magnitude measurements, and the storage and processing circuitry is configured to temporarily disable a given antenna of the plurality of antennas in response to determining that the external object is adjacent to the given antenna; the electronic device further comprising a proximity sensor, wherein the storage and processing circuitry is configured to adjust the transmit power based on the radio-frequency signal phase and magnitude measurements in conjunction with data from the proximity sensor; wherein the proximity sensor is selected from the group consisting of: a camera sensor, a thermal sensor, a resistance sensor, and an acoustic sensor.