of the invention in the embodiment as shown in fig2 an antenna 21 a and an antenna 21 b is located in different positions and the distance between the antenna 21 a and an object 15 is smaller than the distance between the antenna 21 b and the object 15 a detector 23 may be located near the antenna 21 a similarly fig2 only illustrates important components related to the invention and other components are not discussed or shown to begin a control circuit 24 of the portable electronic device 20 may switch to the antenna 21 a to wirelessly transmit or receive a signal during the period the detector 23 transmits a message a 2 to the control circuit 24 when detecting that the distance between an object 25 under test and the detector 23 is smaller than or equal to a predetermined distance when receiving the message a 1 the control circuit 24 switches to another antenna not in use e g the control circuit 24 switches from the antenna 21 a to the antenna 21 b and uses the antenna 21 b to transmit or receive a signal the sar of the portable electronic device 20 and the object 25 can decrease even if the rf conducting power remains unchanged in addition when the antenna 21 b is in use the detector 23 may transmit the message a 2 to the.