An animate body proximate to a radiating antenna may cause the antenna to suffer from de-tuning, increased return loss, and/or other performance degradation.
In addition, the animate body may be exposed to a radio frequency (RF) electromagnetic (EM) field of the antenna, which may impart potentially harmful RF EM energy or radiation to the animate body.
Specific absorption rate (SAR) is a measure of a rate at which energy is absorbed by an animate body when exposed to an RF EM field. SAR may be determined in terms of power absorbed per mass of tissue, such as watts per kilogram (W/kg). SAR may be measured and/or averaged over an entire body or a portion thereof.
The United States Federal Communications Commission (FCC) requires that all wireless communications devices sold in the United States, including portable devices, meet minimum guidelines for human exposure to RF energy. The FCC defines a portable device as “a transmitting device designed to be used so that the radiating structure(s) of the device is/are within 20 centimeters of the body of the user.” (47 C.F.R. Ch. 1, §2.1093). For portable devices transmitting within a frequency range of 100 kHz to 6 GHz, the FCC provides the following SAR limits for general populations:                0.08 W/kg as averaged over the whole-body and spatial peak SAR not exceeding 1.6 W/kg as averaged over any 1 gram of tissue (defined as a tissue volume in the shape of a cube). Exceptions are the hands, wrists, feet and ankles where the spatial peak SAR shall not exceed 4 W/kg, as averaged over any 10 grams of tissue (defined as a tissue volume in the shape of a cube).(47 C.F.R. Ch. 1, §2.1093(2))        
SAR varies based on a distance between an antenna and an animate body. In a portable device, such as a tablet or ultra-book, a user may frequently be within centimeters (cm) or even millimeters (mm) a device antenna, which may hamper or preclude government approval of such devices.
A portable communication device may include an omni-directional or isotropic antenna, such as a planar inverted F antenna (PIFA). The portable communication device may further include a dynamic power reduction (DPR) system to reduce transmit power to a pre-defined lower power level if an animate body is detected proximate to the antenna. In other words, a conventional DPR reduces transmit power in all-directions regardless of the location of a detected body. This may unnecessarily hinder wireless communication. Conventional DPRs also include discrete proximity sensors that occupy relatively considerable space and adversely impact antenna performance, such as when a proximity sensor is within a radiation beam or pattern of the antenna.
In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears.