Correlating Threshold Power With Free-Space Bandwidth for Low-Directivity Antennas

This paper develops a threshold power rationale that can be used to demonstrate inherent compliance for portable wireless devices with specific absorption rate (SAR) limits over the 300 - 6000 - MHz frequency range. This is achieved first by understanding the relationship between basic antenna parameters (bandwidth, operating frequency, and distance to the body) and SAR. From this, an upper limit is determined for the power transmitted by a portable wireless device such that the SAR will not exceed the compliance limit. Based on the presented computational and measured data, an empirical formula is developed for the threshold power as a function of the aforementioned parameters. It is demonstrated that the derived threshold power is conservative for all of the low-directivity antennas studied. Computed results are also compared against practical device data (mobile telephone data collected from the manufacturers) to show that the predicted threshold power data using the proposed formula are conservative. The limitations of the proposed formula are also discussed.