Improved DASY Measurement Protocol to Assess Capacitively Coupled SAR Distribution

Background

Since the publication of the IEC 62209-2 standard in 2010, thinner devices with electrically short antennas and/or feeding networks close to the surface have been developed. These devices are often tested at a separation distance closer than 10-15 mm to the outside surface of the phantom shell. In these cases, the radio-frequency (RF) coupling mechanism may be predominately capacitive rather than inductive, resulting in the generation of evanescence like induced fields or specific absorption rate (SAR) distributions with very localized absorption. In other words, for these configurations, SAR decay is steeper than the presumed exponential decay. Hence, the minimal requirements as specified in the IEC 62209-2 standard are no longer appropriate to accurately determine the peak spatial SAR for devices that are positioned close to or touching the phantom.

SPEAG currently recommends the use of the EX probe in combination with the 6 GHz protocol for devices tested in touch with the phantom shell. During the last twelve months, the IT’IS Foundation has begun to develop validation sources for vector array systems, and the analysis shows that, in extreme cases, even the proposed 6 GHz protocol is insufficiently accurate.

Therefore, the IT’IS Foundation proposes a new recommendation that provides accurate assessment even for very extreme cases.

IT’IS Recommendation for Devices Positioned Closer than 10 mm from the Tissue Simulating Material

When the separation distance between the device under test and the tissue-equivalent liquid (d) is less than 10 mm, the following procedure for the EXD3Vx probe is recommended: horizontal grid step of (d/2 + 1 mm) or less but not larger than 4 mm; minimum zoom scan size of at least 22 mm × 22 mm × 22 mm; probe tip distance from the surface to the geometric center of the dipole sensors shall be set as 1.4 mm; the grid step in the vertical direction has variable spacing and increases by an incremental factor of 1.4 (graded ratio). The preliminary finding of the IT’IS Foundation is that, when this measurement protocol is applied, the current uncertainty budget is valid. Since this new procedure, which is necessary only in rare cases, requires significantly more measurement time, SPEAG plans to implement an adaptive measurement protocol that optimizes the grid size based on the absorption pattern. This protocol will be implemented in cDASY6 V6.2 release.

DASY predefined files are available on request. Please contact us at support@speag.swiss.