Development of a new in situ SAR measurement system, and its use for making SAR measurements inside vehicles

Head SAR is assessed routinely in laboratories using a computer-controlled robot system to map out the internal electric field distribution inside a human head phantom filled with tissue-equivalent liquid. These systems are big, delicate and impractical for the assessment of SAR in situ. The simplest approach to in situ SAR measurement is to use a number of probes positioned at fixed locations within a body phantom, but this would not give any information about localised SAR distribution, and would have made comparison with exposure standards difficult. Instead a system with a scanning probe capable of measuring SAR at any position in the torso and head of a phantom was developed. The first task was the development of the physical whole-body phantom.

Having constructed the phantom, a robot was installed in the upper torso. There is a recessed section in the pelvic section which takes the motors and gearing and which is "dry" (see pictures below). The rest of the phantom is filled with tissue-equivalent liquid. The robot is under computer control and can position the probe almost anywhere on the inner surface of the torso or head. The software that controls the robot and processes the data is based on standard laboratory SAR system control software, and uses the normal measurement procedure of a surface or 2D scan to locate the SAR maximum, followed by a 3D volume scan to return the 10 g average value. The SAR calculation, interpolation and surface extrapolation are the same as in a commercial laboratory SAR system.

The in situ SAR system was validated against both computational predictions of SAR (SAR was calculated in a computational model of the same phantom using a model of the same radio as was used in the measurements) and against laboratory SAR measurement systems by direct comparison of measured data for the same radios using a standard commercial SAR measurement system. The agreement was to within 10%; the uncertainty for the in situ system is comparable with the standard commercial SAR measurement system: less than 30%.

Measurements using the in situ SAR system and calculations were undertaken for real TETRA radios used on and in vehicles. The vehicles were modelled by EMSS ptty for MCL using CAD data supplied by manufacturers, and SARs calculated with 1, 2, 3 or 4 "passengers": these were the MCL whole-body phantoms. MCL made corresponding measurements in the coresponding real vehicles using 1, 2, 3 or 4 whole body phantoms of which one had the in situ SAR system. The measured and calculated SAR predictions could be compared.

Measured and predicted SAR data agree well, and agree with existing SAR data for the radios used.

The in situ SAR measurement system has been manufactured in both seted and standing phantom form; there was also a requirement to make measurements of SAR on a motorcyle, and for this a special variant was constructed with the robot mounted above the shoulders and pointing down into the body. This allowed SAR to be measured in the lower back of the phantom.

Acknowledgements:

This work was funded by the UK Home Office, and logistical support was kindly provided by RFI-Global Ltd of Basingstoke, Hampshire, and by the Police Information Technology Organisation.