Accurate RF dosimetry measurements in simulated humans are difficult to perform, yet they are essential in establishing the compliance of the phone with the relevant compliance standards. The new harmonised SAR test method adopted by CENELEC and IEEE will ensure that the measurement regimes used to test compliance with either the ICNIRP or IEEE exposure standards are largely the same. The following provides only an outline of the complicated process used in measuring the SAR values.
Essentially the SAR measurement system comprises a phantom human head, specially developed liquids, a robot fitted with a measurement probe, and a mobile phone powered up to its maximum certified power level.
In order to achieve a measure of SAR that will include all types of people, a phantom has been produced that is based upon the dimensions of a large adult male head (it has been shown that a larger head is exposed to a higher SAR) as measured on US Army personnel. This phantom has been constructed with compressed thin ears to simulate users with small ears. In addition – right and left model heads are used to ensure that the different exposure areas caused by the asymmetric location of the antenna in many phones are being measured.
The phantom is filled with a liquid that correlates with the dielectric properties of human head tissue.The dielectric properties of ‘head tissue’ have been calculated taking into account the properties of human brain tissue and the matching effects of the outer tissue layers of the head (e.g., skin and skull) to provide a conservative overestimate of the values. Different ‘recipes’ are used for tests on different frequencies.
A robot which consists of a mechanical arm and a special probe is then
used to derive the actual SAR measurements. The measurements are
carried out by establishing a reference point in the phantom and then
scanning a specified area in and around the phantom while the phone is
operating at its maximum certified power level.
This photo is showing an older version of the phantom.
The scan inside the phantom and within the liquid
measures the electric field strength within the head.
This data is then used to calculate the SAR – resulting
in a three dimension map indicating the maximum
SAR value recorded during the test.
The standard specifies two positions for testing, reflecting that consumers hold their phone in different positions while talking. Therefore once the test has been completed in the first position, the phone is adjusted and the tests carried out again.
The process outlined above is then replicated again with the antenna in and out (if applicable); and for the high, medium and low point of each frequency band that the phone is designed to operate on. Therefore for a dual or tri-band phone, the complete testing protocol including accessories can take up to two weeks for each phone.
The end result of this process is the determination of the maximum SAR value for that particular phone and this value is then compared to the exposure limits permitted under the various compliance standards in operation. Once completed, the test results will be used as the basis for the published SAR values that are provided to consumers and to regulators alike.
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The Mobile Manufacturers Forum (MMF)