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ON THE EFFECTIVENESS OF VARIOUS TYPES OF MOBILE PHONE RADIATION SHIELDS |
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SARTest Report 0113 June 2001 ON THE EFFECTIVENESS OF VARIOUS TYPES OF MOBILE PHONE RADIATION SHIELDS MI Manning and M Densley INTRODUCTION Many countries have now defined limits on the maximum body dose from mobile phone handsets. These limits have been set by expert committees, which have examined the health issues. For the UK, a comprehensive review of the current state of understanding is presented in the Stewart Report (Ref 1). Most mobile phone handsets in the UK are on GSM networks and are all expected to be operating within any of the applicable limits for Europe as well as being within the more stringent limits applied, for example, in the US. Nevertheless, it is a user's choice whether to take a precautionary approach to limiting their own personal exposure to RF radiation and various approaches can be considered. The DTI commissioned Sartest to review the devices on the market and test the effectiveness of representative products. Misconceptions about how radiation emanates from mobile phones can lead to false expectations by users of the radiation reductions with some shield devices. Users also need to be made aware that shield devices may interfere with the normal operation of the phones causing reductions in call quality, area coverage and battery life. EXECUTIVE SUMMARY With so many models of mobile phones available and so many types of shielding accessory products on the market, it is impractical to be exhaustive in accessory testing. In this report, we have surveyed the shield products available and classified them using four types: shielded case, earpiece pads and shields, antenna clips and caps, and absorbing buttons. Typical claims made for each category are summarised and certain misconceptions related to how they work are discussed. Devices from each category have been subjected to comparative tests with and without the device in use to measure comparative SAR reduction figures. Many of the devices tested reduce SAR by reducing the radiated power output of the transmitting phone, and so results relating to the reduction in transmitted power have also been obtained. Measurements of the power received by the antenna of the test set controlling the phone are used to provide estimates of the link power reduction due to the devices. However since this measurement is only made at one location at fixed distance and direction from the phones being tested, the transmitted power comparisons are only indicative. The results of SAR tests performed in this study show that many of the shield devices can reduce the maximum SAR from the handset by large amounts. Generally, however, this reduction is due to the device limiting the useful, radiated transmit power from the phone by a similar amount, which has the associated disadvantage of reducing the performance of the phone in weak signal areas or inside buildings. Ideally, shield devices should reduce the SAR without impairing the antenna efficiency of the phone. So, the ratio of the SAR reduction to the transmitted signal reduction is a possible merit indicator for the accessories tested. In this Report, the results obtained for different categories of devices are mapped onto a graph of SAR reduction versus power reduction. The test results show a wide variability in behaviour between devices. To help explain the variations, supporting investigative tests have been performed to aid with explanation of the test results. Investigations have included tests on different shapes of shield (screening) materials placed on the front of the phone. Reports of these tests show that the insertion of screening material between the phone and the head is only effective when the lateral dimensions of the screened area are comparable to the dimensions of the phone. Small shapes of the dimensions typically used for earpiece pads and shields have no significant effect in reducing SAR. Screens of similar dimensions to that of the phone do have significant effects, but it was found that the keypad has to be covered too. For devices mounted on the front of the phone and for shielded cases, the device can increase the separation between the phone and the head. The magnitude of this effect has been investigated. Increasing the separation between the phone and the head also has a secondary benefit as the reduced absorption by the head increases the useful link power. For devices affixed on the antenna of the phone or placed near the antenna, the efficiency of the antenna can be sharply reduced. Such antenna impairment is disadvantageous, because it makes the phone power-limited and reduces the radiated signal in all directions - not just in the direction of the head. A combination of increased distance of the phone from the head and the application of large-dimension screening components between the head and the phone can limit SAR without causing similar reductions in the useful transmitted power from the phone. However, the overall bulk of the phone plus shield may be considerably increased. Personal hands-free kits, which were tested in an earlier study for the DTI (Ref. 5), remain one of the best approaches for SAR reduction. By separating the antenna of the phone from the user's head, SAR is greatly reduced. To avoid exposure to other parts of the body, however, the phone should not be placed close to other parts of the body. Another option available for a user to reduce SAR levels is to select a phone with low SAR. A variability of SAR levels for GSM phones of a factor of 5 over a range of different models has been reported (Ref. 6). This means that a reduction of up to 80% could be achieved by swapping the model of your phone. [...] DISCUSSION OF RESULTS AND RECOMMENDATIONS With so many models of mobile phones available and so many types of shielding accessory products on the market, it is impractical to be exhaustive in accessory testing. In this report, we have surveyed the products available and classified them using four categories: shielded case, earpiece pads and shields, antenna clips and caps, and absorbing buttons. Typical claims made for each category have been summarised and certain misconceptions related to how they work have been examined. Devices from each category have been subjected to comparative tests with and without the device in use to measure comparative SAR reduction figures. Many of the devices tested reduce SAR by reducing the radiated power output of the transmitting phone, and so results relating to the reduction in transmitted power have also been obtained. Measurements of the power received by the antenna of the test set are reported by the test set and these were used to provide estimates of the link power reduction due to the devices. However this measurement is only made at one location at fixed distance and direction from the phones being tested and the transmitted power comparisons are only indicative. SAR test results performed in this study show that many of the shield devices can reduce the maximum SAR from the handset by large amounts. Generally, however, this reduction is due to the device limiting the radiated transmit power from the phone by a similar amount, which has the associated disadvantage of reducing the performance of the phone in weak signal areas or inside buildings. Ideally, shield devices should be reduce the SAR without impairing the antenna efficiency of the phone and the ratio of the SAR reduction to the transmitted signal reduction is a merit indicator for the accessories tested. In Figure 10, the results obtained for different categories of devices are mapped onto a graph of SAR reduction versus power reduction. Increasing the separation between the phone and the head has a secondary benefit. Since the power absorbed by the head is reduced, the useful link power is actually raised. For devices that are affixed on the antenna of the phone or placed near the antenna, the efficiency of the antenna can be sharply reduced. This effect can easily be replicated by coiling a short length of wire around the antenna or, in fact, by placing a finger on the antenna. It would seem that such antenna impairment is disadvantageous, because it makes the phone powerlimited and reduces the radiated signal in all directions - not just in the direction of the head. A combination of increased distance of the phone from the head and the application of large-dimension screening components between the head and the phone can limit SAR without causing similar reductions in the useful transmitted power from the phone. However, the overall bulk of the phone plus shield may be considerably increased. Personal hands-free kits, which were tested in an earlier study for the DTI (Ref. 5), remain one of the best approaches for SAR reduction. By separating the antenna of the phone from the user's head, SAR is greatly reduced. To avoid exposure to other parts of the body, however, the phone should not be placed close to other parts of the body. In this situation, with the phone clipped to a belt or in a pocket, shielding material or additional spacing from the body could be beneficial for SAR reduction. Another option available for a user to reduce SAR levels is to select a phone with low SAR. A variation of SAR levels for GSM phones of a factor of 5 over a range of different models has been reported (Ref. 6). This means that a reduction of up to 80% could be achieved by swapping the model of your phone. REFERENCES 1) Report of the Independent Expert Group on Mobile Phones (Chairman Sir William Stewart), May 2000 (available at http://www.iegmp.org.uk). 2) Manning, MI. 'DTI sponsored measurements of SAR in mobile phone hands-free kits'.Proceedings of conference Approval 2000, Blenheim Palace, Oxford UK, 16th November 2000. 3) CENELEC European Specification ES59005 October 1998 - "Considerations for the evaluation of human exposure to Electromagnetic Fields (EMFs) from Mobile Telecommunication Equipment (MTE) in the frequency range 30 MHz - 6 GHz." 4) 'A study into the effects of mobile radiation shields on the performance of cellular networks', Report by Multiple Access Communications Ltd, Report DTI0399/TDOC/99/001, March 1999. 5) Manning, MI and Gabriel, CHB, 'SAR tests on mobile phones used with and without personal hands-free kits', SARtest Report 0083, July 2000. 6) Article in Sunday Times Newspaper 3rd December 2000.
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| Quelle: http://www.dti.gov.uk/cii/docs/R500016att.pdf | |
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