Please note: In the meantime this project was completed and there are new informations in the German version of this document.

Topic

Development of measurement and calculation methods for the determination of the public exposure due to electromagnetic fields in the vicinity of mobile phone base stations

Start

01.09.2002

End

31.08.2004

Project Management

IMST GmbH, Kamp-Lintfort

Objective

The nation-wide expansion of GSM mobile telephone networks in Germany as well as the development of modern broadband UMTS mobile phone networks is resulting in an increase in electromagnetic field exposure of the public. The discussion of this fact is not only focused on mobile devices (mobile phones) but, above all, on fixed base stations.

The objective of the project was to develop measurement and calculation methods suitable for assessing exposure of the public in the vicinity of mobile telecommunication base stations. The methods must be suitable for compliance testing with regard to limit values. Vicinity is defined as the area up to distance of 200 m from the base station.

Results

Supported by a detailed survey, appropriate methods for meeting the demands were identified. By applying the identified methods, information on the exposure situation in the vicinity of mobile phone base stations and the the various parameters which could influence this situation were obtained. UMTS stations were largely excluded, as they are subject to a separate project within the DMF. Values were obtained in the range of below 1 µW/m2 up to several 100 mW/m2 (the limit values according to 26. BImSchV (Federal regulation) are at 4.5 W/m2 and 9.0 W/m2 respectively). Base station-related as well as location related factors, relevant for the exposure level were identified. It turned out that their interaction is very complex and an isolated observation of individual factors (e.g. distance between base station and location of exposure) generally leads to invalid simplifications. As a result no significant difference was found between the maximum exposure when measuring under indoor and outdoor conditions. When searching for extreme values in order to ensure that limit values are not exceeded, it was observed that the search should concentrate on locations accessible for everyone, which are located at a height comparable to that of the main radiation beam.

If it is necessary to achieve the most accurate determination of the exposure frequency selective devices proved to be more suitable than broadband sensors. However, the use of frequency selective analysers requires qualified personnel for the measurement procedures, as inaccurately selection of resolution and video bandwidths, detector types or sweep times can lead to significant mistakes in assessing exposure. Logarithmic-periodical aerials have proven to be superior to biconic aerials, as they are significantly less influenced by interaction with the user’s body. In regard to carrying out the measurements, the so-called sweeping method was identified as the best compromise between accuracy and effort. However, on the one hand this method requires a particularly careful execution by the person measuring, to capture all polarisations and wave angles in the measured volume. On the other hand, using the sweeping method it is not possible to average the exposures in a measured volume, e.g. to capture the typical volume a person would be exposed to. If this is required, the raster method is an appropriate alternative. However, with this method it was found that the appropriate averaging geometry strongly depend on the individual exposure scenario and it is therefore not possible or useful to define a universally suitable geometry. A precise measuring protocol, containing traceable documentation and measurement analysis, has been developed and suggested by the researcher.

Suitable methods were investigated and selected from those generally available for analytical and numerical calculations. In addition to the precision of the methods, the aspect of practicability was considered. It has been proven to be practical when using computer simulations, to consider the frequency dependency of radiation diagrams and the electrically-adjustable down tilt angle of base station antennas through a synthetic antenna diagram, with an envelope including all possible operational conditions of the antenna. The effort to create a precise model of the 3D environment of a base station was identified as the greatest obstacle in accurately determining exposures.

Whereas the construction plan of a building can be obtained from public registers or satellite pictures, height measurements are usually not available in this way. In the exceptional circumstances where all data is available the prediction of exposures is as accurate as typical measurements are if one includs their uncertainties in the comparison. When considering calculation methods, those based on ray optics prevailed as opposed to field-theoretical and hybrid methods. As a rule, it is possible to achieve rather precise predictions with this method. Underestimation of exposure can however not be completely avoided in certain cases. As a worst case estimate it is suggested to mark up free space propagation by 3 dB to take into account ground reflection. Also the precise modelling of indoor scenarios turned out to be very elaborate. Although the positioning and the type of furniture as well as the movement of people may change the resulting spatial and temporal interference pattern on a small scale, the size of the maxima remains largely unaffected. As a result, a detailed reproduction of interior rooms has proven to be expendable.

The researchers assessed the implementation of an area-wide cadastral with automatic measurement facilities to continuously monitor mobile telecommunication exposures as barely useful. One reason was the lack of sensitivity of broadband measuring instruments, which one would have to use because of financial reasons. In this context, measuring values, recorded on fixed points without searching for maximum values in the immediate local vicinity, were assessed as beig not meaningful, as it was demonstrated that expsoures can be subject to enormous fluctuations of time and space at distances only a few centimetres away from the measuring point

As an alternative, a sequential cadastral can be implemented, in which one or several measuring teams using frequency-selective measuring instruments on a more or less regular grid undertaking short-term measurements. However, time and effort for such an activity is substantial.

The final report can be downloaded as PDF-Datei (4.240 KB) in German.

References

• C. Bornkessel, M. Schubert, M. Wuschek and P. Schmidt: Determination of the general public exposure around GSM and UMTS base stations, Radiation Protection, Vol. 124, No. 1, 2007, pp. 40-47, doi:10.1093/rpd/ncm373

Conclusion

The research project made an important contribution towards presenting the complexity of determining exposures and demonstrated as a result the demands to get reliable data on a professional survey. It further proved, based on real measured values, the inadequacy of simple exposure estimates which are based on the distance to the transmitting facility only. Improved methods for determining exposures were developed. They are applicable in large-scale measuring activities as well as for selective measurements at so-called “sensitive” locations.

The research project demonstrated that typical levels of exposure, even in the immediate vicinity of base stations are significantly below current limit values. No evidence was found that the method used by the Federal Network Agency to notify safe distances is not conservative enough to guarantee the protection of the general public as required by the 26. BImSchV (Federal protection ordinance). Quality standards of future measuring activities can be derived from the project findings. Moreover, important indications on the assessment of exposure in epidemiological studies can also be derived. The findings will be used within the German Mobile Telecommunication Research Programme e.g. to allow the correlation of the scale of exposure from other emitters.