| Determination of the real field distribution from high frequency electromagnetic fields near UMTS transmitters |
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Topic Determination of the real field distribution from high frequency electromagnetic fields near UMTS transmitters Start 01.12.2004 End 31.05.2006 Project Management IMST GmbH, Kamp-Lintfort Objective Background for this project is the construction of UMTS mobile phone networks in Germany since 2002. It can be seen as an addendum to the research project 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, which addressed the development of methods required to measure and calculate electromagnetic fields near GSM mobile phone base stations. The UMTS project was aimed at developing measurement techniques and computational methods (and/or modifying existing methods) which can be used to determine how high frequency electromagnetic fields are distributed in time and space in the vicinity of UMTS base stations. These methods shall be employed to determine and assess the real exposure of the public near such installations. The assessment was based on the reference values set out in the 26thOrdinance for the Implementation of the Federal Immission Control Act (26th BImSchV).
Results As far as the distribution of high frequency electromagnetic fields is to be determined by measurements, three measurement techniques based on different principles could be identified, i.e. broadband, frequency-selective (spectral) and code-selective measurements. The code-selective measurements supplied the most exact and reliable results when compliance was tested with respect to the limits set out in the 26th BImSchV. As required by the 26th BImSchV, this procedure can be used to extrapolate reliably from measurements to the maximum load of the base station irrespective of the actual situation during measurement. This makes it possible to reliably determine the maximum possible exposure at a given measurement point. The sweep method is recommended both for UMTS and GMS networks, if a maximum value is supposed to be determined within a particular measuring volume. It must be noted, however, that the velocity of the sweep procedure has to be adapted to the low decoding speed of the currently available measuring devices. In contrast to the expectations, the fast fading (fading effects within distances of only a few centimetres) of the UMTS fields e.g. in buildings was not less pronounced than in the case of GSM fields in spite of the significantly larger signal bandwidth. The sweep procedure thus has to be conducted with similar diligence in order not to miss any of the locally confined maxima. The researchers recommend to use logarithmic-periodic antennae (log-per antennae) since their directional effect ensures that the measuring results are only to a very small extent influenced by the attending measuring staff. Frequency-selective measurements combined with isotropic antennae, which do not have a distinct directional effect, may be appropriate to determine values averaged over time and space or sequences of measurement values. This method, however, implies a high risk of miscalculations caused by inappropriate settings of the measuring devices. The researchers have elaborated instructions on how to avoid such errors in the course of the research project. It is vital to use RMS detectors and to ensure that the measuring bandwidth and the sweep time are set correctly. The relevant requirements which should be included in a measuring standard were elaborated and put up for discussion. At the 163 measuring points in 11 examined scenarios, which were chosen exemplarily for different network structures and different cell types, exposures were determined in a code-selective measurement procedure and were extrapolated to the maximum capacity of the installation according to the 26th BImSchV. The maximum values were 5.1 V/m and 69.3 mW/m2 (which corresponds to 8.4 % of the electric field strength limit and 0.7 % of the power flux density limit, respectively, as set out in the 26th BImSchV and EU Council Recommendation 1999/519/EC). The lowest values were 58 dB below the maxima (approximately factor 693,000 with respect to the power flux density and factor 832 with respect to the field strength). The exposure values are thus extremely variable. Interestingly, though, most of the highest values were measured in small cell scenarios. This may be due to the fact that the antennae are frequently installed on a rather low level in these scenarios, which makes it possible to come very close to the transmitters, which in turn has lead to increased exposures in spite of the lower transmitter power. On the other hand, the researchers found the lowest exposures in the vicinity of antennae installed on a high level, such as the so-called Ultra High Sites (UHS). It must be noted, however, that the different supply concepts were not examined within this research project with respect to their particular supply quality. As it has already been shown for GSM base stations, the researchers found that in the immediate vicinity of a transmitter (within a radius of up to 100 m) the mere distance to the installation is not a reliable indicator for the exposure. The exposure is significantly influenced by the relevant sight conditions and the position of the particular measuring point with respect to the direction of the antenna’s main lobe. In 85 % of the cases (with respect to the frequency-dependent limits) the GSM exposure values determined for comparison at identical places exceeded the UMTS exposure values. This can be explained by several facts: The GSM base stations were frequently operated at higher transmitter powers, the limit in particular for GSM 900 is lower that that for UMTS and the probability of getting into the main lobe of a GSM antenna is comparably higher since the directional effect is less pronounced for lower frequencies. While the highest UMTS value measured amounted to 8.4 % of the field strength limit, the highest value measured for GSM fields covered 12.8 % of the limit. The median values were 0.72 % for UMTS and 1.75 % for GSM. The mean exposure averaged over all measured values was 2.2 % of the limit for UMTS (with respect to the maximum capacity of the relevant installation). Variations of the exposure level over the day were less frequent for UMTS base stations, one of the reasons is the still rather restrained use of UMTS networks for data and communication traffic. A number of commercially available programmes were examined to assess their suitability for determining the exposure of the public in the vicinity of UMTS base stations. The researchers found that in spite of the large bandwidth of UMTS signals compared with other radio services multifrequential simulations of the field source (i.e. calculations of the exposure for different frequencies within the signal bandwidth) did not deliver more exact results than monofrequential simulations. The additional work and expense is by no means worth the slightly increased exactness. A high conformity of calculations and measurements can be gained, as expected, from scenarios with base station antennae and place of exposure being in a line-of-sight. However, if there are even trees in the line-of-sight between the antenna and the place of exposure and these are not considered in the simulation, the results of the simulation may exceed the measured values by up to 10 dB (factor 10 with respect to the power flux density). If the line-of-sight to the antenna is absolutely free and the directional diagram of the base station transmitter is taken into account, even the simple calculation method for free space distribution plus an additional 3 dB for potential ground reflections may deliver sufficiently exact conservative results. In scenarios with no line-of-sight between the base station antennae and the place of exposure, however, the influence of the surrounding buildings must be taken into account (i.e. the buildings have to be modelled in the simulation). This condition increases considerably the time and effort to be invested in reaching sufficiently exact results so that this method is hardly useful for the determination of exposures. The final report, which also contains all interim reports, is available as three PDF-files in German: Final Report Part 1 (704 kB). Final Report Part 2 (7.5 MB). Final Report Part 3 (10.1 MB). References
Conclusions This research project was able to show that typical exposure levels even in the immediate vicinity of UMTS base stations are far below the currently valid limits. For the time being, the exposures caused by UMTS base stations are frequently lower than those caused by the GSM system. However, it will be necessary to continue to observe this situation in the future since the networks were not completely built up at the time of the research project and only had to deal with a rather limited amount of communication and data traffic. Apart from that, the research work performed made an important contribution to defining the requirements for the appropriate determination of UMTS exposures. |