Toivonen T, Toivo T, Puranen L, Jokela K. Specific absorption rate and electric field measurements in the near field of six mobile phone base stations antennas. Bioelectromagnetics 4 Feb 2009 Ahead of print.
Base station mobile phone antennas, which are often placed on building rooftops, represent a source of occupational exposure to electromagnetic field (EMF) to professionals who have to work in close proximity to the antennas. Examples of such workers include firemen, construction workers, janitors and many other professionals. The specific exposure systematic measurement data for antenna models for the unperturbed fields and the specific absorption rates (SARs) in close proximity (<1m) are still unavailable. The ICNIRP occupational limits consist of the basic restrictions of local SAR as an average whole-body exposure. However, in the very inhomogeneous near field of an antenna, the local exposure is more relevant than who-body average exposure.
This paper studies the radiofrequency (RF) electromagnetic fields in close proximity (distances of 10, 100, and 600 millimeters (mm)) to 6 base station antennas. The study measured the SAR in 800mm X 500mm X 200 mm box phantom as well as unperturbed electric field (E) in air. The results were used to determine whether the measurements of local maximum of unperturbed electric field can be used as compliance check for local exposure.
The methodology consisted of installing 6 different types of antennas on the floor of an anechoic chamber with the main lobe pointing upwards and a continuous wave signal was fed to the antennas from a signal generator. The antenna input power (electric field E max ) was measured with RF power meter. The electric fields were scanned at four different distances (10, 100, 300, and 600 mm) from the antenna’s front surface. The SAR measurements were performed using a rectangular liquid filled box phantom. The measured E max and SAR values were used to calculate the local exposure estimation factor (LEEF).
The results showed that using the simplest exposure assessment method is to measure E max for the unperturbed field and compare it to the reference value (Eref) used by the ICNIRP. It is an applicable method but not a conservative method for estimating local exposure for the 6 antenna types studied. The local exposure becomes less significant compared to the whole-body average when the distance from the antenna increases. The results also showed that the box phantom measurements for SAR underestimate the energy absorption in humans.
Discussion and Conclusion
The results presented in this article show that in the studied group of antennas a compliance check of local exposure can be made by measuring the local maximum of electric field. It was also shown that the ICNIRP basic restriction for local exposure could be exceeded before the basic restriction for whole-body exposure if the distance to the antenna is less than 240 mm. The overall results of this study are in accordance with other published values.