Electromagnetic field strength in proximity of WLAN devices during data and video file transmission

Abstract

Wireless local area network (WLAN) devices emitting in the 2.4 GHz band may raise questions concerning human exposure mainly when a person stands near an antenna. Proximity far-field level variation with distance, in the range 40 cm to 1 m is investigated experimentally in order to extract peculiarities of the radiated field in comparison with theoretical predictions. Electric field strength depends on the method of measurement (not yet standardised for quasi-stochastic signals) and on some key characteristics of transmission. The field level proved to differ from theory: it is lower than predicted and it does not always follow the inverse law with distance. The time-amplitude and the duty-cycle methods of weighting to obtain a realistic field level allowed highlighting the dependence of the field strength on the traffic direction, on the type of the transmitted file and on the actual data rate and its statistical distribution. In the proximity of access points, downloading conducts to the highest fields, whereas near laptops, uploading does this. A data file transfer implies a higher level of exposure than a video file transfer, but one should consider the data rate dispersion. The proximity field does not significantly depend on the distances between the WLAN devices.