Authors
Stefanelli R, Trinchero D. Frequency selective solutions for an efficient non-ionising radiation protection in the radiofrequency and microwave ranges. Radiat Prot Dosimetry. Nov 13, 2009. Ahead of print.

Background
The widespread development and use of mobile phones and digital access networks within urban environments, adjacent to living premises, even in small villages, has increased the number of potentially exposed people, generating a high level of concern for some inhabitants, even if exposure limits are satisfied. This is compared to fewer people who were exposed in the past to non-ionizing radiation in the radiofrequency and microwave bandwidths. Exposure to non-ionizing radiation can be reduced by 5–10 dB within buildings over a specific, selected, even wide radiofrequency bandwidth, maintaining transmission capabilities of the electromagnetic radiofrequency spectrum. This can be done using protective shields.

Objective
This paper shows the development of a protective shield called frequency selective surfaces which are transparent to visible light and it can be adapted to the shielding needs, both in terms of frequency and spatial behaviours.

Methods and Results
The design procedure starts with the identification of a proper dielectric sheet, transparent to visible light, easy to manufacture and inexpensive. Secondly, the most suitable resonating geometry that offers the best independency on the angle of incidence and the larger bandwidth must be selected. As a last step, the resonating elements must be dimensioned, taking into account the presence of the dielectric and the glass itself. This last activity is eased by the approach chosen to carry out the analysis: starts from the equivalent circuit of the full structure, extracted by applying a de-embedding procedure. In this way, the dimensions of the elements, as well as their periodicity, must be calculated. By iterations (with very fast convergence) an optimal solution can be obtained, adapting the geometrical dimensions to the design needs.
Various materials have been evaluated for the construction of the dielectric support. Among all, polypropylene has been found as the one that optimally meets very well the design constrains. It is transparent to visible light, a standard sheet has thickness of 20 mm, and it offers a good tensile strength. Furthermore, it is widespread and used in many industrial processes.

Conclusion
In this paper, a frequency-selective surface for the reduction of human exposure to electromagnetic fields in the microwave bandwidth is reported. Simulations confirmed that the applicability of the proposed method is good and that it offers adequate performance, thus avoiding the cancellation of light transmission from outdoor environments to indoor living premises.


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