Electromagnetic field human exposure of using mobile phone inside metallic elevator

Abstract

Radio frequency (RF) exposure is one of the human safety concerns of using a mobile phone. Due to the reflection and the resonance effects, there is a special concern for mobile phones operating inside a metallic enclosed environment. The biological effect caused by the RF radiation has been proven to be thermal in nature; the specific absorption rate (SAR) is a significant measure of temperature elevation in the human body. Elevators, automobiles and aircraft cabins are typical, electrically large, metallic enclosures. An elevator is a metallic enclosure simply made of metal, without any high dielectric loading. Unlike the seats inside automobiles and aircraft cabins, which act as the dielectric loading, elevators greatly favor the reflection and the resonance effects. These effects could increase the electric field strength induced inside the metallic enclosure and also the level of human exposure. In this thesis, the RF exposure to a human body from using a mobile phone inside a metallic elevator will be studied. The purpose of this thesis is to examine both the electric field distribution and the SAR from the radiation of a 900 MHz mobile phone used inside fully- and partiallyenclosed metallic elevators. The field distribution has been initially investigated by a theoretical analysis using Green’s function. Since the field distribution is complex, a statistical approach, which applies the Monte Carlo method, has been used to characterize the field excited inside the metallic enclosure. Results have indicated that the enclosure can support multiple resonance modes simultaneously due to its electrically large characteristics. The field magnitudes are also Rayleigh distributed, and are supported by experimental measurement. These measurements are greatly increased as compared to the free space, which leads to a concern of intensified human exposure. Based on this safety concern of enhanced RF exposure, the SAR of a mobile phone user inside the metallic elevators has been examined. The localized peak SAR and the whole body average SAR induced in the human head and the body, respectively, were evaluated using the finite-difference time-domain (FDTD) method. A non-uniform meshing technique was used in the FDTD models so as to overcome the electrically large properties. The models have been validated by experimental SAR measurements and the results showed a strong correlation. Numerical simulations have been performed in one fully- and three partially- enclosed metallic elevator models. Results have shown that the resonance effect prevails over the reflection of the electromagnetic field inside the fully enclosed metallic elevator, which makes the level of human exposure even larger. Both the SAR values are greatly increased in the full enclosure, as compared to the free space. Moreover, the average SAR for whole body exposure could also exceed the safety limit given by the IEEE C95.1 Standard and the ICNIRP guidelines by 3.8%. This further indicates the potential hazards of human exposure inside a metallic elevator due to the “hot spots” caused by the resonance effect. In addition, the effects of the mobile phone’s position, the elevator’s dimensions and the aperture sizes on the SAR have also been reviewed in this thesis. These parameters, which are commonly varied for people using mobile phones inside a metallic elevator, are specifically investigated. Results have illustrated that the SAR values, which could be further increased due to these parameters changes, have a close relationship with the field distribution and the resonance modes’ excitation. Based on these findings, the specific mobile phone position and elevator dimensions for a higher RF exposure to human body could be generally estimated.

Date of Award	16 Feb 2009
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sai Wing Peter LEUNG (Supervisor)