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Body EM properties

Bioeffects caused by exposure of a biological body to microwave/RF are related to the internal E and B fields associated with the exposure. The distribution of the internal fields is related to a number of parameters, including the dielectric properties of the tissues in the biological body, the geometrical properties of the body, the orientation of the incident field vectors, whether the exposure is in the near or far field, to mention a few. Although modern numerical simulations are used effectively to determine internal field distribution for complex heterogeneous models, e.g., near-field exposure to hand-held wireless transceivers, simple but important properties of the absorption can be illustrated by simple models.

The complex permittivity is given by:

 

where å o = 8.86 × 10-12 f/m. The penetration depth ä, i.e., the distance from the boundary of a medium to the point at which the field strengths or induced current densities have been reduced to 1/e of their initial boundary value in the medium, is given by (2) for a plane-wave incident on a planar surface.

 

As can be seen in the table and (2), the penetration depth at low RF frequencies is considerably more than 10 cm but rapidly decreases to a millimeter or less at millimeter-wave frequencies. The penetration depth and reflection at the external surface determine how much energy reaches deep into the body. Although the penetration depth found from (2) is large at lower frequencies, the amount of energy that penetrates a conducting body is small because of the shunting of the electric field. For example, for a small spherical object, Schwan has shown that at 60 Hz the internal E-field is nearly six orders of magnitude less than the external E-field, even though the theoretical penetration depth is quite large. Osepchuk [8] estimates that only around the "resonance" frequency of man, i.e., around 100 MHz, is the internal E-field deep in the body within one order of magnitude of the external field. In the millimeter- wave frequency range, the E-field deep in the body is many orders of magnitude below the external field because of small penetration depth.

  
نویسنده : امیر حسین بیک ; ساعت ٢:۳٦ ‎ب.ظ روز یکشنبه ٢٥ بهمن ۱۳۸۸
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