Introduction
In the 2GHz band used for IMT-2000, (while comparing to the current 900 MHz band), electical conductivity of brain tissues in a human body exposed to radiation increases. Moreover as the antenna is miniaturized, the rise of SAR in the human body head is a worrying factor. Therefore, various characteristics of antenna are examined by the FDTD analysis and experiments concerning the loop antenna for 2GHz band portable telephone terminal unit are investigated by arranging the reflection shield in order to decrease the SAR. Thus, the utility of this antenna in the human body neighborhood is shown.
Antenna structure and brain
equivalent model
Figure 1 shows the antenna structure and a brain equivalent model. This time, for convenience of experimental system, antenna has been designed so that the center frequency is 2.45GHz.
Surroundings length of the loop element is about 1λ. By arranging the reflection shield, it is possible to match to 50Ω the system feeding power railway track. For the experiments, a cube of 100×100×100 mm3 is adopted as a brain-equivalent model. The electric constants and density are: εr = 42.5, σ =1.51 S/m, ρ=1030 kg/m3 [1] . The medium is assumed to be one and uniform.
Fig. 1 Antenna composition and brain-equivalent model
Results
Figure 2 shows the calculation value and the measurement value of the radiation directivity in the horizontal plane. Thanks to the unidirectional radiation pattern of this antenna (due to the setting of parasitic elements in array), the electric power absorbed by the human body can be decreased. Moreover, when comparing the radiation patern in the case the antenna is located in the free space or in the vicinity of human body, we can observe in the latter case that the change in the radiation pattern is small. Figure 3 shows the SAR distribution on the surface of a brain-equivalent model and the calculation value of the radiation efficiency. For the comparison, the results are obtained in the case a half wavelength dipole antenna is located at x = 0 with a distance of 15mm from the surface of the brain-equivalent model in the z direction. In the loop antenna with a reflection shield, when a diploe antenna is used, the maximum value of the SAR is decreased of more than 15%. Moreover, concerning the radiation efficiency, in the case of a dipole, it decreases up to about 60% because a lot of electric power is absorbed by the human body. It is also observed to drop off 90% and a high value when it comes to the non fed element of the array loop antenna.
Conclusion
Reference document