Far-field Gain Estimation in Tissue Equivalent Liquid Using Measured Data in Far-field Region

To evaluate the values of the SAR (Specific Absorption Rate) for the mobile communication devices, electric-field probes should be calibrated to relate the field intensity in the tissue equivalent liquid to its output voltages. As a conventional method for 300 MHz-3 GHz, it can be calibrated in the liquid surrounded by the waveguide and the dielectric spacer. Above 3 GHz, the effect of the probe cannot be ignored unless its diameter is smaller than the conventional probe´s one. This would lead to deteriorate the uncertainty of the probe calibration. To overcome this difficulty, we have proposed an alternative method based on an extended Friis transmission formula in the conducting medium. In this method, the gain of the reference antenna operated in the liquid is calibrated and then the probe is calibrated in the radiated field of the calibrated antenna in the liquid. However, the measurement in the far-field region can not be realized by using a vector network analyzer, because the dynamic range of the measurement system is limited whereas the electromagnetic wave is rapidly decayed in the liquid. In this paper, we use the signal generator and spectrum analyzer instead of the vector network analyzer to enhance the dynamic range and measure the power transmission in the far-field region. As a result, the measurable range can be enhanced so that the Friis transmission formula including no contribution of the near field can be applied to the measured power transmission to assure our proposed method.