Gain calibration in near-field region of antenna in tissue-equivalent liquid for SAR assessment

A technique of measuring the gain of the antenna in the tissue equivalent liquid is required to develop alternative method for calibrating SAR probe with no use of the waveguide system. It is based on an extended Friis transmission formula which is valid in the near-field region of the antenna in the liquid, because the attenuation in the liquid is so large that the measurement in the far-field region is impossible. The near-field gain is defined and expressed by polynomial expansion in terms of inverse of the distance between the antennas. In this paper, two or three terms of the expansion, including the far-field gain in dB, should be selected to obtain good result of the curve fitting. The effect of shifting initial position of the antennas is also discussed.