Constant near-field gain for a shielded loop antenna operated in the liquid at 30 MHz

Abstract

For calibrating electric-field probe to evaluate the value of SAR (specific absorption rate) for devices at a frequency of 30 MHz, a shielded loop antenna is used as a reference antenna. To facilitate curve fitting for the calibration, near-field gain should be constant near the loop. In this paper, it is shown that the circumference of the loop can be selected as the near- field gain becomes approximately-constant near the loops. Keywords—near-field gain; shielded loop antenna; tissue- equivalent liquid; SAR; probe calibration I. INTRODUCTION SAR (specific absorption rate) provides a measure of the exposure and is evaluated for mobile communication devices according to IEC standard document (1)(2). Generally, the value of the SAR for the devices under test (DUTs) operated in the GHz band can be calculated by measuring electric-field intensity produced by DUT in tissue-equivalent liquid surrounded by a shell shaped like a half of head, with an electric-field probe or SAR probe (1). Therefore, it is essential to calibrate the SAR probe or to relate the value of the electric- field intensity at the tip of the probe to output voltage of the probe. One of the calibration methods for the SAR probe is to use a reference antenna operated in the liquid (3). In this method, the electric-field intensity produced by the reference antenna or near-field gain of the reference antenna should be calculated as a function of the distance of the observation point from the reference antenna. It is advantageous to use dipole antennas as the reference antennas at frequencies in the GHz band, because electric field is dominant. Conversely, it is advantageous to use loop antennas at frequencies in the MHz band, because magnetic field is dominant. In this paper, the method to use the reference antenna is employed to calibrate the SAR probe and a shielded loop antenna is used as the reference antenna operated in the liquid at frequency of 30 MHz (4). It is known that the near-field gain becomes constant in the far-field region of the antennas so that it can be expressed by an asymptotic series with respect to the distance between the two antennas (3). In practice, measuring S parameters to evaluate the near-field gain of the antenna is limited in the range up to 200 mm, for example, because of the size limitations on the measurement system such as a liquid container. Since the wavelength is 627 mm in the tissue- equivalent liquid at a frequency of 30 MHz, S parameters must be measured in the near-field region of the antennas. The near- field gain may be caused by close coupling between two loop antennas so that the near-field gain cannot converge with the far-field gain in the near-field region. However, the existence of a range that the near-field gain becomes constant is desirable from the viewpoint of fitting asymptotic polynomial to the near-field gain. In this paper, it is shown that the range mentioned above is realized if the circumference of the loops selected well.