Fine Positioning Three-Dimensional Electric-Field Measurements in Automotive Environments

A fine positioning system is introduced that measures the 3-D electric-field space distributions around passenger cars that are equipped with cellular radio antennas. The measurement system, which is constructed of nonmetallic materials to reduce the electric-field fluctuation that is caused by the manipulator, uses an air-motor mechanism that yields a field sensor spatial resolution of better than 10 mm. The uncertainty of the measured electric-field strength is estimated as plusmn4 dB, i.e., variations are within 4 dB. Detailed electric-field distributions inside and outside the passenger car are derived for three antennas: a trunk-lid antenna, rear-window antenna, and roof antenna. The measurement results show that the electric-field strengths in the front and back seats are less than 30 V/m when the antenna input is less than 1 W as the net power. Inside the car, the local peak of the field strength is higher by 2 and 4 dB for the trunk lid and roof antenna, respectively, and approximately 10 dB higher for the rear-window antenna. The electric fields both inside and outside the car do not exceed the Level 4 (30 V/m) specification, which is one of the immunity levels for electronic devices defined in the IEC electromagnetic-compatibility standard. In addition, the measured electric-field strengths are lower than the reference levels for human exposure to RF electromagnetic fields, which are recommended by the International Commission on Non-Ionizing Radiation Protection. At maximum, the field strength of 30 V/m as a specially averaged value at the frequency of 900 MHz corresponds to half of the whole body specific-absorption-rate basic restriction of 0.08 W/kg with respect to specifications for the general public, when assuming conservative estimates for the maximum coupling between the human body and the field. The differences in the far-field distributions of the three antennas outside the car are also estimated