A new antenna calibration method and a selection of a measurement probe with minimal disturbance and sufficient sensitivity for electromagnetic exposure measurements around wireless base stations

To measure electromagnetic fields accurately, the calibration of field probes is necessary. In this paper, an accurate lowcost calibration method is presented. However, calibration does not take into account all influencing factors, especially when measuring fields close to the source (e.g., antenna). While measuring, the electromagnetic field will be disturbed by the measurement probes themselves. So instead of measuring the true free-space field, the disturbed field will be measured. This paper will show that when the disturbance must be limited to a predefined value (e.g., 5 %), a suitable measurement probe with maximum sensitivity can be selected. INTRODUCTION To check whether the exposure (public and occupational) of a base station satisfies the guidelines for electromagnetic exposure, electromagnetic field measurements around base stations must be accurate. To perform electromagnetic measurements, calibration of the measurement probes is needed [1], [2]. Therefore, an accurate low-cost calibration method is developed. Due to the non-negligible dimensions of the measurement probes [3], the electromagnetic field probes will disturb the field that has to be measured. In principle, the calibration should take into account the disturbance of the probe. However, this is only true if the disturbance is equal in the calibration set-up and during the measurement. This is certainly not the case if the measurement is done in the near field of a base station because calibration is mostly done measuring far fields of one or more electromagnetic sources. It is shown that when a maximal disturbance of e.g. 5% is required, different probes for nearand far-field measurements respectively must be selected. CALIBRATION Antenna calibration involves the determination of the antenna factor. The antenna factor is defined as the ratio of the strength of the field in which the antenna is immersed and the output voltage across the load connected to the antenna: