Broadband Optically Modulated Scatterer Probe for Near Field Measurements

In many applications, antenna measurements cannot be made in the far field. Near-field measurements provide an accurate method of antenna diagnostics. However, since diagnostic measurements are made in the very close vicinity of the antennas, the measured fields might be perturbed by the measuring instruments. The Optically Modulated Scatterer technique (OMS) provide a very low perturbation for the field to be measured. In this technique, the field is tagged by inserting a modulated scatterer probe at the position at which the field measurement is desired. The impedance connected to the probe is modulated with an optical fiber thus providing almost perturbation free measurements due to its structural part. In the literature, one can find a low scattering photodiode modulated-probe for microwave near field imaging. The frequency response of the probe is computed at 2.45 GHz. In this paper, however a new formulation for computing the scattered field for low frequencies (from 150 MHz) by a broadband near field probe based on the impedance matrix is developed. In addition, a method to increase the scattered power by controlling matching will be shown.