THEORETICAL AND EXPERIMENTAL STUDIES ON NEAR FIELD/FAR FIELD MEASUREMENTS USING A FREQUENCY INDEPENDENT PROBE

Abstract

In the recent years, the near field measurement of the antennas for the subsequent determination of their far field characteristics is gaining ground over the direct far field measurement due to the many inherent advantages linked with the near field measurements such as controlled environment, no mounting and transportation problems which are usually associated with the large antennas and time and cost effectiveness brought in by numerical computations using computers. In the present work, a dipole antenna which is loaded by a diode has been designed to act as a probe and determine the near fields of the test antennas. The near field patterns for the three test antennas namely the pyramidal horn, microstrip slot line and the_. waveguide slotted array have been observed and recorded. A theoretical insight has been presented for the far field evaluation of an arbitrary antenna and the Lorentz reciprocity theorem has been used to develop a probe compensation criterion, to correct for the directivity of the probe. The experimental determination of near fields is followed by the experimental determination of the far fields of the test antennas and the probe. A computer program has been developed using the FFT algorithm to convert the measured near field data to the far field. The experimentally observed patterns have been compared with the theoretically computed results.