Method to Experimentally Verify Far-Field Conditions in a Direct Far-Field Measurement Setup

In this paper, a method is presented that allows for the experimental verification of far-field conditions in a direct far-field measurement setup. The method is based on a relative-distance sweep (i.e., increasing the distance by linearly translating one antenna) and on the Friis equation. The presented method is only valid for one specified direction and is therefore well suited to assess whether or not far-field conditions are achieved when performing an absolute measurement, such as a maximum gain or effective-isotropic-radiated-power (EIRP) measurement. It is shown that antenna measurement uncertainties due to the finite antenna separation, scattering, positional inaccuracies, drift and noise on the order of hundredths of dBs around 30 GHz for a separation on the order of 1 m can be obtained. Using this method, it is also experimentally shown that whether or not far-field conditions are met depends not on one but on both antennas in a two-antenna measurement setup. This implies that, strictly speaking, the far-field distance cannot be determined by solely considering the largest antenna in a two-antenna measurement setup.