An equivalent source method to determine complex excitation levels of antenna arrays from near-field measurements

Antenna arrays used in mobile communications usually provide only one RF connection that drives all active elements by means of a distribution network. In case defects appear during acceptance tests it is desirable to identify erroneous excitations levels without the pain of measuring each active element individually. Hence a field based algorithm is proposed that enables the determination of the actual complex excitation levels from spherical near-field data that is used to calculate the desired far field employing a near-field to far-field transformation. Due to the limited resolution of the holographic back-projection it is not possible to determine accurate complex excitation levels of antenna arrays employing this technique. However, the use of inverse techniques is usually linked to a lack in stability and/or accuracy. On the other hand it is not desirable to spend an amount of a priori knowledge about the antenna under test (AUT) that would be sufficient to model the antenna using an electromagnetic field solver. The proposed approach requires only one approximate geometric measure if base station antennas that are made of dipoles are under investigation.