INACCURACY IN SPHERICAL NEAR-FIELD ANTENNA MEASUREMENTS DUE TO ANECHOIC CHAMBER REFLECTIVITY

摘要

The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The co-polar and cross-polar directivity patterns are compared at different levels down from the peak directivity.