Antenna Gain Determination by Spherical Near-Field Substitution Method without Full-Sphere Measurement of Reference Gain Antenna

Abstract

In spherical near-field (SNF) antenna measurements, gain of an antenna under test (AUT) is usually determined by substitution method. In this method, a reference antenna with known gain, typically a standard gain horn (SGH), is measured and processed in the same way as the AUT, that is a full-sphere measurement is done for the SGH and this is followed by the spherical near-to-far-field transformation (SNFT). The AUT gain is then determined comparing the calculated levels of the AUT and SGH far-field signals and using the known SGH gain value. It is always emphasized that in the SNF gain determination by substitution, both antennas must be processed in the same way, and thus measuring the full-sphere near-field data for the SGH is unavoidable. However, since typical SGH is not a large antenna, its far-field distance does not exceed few meters, which is a usual measurement distance in many SNF setups. The SGH in these cases is measured in the far field and the SNFT does not change the measured SGH pattern shape. It is interesting to find out, in which conditions it is possible to skip the SNFT for the SGH, and thus also its full-sphere measurement, and use the directly measured SGH data. The SGH measurement can in this case be reduced to a single direction, similar to what is done in the traditional far-field substitution method. In this paper the above question is clarified in detail, paying special attention to various measurement uncertainties which may arise due to the explained simplification of measurement procedure.