Measurement of Antenna System Noise Temperature Using Planar Near-Field Data
Authors: A.C. Newell, P. Pelland, S.F. Gregson, D. Janse Van Rensburg
Publication: AMTA 2017
Copyright Owner: NSI-MI Technologies
This paper presents the results of a new measurement technique to determine antenna system noise temperature using data acquired from a planar near-field measurement. The ratio of antenna gain to system noise temperature (G/T) is usually determined in a single measurement when the antenna is alternately pointed towards the “cold sky” and a hot radio source such as the sun or a star with a known flux density. The antenna gain is routinely determined from nearfield measurements and with the development of this new technique, the system noise temperature can also be determined using the same measurements. The ratio of G/T can therefore be determined from planar near-field data without moving the antenna to an outdoor range. The noise temperature is obtained by using the plane-wave spectrum of the planar near-field data and focusing on the portion of the spectrum in the evanescent or “imaginary space” portion of the spectrum. Near-field data is obtained using a data point spacing of λ/4 or smaller and the plane-wave spectrum is calculated without applying any probe correction or Cos(θ) factor. The spectrum is calculated over real space corresponding to propagating modes of the far-field pattern and also the evanescent or imaginary space region where kx2+ ky2 ≥ k2. Actual evanescent modes are highly attenuated in the latter region and therefore the spectrum in this region must be produced by “errors” in the measured data. Some error sources such as multiple reflections will produce distinct localized lobes in the evanescent region and these are recognized and correctly identified by using a data point spacing of less than λ/2 to avoid aliasing errors in the far-field pattern. It has been observed that the plane wave spectrum beyond these localized lobes becomes random with a uniform average power. This region of the spectrum must be produced by random noise in the near-field data that is produced by all sources of thermal noise in the electronics and radiated noise sources received by the antenna. By analysing and calibrating this portion of the spectrum in the evanescent region the near-field noise power can be deduced and the corresponding noise temperature determined. Simulated and measured data will be presented to illustrate and validate the measurement and analysis techniques.