Novel Method to Improve the Signal-To-Noise Ratio in Far-Field Results Obtained from Planar Near-Field Measurements

Abstract

A method to reduce the noise power in far-field pattern without modifying the desired signal is proposed. Therefore, an important signal-to-noise ratio improvement may be achieved. The method is used when the antenna measurement is performed in planar nearfield, where the recorded data are assumed to be corrupted with white Gaussian and space-stationary noise, because of the receiver additive noise. Back-propagating the measured field from the scan plane to the antenna under test (AUT) plane, the noise remains white Gaussian and space-stationary, whereas the desired field is theoretically concentrated in the aperture antenna. Thanks to this fact, a spatial filtering may be applied, cancelling the field which is located out of the AUT dimensions and which is only composed by noise. Next, a planar field to far-field transformation is carried out, achieving a great improvement compared to the pattern obtained directly from the measurement. To verify the effectiveness of the method, two examples will be presented using both simulated and measured near-field data.

Methods for Noise Reduction in Far-Field Patterns Obtained From Cylindrical Near-Field Antenna Measurements

“…As in the previous method, a field transformation that allows to filter out a portion of noise without modifying the desired signal is required. The transformation used here was already proposed in [13] for noise reduction in far-field results obtained from PNF measurements and consists of back-propagating the measured field from the scan surface to the plane of the AUT. In this new domain, the desired contribution is theoretically concentrated in the antenna aperture, whereas the noise is spread over the whole reconstructed surface.…”

“…When comparing (5) and (6) with (11) and (12) it is deduced that the only difference before and after applying the filtering is the number of terms in the summations. As deduced in (13), that number of terms is less or equal after the filtering stage.…”

“…This idea of reducing the noise by means of a filtering in an auxiliary domain was proposed in [11] and [12] for the spherical near-field (SNF) case. The same idea was used in [13] to improve the signal-to-noise ratio in the far-field pattern of an antenna as measured in PNF. In this last case, however, a spatial filtering on the antenna under test (AUT) plane is employed instead of a modal filtering.…”

Application of postprocessing techniques methods for noise reduction in cylindrical near field antenna measurements

“…As in the previous method, a field transformation that allows filtering out a portion of noise without modifying the desired signal is required. The transformation used here was already proposed in [7] for noise reduction in far-field results obtained from PNF measurements and consists of back-propagating the measured field from the scan surface to the plane of the AUT (Figure 3). In this new domain, the desired contribution is theoretically concentrated in the antenna aperture, whereas the noise is spread over the whole reconstructed surface.…”

“…This idea of reducing the noise by means of a filtering in an auxiliary domain was proposed in [5], [6] for the spherical near-field (SNF) case. The same idea was used in [7] to improve the signal-to-noise ratio in the far-field pattern of an antenna as measured in PNF. In this last case, however, a spatial filtering on the antenna under test (AUT) plane is employed instead of a modal filtering.…”