Abstract
Near-field to far-field transformation (NFFFT) is a frequently-used method in antenna and radar cross section (RCS) measurements for various applications. For weapon systems, most measurements are captured in the near-field area in an anechoic chamber, considering the security requirements for the design process and high spatial costs of far-field measurements. As the theoretical RCS value is the power ratio of the scattered wave to the incident wave in the far-field region, a scattered wave measured in the near-field region needs to be converted into field values in the far-field region. Therefore, this paper proposes a near-field to far-field transformation algorithm based on spherical wave expansion for application in near-field RCS measurement systems. If the distance and angular coordinates of each measurement point are known, the spherical wave functions in an orthogonal relationship can be calculated. If each weight is assumed to be unknown, a system of linear equations as numerous as the number of samples measured in the near electric field can be generated. In this system of linear equations, each weight value can be calculated using the iterative least squares QR-factorization method. Based on this theory, the validity of the proposed NFFFT is verified for several scatterer types, frequencies and measurement distances.
Highlights
In many cases, radar cross section (RCS) measurements for an object under test (OUT) or the radiation patterns of an antenna under test (AUT) are captured in the near-field region inside an anechoic chamber or at an outdoor site
Assuming that the weight multiplied by each spherical wave is unknown, a system of linear equations as numerous as the number of samples measured in the near electric field can be generated
A spherical wave expansion (SWE)-Near-field to far-field transformation (NFFFT) method was proposed for arbitrary measurement points that do not belong to canonical surfaces, and it was proven that the proposed method is applicable to near-field
Summary
Radar cross section (RCS) measurements for an object under test (OUT) or the radiation patterns of an antenna under test (AUT) are captured in the near-field region inside an anechoic chamber or at an outdoor site. To overcome the limitations discussed above, an NFFFT algorithm that can flexibly convert an arbitrary near-field scanning surface measurement value into a far-field RCS within various frequency bands is required. A far-field RCS prediction method based on the spherical wave expansion (SWE) method that adapts modal expansion techniques away from canonical surfaces is proposed. Assuming that the weight multiplied by each spherical wave is unknown, a system of linear equations as numerous as the number of samples measured in the near electric field can be generated. The validity of SWE-based NFFFT for arbitrary scanning surfaces was confirmed based on the extracted far-field RCS results from scanning near-field samples for actual missile type OUTs with a larger electrical size
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