A generalized electromagnetic model is presented in order to predict the response\udof forward scatter radar (FSR) systems for air-target surveillance applications in both far-field\udand near-field conditions. The relevant scattering problem is tackled by developing the\udHelmholtz–Kirchhoff formula and Babinet’s principle to express the scattered and the total fields\udin typical FSR configurations. To fix the distinctive features of this class of problems, our approach\udis applied here to metallic targets with canonical rectangular shapes illuminated by a plane wave,\udbut the model can straightforwardly be used to account for more general scenarios. By exploiting\udsuitable approximations, a simple analytical formulation is derived allowing us to efficiently describe\udthe characteristics of the FSR response for a target transitioning with respect to the receiver from\udfar-field to near-field regions. The effects of different target electrical sizes and detection distances on\udthe received signal, as well as the impact of the trajectory of the moving object, are evaluated and\uddiscussed. All of the results are shown in terms of quantities normalized to the wavelength and can\udbe generalized to different configurations once the carrier frequency of the FSR system is set. The\udrange of validity of the proposed closed-form approach has been checked by means of numerical\udanalyses, involving comparisons also with a customized implementation of a full-wave commercial\udCAD tool. The outcomes of this study can pave the way for significant extensions on the applicability\udof the FSR technique.
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