Bistatic Far-Field Fraunhofer Distance Versus the Transmitter and Receiver Spherical Angles and the Target Parabolic Profile Features

For radiation or scattering problems, the Fraunhofer distance allows us to define the far-field (FF) zone. It depends on the largest dimension of the target or aperture and the wavelength. This article shows that this distance can be expressed from this conventional Fraunhofer distance multiplied by a new function. It depends on the transmitter and receiver spherical angles and the target or aperture profile features, assumed to be a paraboloid. This function is positive and smaller than or equal to one, which means that the conventional Fraunhofer distance can be overestimated. In addition, the criterion is generalized to the case where the receiver and the transmitter must be both in FF. From the Rao-Wilton–Glisson Galerkin method of moments (MoMs), the comparison of the scattered field computed both in near and far fields validates the closed-form expressions of the new Fraunhofer criterion.