Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

Homogeneous and evanescent contributions in scalar near-field diffraction

Abstract: The contributions of homogeneous and evanescent waves to two-dimensional near-field diffraction patterns of scalar optical fields are examined in detail. The total plane-integrated intensities of the two contributions are introduced as convenient measures of their relative importance. As an example, the diffraction of a plane wave by a slit is considered.

Surface diagnosis of large reflector antennas using microwave holographic metrology: An iterative approach

Abstract: The radiation pattern of an antenna can be significantly affected by reflector surface errors A minimization of losses due to surface inaccuracies is, therefore, required A suitable procedure for the determination of the deviations of the surface from its ideal geometry is based on the microwave holographic technique In this procedure, the complex pattern of the antenna is measured, and the Fourier transform relationship, existing between the radiation pattern and the function related to the current distribution on the reflector surface, is applied It is naturally desirable to limit the amount of measured data The present investigation is concerned with the concept of the Fourier transform relationship between the induced current and the far field It is demonstrated that the relationship, in general, can be interpreted as a summation of many Fourier transforms An iterative scheme is employed to continue the far-field pattern outside the measurement window

Far-field-current relationship based on the TL model for lightning return strokes to elevated strike objects

Abstract: New general expressions relating lightning return stroke currents and far radiated electric and magnetic fields are proposed, taking into account the effect of an elevated strike object, whose presence is included as an extension to the transmission line (TL) model. Specific equations are derived for the case of tall and electrically short objects. The derived expressions show that, for tall structures (when the round-trip propagation time from top to bottom within the tower is greater than the current zero-to-peak risetime), the far field is enhanced through a factor with respect to an ideal return stroke initiated at ground level. The enhancement factor can be expressed in terms of the return stroke wavefront speed v, the speed of light in vacuum c, and the current reflection coefficient at the top of the elevated strike object. For typically negative values of this top reflection coefficient, lightning strikes to tall towers result in a significant enhancement of the far electromagnetic field. Expressions relating the far electromagnetic field and the return stroke current are also presented for electrically short towers and for very long return stroke current wavefronts. For the case of return strokes initiated at ground level (h=0), these expressions represent a generalization of the classical TL model, in which the reflections at the ground are now taken into account. We describe also simultaneous measurements of return stroke current and its associated electric and magnetic fields at two distances related with lightning strikes to the 553-m-high Toronto Canadian National (CN) Tower performed during 2000 and 2001. The derived expressions for tall strike objects are tested versus obtained sets of simultaneously measured currents and fields associated with lightning strikes to the CN Tower, and a reasonable agreement is found. Additionally, it is shown that the peak of the electromagnetic field radiated by a lightning strike to a 553-m-high structure is relatively insensitive to the value of the return stroke velocity, in contrast to the lightning strikes to ground.

Near-Field Thermal Radiation between Metasurfaces

Abstract: Evanescent waves induced by thermal fluctuations can tunnel through nanoscale gap spacing, leading to super-Planckian thermal radiation. However, investigations of near-field thermal radiation of macroscopic objects have been limited to simple planar or effectively planar geometries until recently. Based on exact formulations including the scattering theory and Green’s function method, patterning thin films into 1D and 2D metasurfaces is found to increase the radiative heat flux by more than 1 order of magnitude in a certain range of thicknesses. The underlying mechanism of this counterintuitive phenomenon lies in the excitation of hyperbolic modes supporting high local density of states for broad frequency and k-space regimes. The radiative heat flux of a 2D metasurface increases monotonically with the thickness, while the heat flux of a 1D metasurface is not so sensitive to the thickness and is surprisingly higher than that of its 2D counterparts. The stark difference is attributed to the rapid-decay su...