The specific problems encountered in the design of near-field focused planar microstrip arrays for RFID (Radio Frequency IDentification) readers are described. In particular, the paper analyzes the case of a prototype operating at 2.4 GHz, which has been designed and characterized. Improvements with respect to conventional far-field focused arrays (equal phase arrays) are discussed and quantified

Abstract. A hybrid uniform geometrical theory of diffraction (UTD)-moment method (MOM) approach is introduced to provide an efficient analysis of the electromagnetic radiation/scattering from electrically large, finite, planar periodic arrays. This study is motivated by the fact that conventional numerical methods become rapidly inefficient and even intractable for the analysis of electrically large arrays containing many antenna or frequency-selective surface (FSS) elements. In the present hybrid UTD-MOM approach, the number of unknowns to be solved is drastically reduced as compared to that which is required in the conventional MOM approach. This substantial reduction in the MOM unknowns is essentially made possible by introducing relatively few, special ray-type (or UTD) basis functions to efficiently describe the unknown array currents. The utility of the present hybrid approach is demonstrated here for the simple case of a large rectangular phased array of short and thin metallic dipoles in air, which are excited with a uniform amplitude and linear phase distribution. Some numerical results are presented to illustrate the efficiency and accuracy of this hybrid method.

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Poisson sum formulas have been previously presented and utilized in the literature [1]-[8] for converting a finite element-by-element array field summation into an alternative representation that exhibits improved convergence properties with a view toward more efficiently analyzing wave radiation/scattering from electrically large finite periodic arrays. However, different authors [1]-[6] appear to use two different versions of the Poisson sum formula; one of these explicitly shows the end-point discontinuity effects due to array truncation, whereas the other contains such effects only implicitly. It is shown here, via the sifting property of the Dirac delta function, that first of all, these two versions of the Poisson sum formula are equivalent. Second, the version containing implicit end point contributions has often been applied in an incomplete fashion in the literature to solve finite-array problems;it is also demonstrated here that the latter can lead to some errors in finite-array field computations.Index Terms-Array antennas, Poisson sum formula.

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