1,721,216 research outputs found
Truncation Effects in a Semi-infinite Periodic Array of Thin Strips: A discrete Wiener-Hopf Formulation
Full text linkrigorous solution for the current induced on a semi-infinite array of narrow metallic
strips is obtained using the Wiener-Hopf factorization method in the Z-transformed
domain. The method can be applied to arrays with fixed current shape on each element
(e.g, single mode elements), and shows rigorously the physics of waves associated to
truncated periodic structures. The solution is obtained via a rigorous factorization, that is
improved by using a closed form result based on an approximated factorization. The
current on the truncated array is rigorously represented as the sum of the current pertaining
to the infinite array plus a contribution induced by the truncation of the array. Asymptotics
shows that the truncation-induced current contribution has a diffractive behavior decaying
algebraically with the element number, away from the truncation. Uniform asymptotics
shows that this diffractive current is effectively represented in terms of Fresnel functions,
permitting also a closed form representation in proximity of and at transverse inward
resonance, i.e., when a grazing grating lobe points toward the array. Illustrative examples
and comparisons with a method of moment solution show the accuracy of our results
Planar metamaterial transverse equivalent network and its application to low-profile antenna designs
Full text linkWe present a lumped circuit description of a novel metamaterial layer made of arrayed pairs of tightly coupled conductors (dogbones or Jerusalem crosses). This lumped element network is synthesized to exhibit the same frequency response of the metamaterial layer when inserted in the plane-wave equivalent transmission line. The metamaterial and its transverse equivalent network (TEN) model is then applied to the design of a high impedance surface for low profile dipoles and a partially reflective superstrate for highly directive Fabry-Perot cavity antennas. Numerical results illustrating the radiation properties of such antennas are provided
Array thinning by using antennas in a Fabry-Perot cavity for gain enhancement
No full textA Fabry-Perot cavity (FPC) between a ground plane and a partially reflective surface (PRS) is used here to design array antennas with large distance between the radiating elements. This configuration provides some advantages: i) a reduction of the number of array elements to achieve high directivity; ii) large space between contiguous elements that may host a bulky feeding network as required for dual polarization or active antennas; iii) small coupling and easy feeding network designs because of the smaller number of elements with larger inter-element distance. We show that when designing the FPC antenna a frequency shift of the gain maximum may occur, especially in this sparse array configuration. We also show the existence of preferred distances between elements that controls both the directivity and the side lobe level, and how the presence of the FPC and the relaxed requirement of the interelement distance result in a lower interelement coupling. The presented dual polarized antenna comprises two interleaved 2 × 2 arrays placed in a 2-layer FPC, and exhibits a 19 dBi gain and 30 dB of isolation between the two ports over an operating bandwidth of approximately 5.7%, i.e., typical for patch antennas. © 2006 IEEE
ITD formulation for the currents on a plane angular sector
Full text linkApproximate high-frequency expressions for the
currents induced on a perfectly conducting plane angular sector
are derived on the basis of the incremental theory of diffraction
(ITD). These currents are represented in terms of those predicted
by physical optics (PO) plus fringe contributions excited by singly
and doubly diffracted (DD) rays at the two edges of the angular
sector. For each of these two contributions, additional currents
associated to vertex diffracted rays are introduced that provide
continuity at the relevant shadow boundary lines. The transition
region of DD rays is described by a transition function involving
cylinder parabolic functions. The asymptotic solution presented
here is constructed in such a way to satisfy far from the vertex
the expected edge singularities, which tend to be the same as
those predicted by the exact solution of the half plane. Numerical
results are compared with the exact solution of the same problem
and with moments method results for scattering from polygonal
plates
EBG superstrates for dual polarized sparse arrays
Full text linkA superstrate of an EBG material (or equivalently a Fabry-Perot cavity) is used to design array antennas with large distance between the radiating elements. This configuration provides some advantages: (i) a reduction of the number of array elements to achieve high directivity; (ii) large space between contiguous elements decreases their coupling and permits an easy arrangement for a complicated feeding network (as needed for dual polarization), also on the same plane of the radiating elements. These possibilities are clearly shown in a few examples treated here and in the design of dual polarized antennas with two interleaved arrays. Furthermore, we indicate that in these designs there are optimum distances between elements that either maximize the directivity or minimize the sidelobe level. It is also shown that due to the fact that the radiating elements have larger-than-usual mutual distances it is easy to achieve -40dB of isolation between the two excitation ports, for the two polarizations
Transmission line model with X-circuit for a metamaterial layer made of pairs of dogbone-shaped planar conductors
Full text linkIn this paper we analyze the propagation through a metamaterial in planar technology recently proposed. The metamaterial basic constituent cell consists of a pair of tightly coupled conductors that supports two main resonance modes corresponding to either a symmetric or an antisymmetric current distribution in the pair. We present an equivalent X-shaped lumped circuit network to be interposed in the transmission line (TL) model of propagation across a metamaterial layer to reproduce its electric and magnetic resonances. We show that reflection and transmission features of a periodic array of dogbone pairs as well as its dispersion diagram are accurately predicted by this simple but effective model
EM characterization of Raspberry-like nanocluster metamaterials
Full text linkThis work investigates certain electromagnetic (EM) properties of metamaterials formed by densely arrayed nanoclusters of plasmonic nanoparticles. An approximate model based on the single dipole approach in conjunction with the multipole expansion of the scattered field is used to evaluate the electric and magnetic polarizabilities of the nanocluster. Then, the permittivity and permeability of the composite medium are estimated by the Maxwell Garnett homogenization model. Results obtained from these approximatations are compared with data from full-wave simulations, focusing on the characterization of the nanocluster resonant isotropic electric and magnetic responses to an incident wave field, and the possibility to realize an isotropic negative index materials at optical frequencies.
Time domain double diffraction at a pair of coplanar skew edges
Full text linkThis study aims at describing the field propagation in terms of pulsed rays, that are particularly advantageous when dealing with short-pulse excitations. In the framework of the Geometrical Theory of Diffraction we augment Geometrical Optics and uniform singly diffracted field solutions available in the time domain (TD), by TD doubly diffracted (DD) rays, that are expressed in simple closed forms. Impulsive double diffraction at a pair of coplanar edges is here formulated directly in the TD, as a double superposition of impulsive spherical waves. Nonuniform and uniform wavefront approximations for TD-DD fields are determined in closed form, defining two novel TD transition functions. The scalar case with either hard or soft boundary conditions is analyzed first, and then used to build an electromagnetic dyadic DD coefficient for a pair of coplanar edges with perfectly conducting faces. Particular attention is given to the definition of TD transition regions, i.e., the elliptical regions where the TD-DD field does not exhibit a ray optical behavior. The compensation mechanism by which the TD-DD fields repair the discontinuity introduced by singly diffracted fields at their shadow boundaries is also analyzed in detail. Our result for the TD-DD field excited by an impulsive spherical wave is valid only for early times, at and close to (behind) the DD ray wavefront. The TD-DD field response to a more general pulsed excitation is obtained via convolution, and if the exciting signal has no low-frequency components the range of validity of the resulting pulsed response is enlarged to later observation times behind the wavefront
Asymptotic high-frequency Green's function for a planar phased sectoral array of dipoles
Full text linkThis paper deals with the derivation and physical interpretation of a uniform high-frequency Green's function for a planar right-angle sectoral phased array of dipoles. This high-frequency Green's function represents the basic constituent for the full-wave description of electromagnetic radiation from rectangular periodic arrays and scattering from rectangular periodic structures. The field obtained by direct summation over the contributions from the individual radiators is restructured into a double spectral integral whose high-frequency asymptotic reduction yields a series of propagating and evanescent Floquet waves (FWs) together with corresponding FW-modulated diffracted fields, which arise from FW scattering at the array edges and vertex. Emphasis is given to the analysis and physical interpretation of the vertex diffracted rays. The locally uniform asymptotics governing this phenomenology is physically appealing, numerically accurate, and efficient, owing to the rapid convergence of both the FW series and the series of corresponding FW-modulated diffracted fields away from the array plane. A sample calculation is included to demonstrate the accuracy of the asymptotic algorithm
Three-dimensional Green’s function for planar rectangular phased dipole arrays
Full text linkThis paper deals with the construction, physical interpretation and application of a uniform high-frequency representation of array Green’s functions (AGFs) for planar rectangular phased arrays of dipoles. An AGF is the basic constituent for the full-wave description of electromagnetic radiation from large periodic structures. For efficient treatment of high-frequency phenomena, the AGF obtained by direct summation over the contributions from the individual radiators is globally restructured via the Poisson sum formula into a series of propagating and evanescent Floquet waves (FWs) together with corresponding FW-modulated diffracted waves, which arise from FW scattering at the array edges and vertexes. These results are obtained by high-frequency uniform asymptotics applied to the wave integrals generated by Poisson summation in the spatial or spectral domains. The final algorithm is physically appealing, numerically accurate, and efficient, owing to the rapid convergence of both the FW series and the series of corresponding FW-modulated diffracted fields away from the array plane. The use of the asymptotic AGF in the full-wave analysis of large slot arrays is discussed, with the inclusion of numerical results
- …
