Theory and Applications of Infinitesimal Dipole Models for Computational Electromagnetics

The recently introduced quantum particle swarm optimization (QPSO) algorithm is employed to find infinitesimal dipole models (IDM) for antennas with known near-fields (measured or computed). The IDM can predict accurately both the near-fields and the far- fields of the antenna. A theory is developed to explain the mechanism behind the IDM using the multipole expansion method. The IDM obtained from single frequency solutions is extrapolated over a frequency range around the design frequency. The method is demonstrated by analyzing conductingand dielectric- type antennas. A calibration procedure is proposed to systematically implement infinitesimal dipoles within existing MOM codes. The interaction of the IDM with passive and active objects is studied through several examples. The IDM proved to predict the interaction efficiently. A closed-form expression for the mutual admittance between similar or dissimilar antennas, with arbitrary orientations and/or locations, is derived using the reaction theorem

On the Spatial Structure of the Antenna Electromagnetic Near Field

Abstract This paper presents a general theory of the antenna near field in the spatial domain. The approach is based on using the Wilcox expansion of the radiated field to define a set of asymptotic spherical regions covering the entire exterior space of the antenna problem. Careful examination of the energy expression within this picture revealed the rich and complex interaction mechanisms between the various spherical regions indicated above. The multipole expansion is then utilized to construct nonrecursively the full near field in the exterior region starting from the far field only. The analysis led to interesting theorems regrading energy exchange processes in the near zone and also to a completely analytical evaluation of the antenna reactive energy in terms of the TE and TM modes of the antenna system. The Structure of the Antenna Near Field in the Spatial Domain We assume that an arbitrary electric current J(r) exists inside a volume V 0 enclosed by the surface S 0 . Let the antenna be surrounded by an infinite, isotropic, and homogenous space with permittivity ε and permeability µ. The antenna current will radiate electromagnetic fields everywhere and we are concerned with the region outside the source volume V 0 . We consider two characteristic regions. The first is the region V enclosed by a spherical surface S and this will be the setting for the near fields. The second region V ∞ is the one enclosed by the spherical surface S ∞ taken at infinity and it corresponds to the far fields. The complex Poynting theorem states that ∇ · S = −(1/2)J * · E + 2iω (w h − w e ), where the complex Poynting vector is defined as S = (1/2) E × H * and the magnetic and electric energy densities are given, respectively, by w e = (1/4)εE · E * and w h = (1/4)µH · H * , and ω is the radian frequency where the radiated power is defined as P rad = Re S ds 1 2 (E × H * ). The result above will be taken up again in Section 3. In the remaining part of this section we try to gain a more detailed insight into the nature of the near field that goes beyond the simple picture presented by (1). We consider the fields generated by the antenna lying in the intermediate zone, i.e., the interesting case between the far zone kr → ∞ and the static zone kr → 0. We aim to attain a conceptual insight into the nature of the near field by mapping out its inner structure in details. Since the fields in the exterior region satisfy the homogenous Helmholtz equation, we can expand the electric and magnetic fields as [1] where A n and B n are vector angular functions dependent on the far-field radiation pattern of the antenna and k = ω √ εµ is the wavenumber. 1 Let us then divide the entire exterior region surrounding the antenna into an infinite number of spherical layers. The outermost layer R 0 is identified with the far zone while the innermost layer R ∞ is defined as the minimum sphere totally enclosing the antenna current distribution. In between these two regions, an infinite number of layers exists, each corresponding to a term in the Wilco