소형 안테나 ( Small Antennas )

Three-dimensional modeling of marine controlled-source electromagnetic (CSEM) data is vital to improve the understanding of electromagnetic (EM) responses collected in increasingly complex geologic settings. A modeling tool for simulating 3D marine CSEM surveys, based on a finite-difference discretization of the Helmholtz equation for the electric fields, has been developed. Optimizations for CSEM simulations include the use of a frequency-domain technique, a staggering scheme that reduces inaccuracies especially for horizontal electric-dipole sources located near the seafloor, and a new interpolation technique that provides highly accurate EM field values for receivers located in the immediate vicinity of the seafloor. Source singularities are eliminated through a secondary-field approach, in which the primary fields are computed analytically for a homogeneous or a 1D layered background; the secondary fields are computed using the finite-difference technique. Exploiting recent advances in computer technology and algorithmic developments, the system of finite-difference equations is solved using the MUMPS direct-matrix solver. In combination with the other optimizations, this allows accurate EM field computations for moderately sized models on small computer clusters. The explicit availability of matrix factorizations is advantageous for multisource modeling and makes the algorithm well suited for future use within an inversion scheme. Comparisons of simulated data for (1) 1D models to data generated using a 1D reflectivity technique and (2) 3D models to published 3D data demonstrate the accuracy and benefits of the approach.

3D finite-difference frequency-domain modeling of controlled-source electromagnetic data: Direct solution and optimization for high accuracy

We introduce the novel concept of superdirective nanoantennas based on the excitation of higher-order magnetic multipole moments in subwavelength dielectric nanoparticles. Our superdirective nanoantenna is a small Si nanosphere containing a notch, and is excited by a dipole located within the notch. In addition to extraordinary directivity, this nanoantenna demonstrates efficient radiation steering at the nanoscale, resulting from the subwavelength sensitivity of the beam radiation direction to variation of the source position inside the notch. We compare our dielectric nanoantenna with a plasmonic nanoantenna of similar geometry, and reveal that the nanoantenna's high directivity in the regime of transmission is not associated with strong localization of near fields in the regime of reception. Likewise, the absence of hot spots inside the nanoantenna leads to low dissipation in the radiation regime, so that our dielectric nanoantenna has significantly smaller losses and high radiation efficiency of up to 70%.

Superdirective dielectric nanoantennas.

The high Q-factor (low bandwidth) and low efficiency make the design of small antennas challenging. Here, convex optimization is used to determine current distributions that provide upper bounds on the antenna performance. Optimization formulations for maximal gain Q-factor quotient, minimal Q-factor for superdirectivity, and minimum Q for given far-fields are presented. The effects of antennas embedded in structures are also discussed. The results are illustrated for planar geometries.

Optimal Antenna Currents for Q, Superdirectivity, and Radiation Patterns Using Convex Optimization

Electrically small electric and magnetic dipole antennas are introduced that are based on electric and magnetic near-field resonant parasitic (NFRP) elements that are electrically coupled to the driven element. By properly combining these two NFRP antenna types, electrically small Huygens sources are realized. The proposed Huygens sources have one feed point, are well matched to 50 Ω, and have high radiation efficiencies. Their predicted directivity patterns approach the known Huygens source theoretical limit.

Metamaterial-Inspired, Electrically Small Huygens Sources

[1] The theory, computer simulations, and experimental measurements are presented for electrically small, two-element supergain arrays with near-optimal end-fire gains of 7 dB. We show how the difficulties of narrow tolerances, large mismatches, low radiation efficiencies, and reduced scattering of electrically small parasitic elements are overcome by using electrically small resonant antennas as the elements in both separately driven and singly driven (parasitic), two-element, electrically small supergain end-fire arrays. Although rapidly increasing narrow tolerances prevent the practical realization of the maximum theoretically possible end-fire gain of electrically small arrays with many elements, the theory and preliminary numerical simulations indicate that near-maximum supergains are also achievable in practice for electrically small arrays with three (and possibly more) resonant elements if the decreasing bandwidth with increasing number of elements can be tolerated.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007RS003747

Electrically small supergain end-fire arrays

In principle, the end-fire directivity of a linear periodic array of N isotropic radiators can approach N/sup 2/ as the spacing between elements decreases, provided the magnitude and phase of the input excitations are properly chosen. Thus, the directivity of a two-element array of isotropic radiators would approach a value of four, that is, 6 dB higher than that of a single isotropic radiator. We have conducted a theoretical, computational, and experimental study for a two-element superdirective array of resonant monopoles. In agreement with the theoretical and computational curves, the measured gain of the monopole array does indeed continually increase with decreasing spacing of the monopoles, provided the relative magnitudes and phases are maintained. However, for very small separation, maximum achievable gain is not reached due to the presence of ohmic loss.

A monopole superdirective array

Electrically small superdirective arrays that are theoretically possible have been shown feasible for two and three element arrays. However the individual elements must be fed separately with the correct input current magnitudes and phases to achieve superdirectivity. In this paper, almost equivalent superdirectivity was obtained with two-element arrays by feeding only one element and shorting the second "parasitic" element. This parasitic array exhibits a new resonant frequency f/sub a0/ which is usually lower than the single-element resonance, and two "superdirective" frequencies emerge on either side of f/sub a0/ at which the parasitic element acts either as a director or a reflector. It was found, in general, that parasitic superdirectivity is optimized for electrically small elements by using the parasitic element as a reflector and by choosing the correct element separation and element orientation (for asymmetrical elements). In addition, the use of relatively low Q elements allows for smaller separation distances, resulting in increased directivity while achieving wider bandwidths. A brief overview of superdirective arrays was presented and separately fed and parasitic two-element superdirective arrays were compared.

Electrically small superdirective arrays using parasitic elements

The theory, computer simulations, and experimental measurements are presented for electrically small two-element supergain arrays with near optimal endfire gains of 7 dB. We show how the difficulties of narrow tolerances, large mismatches, low radiation efficiencies, and reduced scattering of electrically small parasitic elements are overcome by using electrically small resonant antennas as the elements in both separately driven and singly driven (parasitic) two-element electrically small supergain endfire arrays. Although rapidly increasing narrow tolerances prevent the practical realization of the maximum theoretically possible endfire gain of electrically small arrays with many elements, the theory and preliminary numerical simulations indicate that near maximum supergains are also achievable in practice for electrically small arrays with three (and possibly more) resonant elements if the decreasing bandwidth with increasing number of elements can be tolerated.

Electrically Small Supergain Arrays

With proper adjustment of the amplitude and phase of the element excitations, the directivity of an N-element end-fire array of isotropic radiators can approach N2 as the spacing between the elements approaches zero. To achieve this end-fire superdirectivity for a 2-element array, the excitation phase difference between the elements must closely approach 180deg. When closely spaced elements are driven nearly 180deg out-of-phase, the element radiation resistances approach zero, resulting in a high input Voltage Standing Wave Ratio (VSWR) and reduced radiation efficiency. At the same time, there is also a significant decrease in the operating bandwidth. In this letter we present the design of a 2-element, superdirective multiple arm folded monopole array that achieves a near 50 Omega input radiation resistance at each element, resulting in a matched input VSWR, higher radiation efficiency and therefore, a substantial increase in realized or overall efficiency.

Terry H. O'Donnell

Papers

Electrically small supergain end-fire arrays

A monopole superdirective array

Electrically small superdirective arrays using parasitic elements

Electrically Small Supergain Arrays

An Impedance-Matched 2-Element Superdirective Array