Antenna Excitation Research Articles

Numerical Study of the Near-Field and Far-Field Properties of Active Open Cylindrical Coated Nanoparticle Antennas

A very electrically small, active open cylindrical coated nanoparticle model is constructed, and its electromagnetic properties are investigated in the visible frequency band. Its optical response under both planewave and electric dipole antenna excitations shows very strong dipole behavior at its lowest resonance frequency. The scattering cross section at that dipole resonance frequency is increased by more than +50 dBsm for the planewave excitation. When the open structure is excited by a small current (I0 = 1 ×10-3A) driven dipole antenna, the maximum radiated power of the composite nanoantenna can be increased by +83.35 dB over its value obtained when the dipole antenna radiates alone in free space. The behaviors under various locations and orientations of the dipole are explored. Dipole orientations along the cylinder axis and symmetric locations of the dipole produced the largest radiated power enhancements.

RF B1 field localization through convex optimization

AbstractA 9‐cm wavelength in the human brain at the 400‐MHz Larmor frequency for 9.4‐T leads to pronounced B1 field contours and consequential image inhomogeneity. The objective of this study was to develop a new technique to control this nonuniformity by designing the phase and magnitude of radiofrequency (RF) power emanating from RF coil antenna elements in a multichannel transceiver array. Nonuniformity was used to steer a constructively interfering B1 field node to spatially correlate with an anatomic region of interest. This work outlines a convex (quadratically constrained quadratic problem) formulation of the B1 localization problem and the benefits of such a formulation. This convex formulation was used to design antenna excitation magnitude and phase. Localization is demonstrated in simulated finite difference time domain (FDTD) B1 field human head distributions and human head phantom measurement. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:31–37, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26456

Effects of two-photon absorption on terahertz radiation generated by femtosecond-laser excited photoconductive antennas

Terahertz (THz) radiation can be generated more efficiently from a low-temperature-grown GaAs (LT-GaAs) photoconductive (PC) antenna by considering the two-photon absorption (TPA) induced photo-carrier in the photoconductor. A rate-equation-based approach using the Drude-Lorentz model taking into account the band-diagram of LT-GaAs is used for the theoretical analysis. The use of transform-limited pulses at the PC antenna is critical experimentally. Previously unnoticed THz pulse features and anomalously increasing THz radiation power rather than saturation were observed. These are in good agreement with the theoretical predictions. The interplay of intensity dependence and dynamics of generation of photoexcited carriers by single-photon absorption and TPA for THz emission is discussed.

A novel electrical model to an antenna array

In this paper, an electrical model is developed to represent the input admittance of an antenna array with a finite number of elements. This model consists of an RLC bock component to represent the input admittance of each elementary antenna element and a capacity component to represent different degrees of antenna coupling effects. The equations based on cavity model are developed to represent physical meaning of each model. Numerical results show that good accuracy for the simulation results can be obtained by using this electrical model to the results obtained by using HFSS. As the array is large and sparse, a very small amount of computation can yield good accuracy. This model is shown not only to be numerically efficient compared to the full wave analysis using the moment method, but also to give physical insight into the antenna array mutual coupling mechanism. Furthermore, this model has no limitation on antenna array geometry and excitation. Key words: Antenna array, electrical model, coupling capacity.

THz pulse shaping and improved optical-to-THz conversion efficiency using a binary phase mask

We demonstrate improved optical-to-terahertz (THz) conversion efficiency and THz pulse shaping from an interdigitated GaAs large area photoconductive antenna by using a binary phase mask. The binary phase mask results in a time-delayed excitation of the adjacent antennas, which allows subsequent antennas to produce an additive field, thus resulting in a quasi-single-cycle THz pulse. We demonstrate control over the temporal profile of the THz waveform to maximize optical-to-THz conversion efficiency.

Optical properties and luminescence dynamics ofEu3 + -doped terbium orthophosphate nanophosphors

The development of luminescent inorganic nanocrystals (NCs) doped withrare-earth (RE) ions has attracted increasing interest owing to their distinctoptical properties and versatile applications in time-resolved bioassays,multiplex biodetection, DNA hybridization and bioimaging. HexagonalTbPO4:Eu3 + NCs (10–30 nm) were synthesized via a facile hydrothermal method assisted with oleic acid (OA)surfactants, which exhibit tunable emissions from green to red by varying the concentration ofEu3 + . TheTb3 + -to-Eu3 + energy transfer efficiency observed reaches up to 94%. Different from theirbulk counterparts, a new interface-state band (316 nm) in addition to thecommonly observed spin-forbidden 4f–5d transition band (265 nm) ofTb3 + wasfound to be dominant in the excitation spectrum of NCs due presumably to the formation of surfaceTbPO4/OA complexes,which provides an additional excitation antenna in practical utilization. Two kinds of luminescence sitesof Eu3 + inTbPO4 NCs, with the sitesymmetry of C2 or C1, were identified based on the emission spectra at 10 K and roomtemperature. Furthermore, the photoluminescence (PL) dynamics ofTb3 + ions inpure TbPO4 NCs have been revealed. Compared to the exponential PL decay in bulk counterpartsinduced by very fast energy migration, the non-exponential decay from 5D4 ofTb3 + inTbPO4 NCsis mainly attributed to the diffusion-limited energy migration due to more rapid energy transfer fromTb3 + to defects than theenergy migration among Tb3 + .

Modeling of Electron Cyclotron Heating in Large Tokamaks and ITER with the 3-D Full-Wave Code STELEC

We present modeling results of basic electron cyclotron heating (ECH) scenarios in several tokamaks and ITER performed with the most recent version of the three-dimensional (3-D) full-wave STELEC code (stellarator ECH, including tokamaks as a special case). This code includes all basic wave physics such as interference, diffraction, wave tunneling, mode conversion to electron Bernstein waves at the upper hybrid resonance (UHR), and appropriate boundary conditions. The code solves the wave equations in real 3-D magnetic geometry and thanks to the use of massive parallel teraflop computers, it is the first to provide full-wave solutions of the problem in toroidal plasmas. Several important new results are thus obtained that cannot be predicted with codes based on ray-tracing techniques, such as the influence of diffraction effects and the importance of the UHR for both X- and O-mode antenna excitation at fundamental harmonic. This last result also shows that the so-called “O and X” modes are coupled solutions. The coupling of these modes, partly supported by experiments in the DIII-D tokamak showing similar heating efficiencies for both radiated modes, leads to different power deposition profiles and spatial distribution, compared to results from ray-tracing codes. Coupling between the O-mode and the X-mode (launched at the low-field side) reveals the importance of electron Bernstein waves in ECH calculations for high-density ITER plasmas. These results not only could influence the predictions for neoclassical tearing mode suppression for ITER using electron cyclotron current drive but could also lead to important simplifications in ECH hardware (converters, polarization, etc.) and to a reduced cost of the ECH system on ITER. The code also allowed investigation of the urgent issue of the O-X-B ECH scenario for overdense tokamak/stellarator plasmas.

Simple printed monopole slot antenna for penta-band wireless wide area network operation in the mobile handset

A simple printed monopole slot antenna for penta-band wireless wide area network operation (824–960/1710–2170 MHz) in the mobile handset is presented.The monopole slot has a length of 50 mm only and is embedded close to the bottom edge of the system ground plane of the handset with a small distance of 11 mm. By adding a small-size C-shaped strip connected orthogonal to the bottom edge of the system ground plane, good excitation of the monopole slot antenna at its fundamental (0.25 wavelength) and higher-order resonant modes can be obtained. The excited resonant modes form two wide operating bands to cover the GSM850/900 operation (824–960 MHz) and GSM1800/1900/UMTS operation (1710–2170 MHz). Detailed operating principle and radiation characteristics including the specific absorption rate results of the proposed monopole slot antenna are described. Measured results of the fabricated antenna are also presented. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1399–1404, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26017

Influence of the Aperture Edge Diffraction Effects on the Mutual Coupling Compensation Technique in Small Planar Antenna Arrays

Influence of the Aperture Edge Diffraction Effects on the Mutual Coupling Compensation Technique in Small Planar Antenna Arrays In this paper the quality of a technique to compensate for mutual coupling (and other phenomena) in small linear antenna arrays is investigated. The technique consists in calculation of a coupling matrix, which is than used to determine corrected antenna array excitation coefficients. Although the technique is known for more than 20 years, there is still very little information about how different phenomena existing in a real antenna arrays influence its performance. In this paper two models of antenna arrays are used. In the first model the effect of mutual coupling is separated from the aperture edge diffraction. In the second model antenna both mutual coupling and aperture edge diffraction effects are included. It is shown that mutual coupling itself can be compensated very well and an ultralow sidelobe level (i.e. -50 dB) could be achieved in practice. In the presence of diffraction effects -46.3 dB sidelobe level has been attained, but radiation pattern can be controled only in narrow angle range (i.e. up to ±60°).

SYNTHESIS OF THINNED PLANAR CONCENTRIC CIRCULAR ANTENNA ARRAYS --- A DIFFERENTIAL EVOLUTIONARY APPROACH

Circular antenna array design is one of the most important electromagnetic optimization problems of current interest. The problem of designing a large multiple concentric planar thinned circular ring arrays of uniformly excited isotropic antennas is considered in this paper. This antenna must generate a pencil beam pattern in the vertical plane along with minimized side lobe level (SLL). In this paper, we present an optimization method based on an improved variant of one of the most powerful real parameter optimizers of current interest, called Difierential Evolution (DE). Two sets of difierent cases have been studied here. First set deals with thinned array design with the goal to achieve number of switched ofi elements equal to 220 or more. The other set contains design of array while maintaining side lobe level (SLL) below a flxed value. Both set contains two types of design, one with uniform inter-element spacing flxed at 0.5‚ and the other with optimum uniform inter-element spacing. The half-power beam width of the synthesized pattern is attempted to maintain flxed at the value equal to that of a fully populated array with uniform spacing of 0.5‚. Simulation results of the designed thinned arrays are compared with a fully populated array for all the cases to illustrate the efiectiveness of our proposed method.

Lamp Impedance Analysis of the Compact HID Lamps Sustained by Antenna Excited Microwave Discharge

A compact metal halide lamps have showed high luminous efficacy and long lamp life time, when they are ignited by an antenna-excited microwave discharge (hereafter abbreviated by AEMD). The impedance of the lamp in the steady state ignition has been derived to discuss properties and structures of the lamp, using the equivalent electric circuits for the microwave circuit and the lamp. The expressions for the resistance and reactance of the lamp impedance are applied to deduce the matching and resonance conditions to efficiently feed the microwave power into the lamp. It is found that these expressions not only explain measured lamp properties but also contribute to design the HID lamp structures.

FIRE FLY AND ARTIFICIAL BEES COLONY ALGORITHM FOR SYNTHESIS OF SCANNED AND BROADSIDE LINEAR ARRAY ANTENNA

This paper describes the application of two recently developed metaheuristic algorithms known as flre ∞y algorithm (FFA) and artiflcial bees colony (ABC) optimization for the design of linear array of isotropic sources. We present two examples: one for broad side arrays and the other for steerable linear arrays. Three instances are presented under each category consisting of difierent numbers of array elements and array pattern directions. The main objective of the work is to compute the radiation pattern with minimum side lobe level (SLL) for specifled half power beam width (HPBW) and flrst null beam width (FNBW). HPBW and FNBW of a uniformly excited antenna array with similar size and main beam directions are chosen as the beam width constraints in each case. Algorithms are applied to determine the non-uniform excitation applied to each element. The efiectiveness of the proposed algorithms for optimization of antenna problems is examined by all six sets of antenna conflgurations. Simulation results obtained in each case using both the algorithms are compared in a statistically signiflcant way. Obtained results using flre ∞y algorithm shows better performances than that of artiflcial bees colony optimization technique provided that the same number of function evaluations has been considered for both the algorithms.

Gyrokinetic particle simulations of reversed shear Alfvén eigenmode excited by antenna and fast ions

Global gyrokinetic particle simulations of reversed shear Alfvén eigenmode (RSAE) have been successfully performed and verified. We have excited the RSAE by initial perturbation, by external antenna, and by energetic ions. The RSAE excitation by antenna provides verifications of the mode structure, the frequency, and the damping rate. When the kinetic effects of the background plasma are artificially suppressed, the mode amplitude shows a near-linear growth. With kinetic thermal ions, the mode amplitude eventually saturates due to the thermal ion damping. The damping rates measured from the antenna excitation and from the initial perturbation simulation agree very well. The RSAE excited by fast ions shows an exponential growth. The finite Larmor radius effects of the fast ions are found to significantly reduce the growth rate. With kinetic thermal ions and electron pressure, the mode frequency increases due to the elevation of the Alfvén continuum by the geodesic compressibility. The nonperturbative contributions from the fast ions and kinetic thermal ions modify the mode structure relative to the ideal magnetohydrodynamic (MHD) theory. The gyrokinetic simulations have been benchmarked with extended hybrid MHD-gyrokinetic simulations.

Gyrokinetic particle simulation of beta-induced Alfvén eigenmode

The beta-induced Alfvén eigenmode (BAE) in toroidal plasmas is studied using global gyrokinetic particle simulations. The BAE real frequency and damping rate measured in the initial perturbation simulation and in the antenna excitation simulation agree well with each other. The real frequency is slightly higher than the ideal magnetohydrodynamic (MHD) accumulation point frequency due to the kinetic effects of thermal ions. Simulations with energetic particle density gradient show exponential growth of BAE with a growth rate sensitive to the energetic particle temperature and density. The nonperturbative contributions by energetic particles modify the mode structure and reduce the frequency relative to the MHD theory. The finite Larmor radius effects of energetic particles reduce the BAE growth rate. Benchmarks between gyrokinetic particle simulation and hybrid MHD-gyrokinetic simulation show good agreement in BAE real frequency and mode structure.

Active excitation and damping rate measurement of intermediate-n toroidal Alfvén eigenmodes in JET, C-Mod and MAST plasmas

The stability properties of Alfvén eigenmodes (AEs) are investigated directly using external antenna excitation and detection of stable modes in a variety of plasma configurations in different devices. Dedicated methods to measure the AE damping rate separately from the fast ion drive have been pioneered at JET, using low toroidal mode number internal saddle coil antennas. Other experiments have since installed localized in-vessel antennas to drive and detect MHD modes in the Alfvén frequency range, first on C-Mod, then on MAST. Experiments on C-Mod proved for the first time that intermediate-n toroidal AEs can be driven and detected, and point out significant differences with respect to the low-n regime on JET in the values and scaling of the damping rate with plasma parameters, e.g. the edge shape. On JET, a new antenna system, comprising two assemblies of four toroidally spaced coils each, was developed to replace the low-n saddle coil structure and excite AEs in the toroidal mode number range that is expected to be most unstable in ITER, with n up to ∼10. Experiments with the new JET antennas confirm that excitation is possible in a large volume plasma, together with real-time tracking of core modes throughout the limiter and divertor phases of high performance discharges, with significant additional heating. The similarities and differences between the active MHD antenna systems as well as a comparison of the results on C-Mod, JET and MAST are illustrated. Both C-Mod and JET results underline the fact that a precise reconstruction of the mode structure and its spectrum, important for a quantitative comparison with theoretical models, represents a significant challenge in the intermediate-n range and in the presence of several modes.

Exciton dynamics in photosynthetic complexes: excitation by coherent and incoherent light

In this paper, we consider the dynamics of a molecular system subjected to external pumping by a light source. Within a completely quantum mechanical treatment, we derive a general formula, which enables us to assess the effects of different light properties on the photo-induced dynamics of excitations in a molecular system. We show that, once the properties of light are known in terms of a certain two-point correlation function, the only information needed to reconstruct the system dynamics is the reduced evolution superoperator. The latter quantity is, in principle, accessible through ultrafast nonlinear spectroscopy. Considering a direct excitation of a small molecular antenna by incoherent light, we find that excitation of coherences is possible due to the overlap of homogeneous line shapes associated with different excitonic states. In Markov and secular approximations, the amount of coherence is significant only under fast relaxation, and both the populations and coherences between exciton states become static at long times. We also study the case when the excitation of a photosynthetic complex is mediated by a mesoscopic system. We find that such a case can be treated by the same formalism with a special correlation function characterizing ultrafast fluctuations of the mesoscopic system. We discuss bacterial chlorosome as an example of such a mesoscopic mediator and propose that the properties of energy-transferring chromophore–protein complexes might be specially tuned to the fluctuation properties of their associated antennae.

Large-area high-density helicon plasma sources

High-density (1012–1013 cm−3) helicon plasmas produced using two different but similar devices whose diameters are very large, 40 cm and 74 cm, respectively, have been characterized. A scaling of the particle production efficiency has been investigated in detail into a low aspect ratio (the ratio of the diameter to the axial length) region down to 0.075. It has been found that reducing this ratio manifests the standing wave-like patterns of the excited radio frequency (rf) fields. Spatial profiles of the electron density and the rf wave fields are measured, showing the effectiveness of the control of these profiles by changing the magnetic field configurations near the excitation antenna. Finally, high-beta plasmas with up to β ∼ 0.8, that exhibit diamagnetic character, where β is the ratio of the plasma pressure to the magnetic pressure, have been produced with low magnetic fields.

Imaging Individual Proteins and Nanodomains on Intact Cell Membranes with a Probe‐Based Optical Antenna

Optical antennas that confine and enhance electromagnetic fields in a nanometric region hold great potential for nanobioimaging and biosensing. Probe-based monopole optical antennas are fabricated to enhance fields localized to <30 nm near the antenna apex in aqueous conditions. These probes are used under appropriate excitation antenna conditions to image individual antibodies with an unprecedented resolution of 26 +/- 4 nm and virtually no surrounding background. On intact cell membranes in physiological conditions, the obtained resolution is 30 +/- 6 nm. Importantly, the method allows individual proteins to be distinguished from nanodomains and the degree of clustering to be quantified by directly measuring physical size and intensity of individual fluorescent spots. Improved antenna geometries should lead to true live cell imaging below 10-nm resolution with position accuracy in the subnanometric range.

INPUT IMPEDANCE OF A CYLINDRICAL MICROSTRIP ANTENNA WITH AN ARBITRARY GEOMETRY RADIATOR EXCITED BY A COAXIAL LINE

The problem of a cylindrical microstrip antenna excitation by a coaxial line (rod) is considered. The coaxial line is modeled by a finite-length filament of electric current. The problem is solved by the method of moments in the spectral domain using piece-wise specified functions. Such an approach allows the class of possible problems to be essentially expanded, specifically, it makes possible to analyze antennas with radiators of arbitrary geometries. The frequency dependence of the return loss amount is considered for the antenna with an E-shaped radiator.

Optical antenna thermal emitters

Optical antennas are a critical component in nanophotonics research1 and have been used to enhance nonlinear2,3 and Raman4 cross-sections and to make nanoscale optical probes5. In addition to their ‘receiving’ properties, optical antennas can operate in ‘broadcasting’ mode, and have been used to modify the emission rate6 and direction7 of individual molecules. In these applications the antenna must operate at frequencies given by existing light emitters. Using thermal excitation of optical antennas, we bypass this limitation and realize emitters at infrared frequencies where sources are less readily available. Specifically, we show that the thermal emission from a single SiC whisker antenna is attributable to well-defined, size-tunable Mie resonances8. Furthermore, we derive a fundamental limit on the antenna emittance and argue theoretically that these structures are nearly ideal black-body antennas. Combined with advancing progress in antenna design, these results could lead to optical antenna emitters operating throughout the infrared frequency range. Single SiC whiskers can be made into infrared emitters by thermal excitation. The broadband thermal emission is coupled to the electromagnetic resonances of the whisker, allowing relatively narrowband emission at infrared frequencies. The emission frequency can be tuned by adjusting the size of the whiskers.