The problem of the synthesis of optimal continuous aperture sources to optimally realize the satellite multibeam coverage of Earth is stated and solved. The design approach relies on a far-field representation which exploits at best the degrees of freedom arising from the geometrical structure of the well-known four-colors colour coverage map. The overall synthesis is stated as a convex-programming problem wherein the fast achievement of the (unique) globally optimal solution is guaranteed. The introduced tools allow stating the ultimate theoretical radiation performances achievable by any circular-aperture antenna of fixed size and, at the same time, can be exploited as a reference in the synthesis of isophoric direct-radiating arrays. Numerical examples concerning a mission scenario recently proposed by the European Space Agency are provided.
(Aim) Pathological brain detection (PBD) systems aim to assist and even replace neuroradiologists to make decisions for patients. This review offers a comprehensive and quantitative comparison for PBD systems by artificial intelligence in magnetic resonance imaging (MRI) scanning. (Method) We first investigated four categories of brain diseases, including neoplastic disease, neurodegenerative disease, cerebrovascular disease, and inflammation. Next, we introduced important MRI techniques, such as the shimming, water and fat suppression, and three advanced imaging modalities (functional MRI, diffusion tensor imaging, and magnetic resonance spectroscopic imaging). Then, we discussed four image preprocessing techniques (image denoising, slice selection, brain extraction, spatial normalization, and intensity normalization), seven feature representation techniques (shape, moment, wavelet, statistics, entropy, gray level co-occurrence matrix, and Fourier transform), and two dimension reduction techniques (feature selection and feature extraction). Afterwards, we studied classification related methods: six learning models (decision tree, extreme learning machine, k-nearest neighbors, naive Bayes classifier, support vector machine, feed-forward neural network), five kernel functions (linear, homogeneous and inhomogeneous polynomial, radial basis function, and sigmoid), and three types of optimization methods (evolutionary algorithm, stochastic optimization, and swarm intelligence). (Results) We introduced three benchmark datasets and used Kfold stratified cross validation to avoid overfitting. We presented a detailed quantitative comparison among 44 state-of-the-art PBD algorithms and discussed their advantages and limitations. (Discussions) Artificial intelligence is now making stride in the PBD field and enjoys a fair amount of success. In the future, semi-supervised learning and transfer learning techniques may be potential breakthroughs to develop PBD systems.
We deal with one of the computationally most critical steps of the Phase-Only synthesis of aperiodic reflectarrays, namely the fast evaluation of the radiation operator. We present an approach exploiting the use of a fast numerical algorithm using 2D Non-Uniform FFTs (NUFFTs) of NED (Non-Equispaced Data) and NER (non equispaced results) type and of parallel processing on Graphic Processing Units (GPUs). We extend the approach in K. Fourmont, J. Fourier Anal. Appl., vol. 9, n. 5, pp. 431-540, 2013 for implementing NUFFT routines to the 2D case and illustrate the parallel strategies to accelerate the approach. In particular, we show how the two levels of parallelism intrinsic in the interpolation step of the 2D NED-NUFFT can be fruitfully exploited by adopting dynamic parallelism, a feature made available in one of the latest architecture of NVIDIA cards. The presented synthesis results show that the introduction of further degrees of freedom (positions) allows improving the performance with respect to periodic reflectarrays. Also, the possibility of adopting aperiodic reflectarrays of reduced number of elements for fixed performance is demonstrated.
The high frequency scattering problems of electromagnetic fields scattered from electrically large scatterers are important and challenging. On the calculation of the reflected and diffracted wave fields, the high frequency methods could be classified into the current based method and the ray based method. In this paper, first, we give a review on the progress of the modern high frequency methods for solving the electromagnetic scattering problems. Next, due to the highly oscillatory property of the high frequency electromagnetic scattered fields, we propose the numerical steepest descent path method. Finally, we comprehensively address the high frequency wave physics, including the high frequency critical point contributions, the Keller's cone, the shadow and reflection boundaries and the creeping wave fields.
This paper presents a comprehensive method to analyze negative group delay (NGD) phenomenon at microwave frequency. This method is based on a coupling matrix with finite unloaded quality factor resonators. Unlike conventional NGD circuit topologies that use a lumped resistor R along with bandstop resonators, the proposed topology does not require any R for generating NGD and therefore, provides fully distributed circuit realization. The proposed topology has both source to load and inter-resonator coupling structures. Analytical design equations are provided to obtain predefined NGD with matched input/output ports; the proposed structure therefore does not require any extra matching networks. From analytical analysis, it is also found that the NGD bandwidth as well as magnitude flatness can be controlled by inter-resonator couplings. The proposed design theory is proven through fabrications of NGD circuit at a center frequency of 2.14 GHz. The measurement results are in good agreement with simulations and predicted theoretical results.
In this paper, a methodology to design non-50 Ω antennas for energy harvesting is presented. Two prototypes are simulated and realized on an epoxy substrate: one operating at 433 MHz and 900 MHz, the other at 900 MHz and 2.4 GHz. These antennas are designed to match the input impedances of an integrated radio-frequency harvester for an output voltage of 1 V, value chosen considering the voltage needed to power the new generation of micro-controllers and electronic circuits for the Internet of Things. The measurement results indicate a reflection coefficient below -10 dB at the frequencies of interest, validating the methodology.
A circular sector patch antenna loaded with a periodic metamaterial topology is presented. Several shapes of the circular sector patch are analyzed, and the input impedances and radiation patterns are compared. The topology reveals a nearly constant resonant frequency at zeroth-order resonance (ZOR) while the radiation performance approaches the one of the ZOR full circular patch antenna. Compared with rectangular and circular patch antennas, the sector patch offers more tuning possibilities. A matching network can be easily introduced to enhance the impedance bandwidth. Apart from the ZOR characteristics, this topology can also support a quasi-monopolar pattern at multiple modes. A semicircular patch operating at 4.1 GHz together with an impedance matching network and a dual-band semicircular patch antenna are fabricated and measured.
Planar gigahertz (GHz) inductors were fabricated based on high crystalline-anisotropy Zn0.13Co0.04Ni0.63Fe2.2O4 (Zn-Co-Ni ferrite) and Ba3Co2Fe24O41 (Co2Z hexaferrite) and characterized for inductance (L) and quality (Q) factor. The planar ferrite inductors show an L of 4.5 nH (Zn-Co-Ni), 5.6 nH (Zn-Co-Ni + low Hk and fFMR Co2Z:), and 4.8 nH (Zn-Co-Ni + high Hk and fFMR Co2Z:) at 2 GHz. The corresponding L-densities are 18.0, 22.4, and 19.2 nH/mm2, which are greater than 16.8 nH/mm2 of the air-core inductor. With respect to the Q factor, the air-core and ferrite inductors exhibit Q factors of 6.7 (air-core), 4.8 (Zn-Co-Ni), 2.8 (Zn-Co-Ni + low Hk Co2Z), and 4.0 (Zn-Co-Ni + high Hk Co2Z) at 2 GHz. The tan δμ of the ferrites caused a reduction in the Q factor. Nevertheless, the high Hk and fFMR Co2Z ferrite inductor demonstrates a higher Q factor than that of the low Hk and fFMR Co2Z inductor. It is, therefore, suggested that high-resistivity, -anisotropy, -magnetization ferrite can produce large L-density and Q-factor GHz inductors.
A low-profile three-current model is proposed to guide the design of an on-board antennas with a broad beam. Based on this model, an on-board three-element antenna is designed. When the side length of the ground is infinite, the 3-dB beamwidths in the xz plane and yz plane are all 180°, and the radiation pattern in the xy plane has a gain fluctuation less than 3 dB. When the side length of the ground is 2λ, the measured 3-dB beamwidths in the xz plane and yz plane are 141° and 148°, respectively. A quasi-hemispherical radiation pattern can be obtained based on the proposed three-current model.
Current microwave hyperthermia applicators are not well suited for uniform heating of large tissue regions. The objective of this research is to identify an optimal microwave antenna array for clinical use in hyperthermia treatment of cancer. For this aim we present a novel 434 MHz applicator design based on a metamaterial zeroth order mode resonator, which is used to build larger array configurations. These applicators are designed to effectively heat large areas extending deep below the body surface and in this work they are characterized with numerical simulations in ahomogenous muscle tissue model. Their performance is evaluated using three metrics: radiation pattern-based Effective Field Size (EFS), temperature distribution-based Therapeutic Thermal Area (TTA), and Therapeutic Thermal Volume (TTV) reaching 41-45°C. For 2×2 and 2×3 array configurations, the EFS reaching > 25% of maximum SAR in the 3.5 cm deep plane is 100% and 91% of the array aperture area, respectively. The corresponding TTA for these arrays is 95% and 86%, respectively; and the TTV attaining > 41°C is over 85% of the aperture area toa depth of over 3 cm in muscle, using either array configuration. With theoretical heating performance exceeding that of existing applicators, these new metamaterial zero order resonator arrays show promise for future applications in large area superficial hyperthermia.