Benjamin Fuchs

Synthesis of Sparse Arrays With Focused or Shaped Beampattern via Sequential Convex Optimizations

An iterative procedure for the synthesis of sparse arrays radiating focused or shaped beampattern is presented. The algorithm consists in solving a sequence of weighted l1 convex optimization problems. The method can thus be readily implemented and efficiently solved. In the optimization procedure, the objective is the minimization of the number of radiating elements and the constraints correspond to the pattern requirements. The method can be applied to synthesize either focused or shaped beampattern and there is no restriction regarding the array geometry and individual element patterns. Numerical comparisons with standard benchmark problems assess the efficiency of the proposed approach, whose computation time is several orders of magnitude below those of so-called global optimization algorithms.

A Printed Transition for Matching Improvement of SIW Horn Antennas

The substrate integrated waveguide (SIW) technology allows to construct several types of commonly used antennas in a planar way. However, frequency limitations associated to commercial substrates appear in the implementation of certain types of antennas, e.g., SIW horn antennas are not well matched when the substrate thickness is much smaller than the wavelength. A printed transition is proposed to overcome this problem. Differently from current solutions, no bulky elements are required allowing to maintain the most important features of this technology namely its compactness and ease of manufacturing. In order to quickly analyze and design the transition, both a coupled resonator and a transmission line models are developed, together with design guidelines. The proposed transition is designed to match a H-plane SIW horn antenna built in a thin substrate

The Effect of Insulating Layers on the Performance of Implanted Antennas

({\rm thickness}<\lambda_{0}/10)

Design and characterization of half Maxwell fish-eye lens antennas in millimeter waves

at different frequency bands at the Ku-band. Experimental results for 3 different transitions show that the matching characteristics are efficiently improved compared with the conventional SIW horn antenna and validates the proposed models.

Comparative Design and Analysis of Luneburg and Half Maxwell Fish-Eye Lens Antennas

This work presents the analysis of the influence of insulation on implanted antennas for biotelemetry applications in the Medical Device Radiocommunications Service band. Our goal is finding the insulation properties that facilitate power transmission, thus enhancing the communication between the implanted antenna and an external receiver. For this purpose, it has been found that a simplified model of human tissues based on spherical geometries excited by ideal sources (electric dipole, magnetic dipole and Huygens source) provides reasonable accuracy while remaining very tractable due to its analytical formulation. Our results show that a proper choice of the biocompatible internal insulation material can improve the radiation efficiency of the implanted antenna (up to six times for the investigated cases). External insulation facilitates the electromagnetic transition from the biological tissue to the outer free space, reducing the power absorbed by the human body. Summarizing, this work gives insights on the enhancement of power transmission, obtained with the use of both internal, biocompatible and external, flexible insulations. Therefore, it provides useful information for the design of implanted antennas.

Application of Convex Relaxation to Array Synthesis Problems

This paper presents the performance of multilayered half Maxwell fish-eye (HMFE) lenses fed by aperture-coupled microstrip patch antennas. Manufacturing techniques are reviewed and the shell technique is retained. Many lens configurations are investigated and compared using a full-wave electromagnetic software at 50 GHz. We report the effects of the number of shells, diameter of the lens, and distance between the primary source and the lens on the input impedance, broadside directivity, and aperture efficiency. Thus, we show that aperture efficiencies up to 95% can be obtained for a one-wavelength-diameter lens with only three shells, justifying the interest in such lenses. An analytical optimization method is also proposed and detailed to choose the thickness and permittivity of a three-shell HMFE lens to approach the radial permittivity law as well as possible. Simulations of lens antennas whose shell characteristics are determined by various ways show that the optimized lens is the one that provides the highest broadside directivities. Finally, measurements done with a three-shell four-wavelength diameter lens fed by a 2 times 2 patch antenna array show the validity of these simulations. To our knowledge, this represents the first layered HMFE lens carried out in the millimeter-wave frequency range

Influence of the implanted pulse generator as reference electrode in finite element model of monopolar deep brain stimulation

The design and performances is described of half Maxwell fish-eye (HMFE) and Luneburg lens antennas. Techniques to estimate the gradient index lens weights are first proposed and compared to measurements. Thanks to an analytical method based on spherical modal expansion, the radiation performances of Luneburg and HMFE lens antennas are then quantified and compared. The effects of the shell number, the lens diameter and the air gaps on the lens antenna directivity are investigated. Finally, a minmax optimized 9-shell 60 mm-diameter HMFE lens antenna is characterized at both 77 GHz and 110 GHz for on- and off- axis configurations to show its frequency behavior, focalization and beam scanning properties.

Optimal Polarization Synthesis of Arbitrary Arrays With Focused Power Pattern

A general procedure to solve efficiently non convex array synthesis problems is presented. It is based on the SemiDefinite Relaxation (SDR) technique. The way to properly relax the constraints in order to formulate the synthesis of shaped beams, phase-only arrays and reconfigurable arrays as semidefinite programming problems is detailed. These so-approximated array synthesis problems are then convex, easy to implement and can be efficiently solved using off-the-shelf numerical routines. The conditions under which the relaxed problems provide the optimal solution to the original non convex synthesis problems are specified. Various representative numerical comparisons with arrays designed by other approaches show the validity of the proposed method and illustrate its potentialities.

Scattering of Spherically and Hemispherically Stratified Lenses Fed by Any Real Source

Electrical deep brain stimulation (DBS) is an efficient method to treat movement disorders. Many models of DBS, based mostly on finite elements, have recently been proposed to better understand the interaction between the electrical stimulation and the brain tissues. In monopolar DBS, clinically widely used, the implanted pulse generator (IPG) is used as reference electrode (RE). In this paper, the influence of the RE model of monopolar DBS is investigated. For that purpose, a finite element model of the full electric loop including the head, the neck and the superior chest is used. Head, neck and superior chest are made of simple structures such as parallelepipeds and cylinders. The tissues surrounding the electrode are accurately modelled from data provided by the diffusion tensor magnetic resonance imaging (DT-MRI). Three different configurations of RE are compared with a commonly used model of reduced size. The electrical impedance seen by the DBS system and the potential distribution are computed for each model. Moreover, axons are modelled to compute the area of tissue activated by stimulation. Results show that these indicators are influenced by the surface and position of the RE. The use of a RE model corresponding to the implanted device rather than the usually simplified model leads to an increase of the system impedance (+48%) and a reduction of the area of activated tissue (-15%).

Fast Antenna Array Diagnosis from a Small Number of Far-Field Measurements

The joint synthesis of the spatial power pattern and polarization of arbitrary arrays is addressed. Specifically, the proposed approach gives the solution to a frequently encountered problem, namely the array design (i.e., the determination of the radiating element weightings) to achieve a pattern that is arbitrarily upper bounded, while its polarization is optimized in a given angular region. Any state of polarization (elliptical, circular and linear) can be synthesized and there is no restriction regarding the array geometry and element patterns. The synthesis problem is rewritten as a convex optimization problem, that is efficiently solved using readily available software. This ensures the optimality of the proposed solution. Various numerical results are presented to validate the proposed method and illustrate its potentialities. The synthesis of a sequentially rotated array is first addressed. Then a linear array of equispaced randomly oriented dipoles is considered. Finally, a conformal and a planar array of patches, where the mutual coupling effects are considered, are synthesized to radiate a linear and a circular polarization.