Spherical lens antenna designs with particle swarm optimization

A design procedure for spherical lens antennas is described. A particle swarm optimization (PSO) algorithm is coupled to a mode matching technique based on spherical wave expansion to analyze the lens antennas. The proposed methodology is applied to three optimization problems using real-number and binary PSO. First, the maximization of the directivity of Luneburg lens antennas is addressed. Then, amplitude shaped radiation patterns are synthesized by optimizing, both amplitude and position of each element of an array that illuminates a lens. Finally, a dual-beam reconfigurable lens antenna is optimized. By only switching properly the elements of an array, the lens antenna radiates either a directive or a sectoral beam. Numerical comparisons with a full wave commercial software successfully validate the proposed design procedure. (C) 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1655-1659, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25278

Low frequency electromagnetic wave propagation in 3D plasma configurations

Pavel Popovitch

We investigate low-frequency electromagnetic wave propagation and absorption properties in 2D and 3D plasma configurations. For these purposes, we have developed a new full-wave 3D code LEMan that determines a global solution of the wave equation in bounded stellarator plasmas excited with an external antenna. No assumption on the wavelength compared to the plasma size is made, all the effects of the 3D geometry and finite plasma extent are included. The equation is formulated in terms of electromagnetic potentials in order to avoid numerical pollution effects. The code utilises linear and Hermite cubic finite element discretisation in the radial direction and Fourier series in the poloidal and toroidal variables. The full cold plasma model including finite electron inertia and, thus, mode conversion effects is implemented. The code uses Boozer magnetic coordinates and has an interface to the TERPSICHORE code. Special care is taken to treat the magnetic axis and to ensure the unicity of the numerical solution. The discretisation, interpolation and numerical derivation methods specifically adapted for our problem avoid the energy sink in the origin and provide a very good local and global energy conservation. A special algorithm has been developed to analytically expand the wave equation coefficients in the full 3D stellarator geometry. The code has been specifically optimised for vector computing platform, reaching close to maximum average performances on the NEC SX5 machine. The code has been applied in 1D, 2D, and 3D geometries. No unphysical solutions have been observed. LEMan successfully recovers all the fundamental properties of the Alfvén spectrum (gaps, eigenmodes). Benchmarks have been made against the 2D LION code and JET experimental measurements, showing a good agreement between the results.

EPFL2004

Challenges in Computational Electromagnetics

Ruzica Golubovic

To meet strict requirements of the information society technologies, antennas and circuit elements are becoming increasingly complex. Frequently, their electromagnetic (EM) properties cannot be anymore expressed in closed-form analytical expressions mainly because of the multitude of irregular geometries found in actual devices. Therefore, accurate and efficient (in terms of computational time and memory) electromagnetic models coupled with the robust optimization techniques, are needed in order to be able to predict and optimize the behavior of the innovative antennas in complex environments. The contribution of this thesis consists in the development and improvement of accurate electromagnetic modeling and optimization algorithms for an ubiquitous class of antennas, the planar printed antennas. The approach most commonly used to model and analyze this type of structures is the Integral Equation (IE) technique numerically solved using the Method of Moments (MoM). From the computational point of view, the main challenge is to develop techniques for efficient numerical evaluation of spatial-domain Green's functions, which are commonly expressed in terms of the well-known Sommerfeld integrals (SIs), i.e., semi-infinite range integrals with Bessel function kernels. Generally, the analytical solution of the SIs is not available, and their numerical evaluation is notoriously difficult and time-consuming because the integrands are both oscillatory and slowly decaying, and might possess singularities on and/or near the integration path. Due to the key role that SIs play in many EM problems, the development of fast and accurate techniques for their evaluation is of paramount relevance. This problem is studied in detail and several efficient methods are developed. Finally, the applicability of one of these methods, namely the Weighted Averages (WA) technique, is extended to the challenging case appearing in many practical EM problems: the evaluation of semi-infinite integrals involving products of Bessel functions. However, the development of effective analysis codes is only one aspect. At least equally important is the availability of reliable optimization techniques for an adequate design of antennas. For that purpose, the Particle Swarm Optimization (PSO) algorithm is introduced in the context of our analysis codes. Moreover, the innovative hybrid version of the PSO algorithm, called the Tournament Selection PSO, has been proposed with the aim of even further improving convergence performances of the classical PSO algorithm. Detailed theoretical description of this socially inspired evolutionary algorithm is given in the thesis. Finally, the characteristics of both algorithms are compared throughout several EM optimization problems.

EPFL2011

Reconfigurable Antennas for Beam-Space MIMO Transmission with a Single Radio

Mohsen Yousefbeiki

MIMO techniques allow remarkable improvements in the reliability and/or transmission rate of wireless communication systems. However, there are several major challenges towards the implementation of conventional MIMO concept in terminals with size, cost, and power constraints. Firstly, insufficient space impedes the design of efficient and decorrelated MIMO antennas. Second, MIMO traditionally demands each antenna to be fed by its own RF chain, which in turn results in greater hardware complexity, larger power consumption, and higher implementation cost. Among all reduced-complexity and antenna-decoupling schemes proposed so far, the so-called beam-space MIMO has attracted a great deal of interest as a potential solution for addressing both problems concurrently. The key idea therein is to engineer the radiation pattern of a single-feed antenna structure for each symbol period, such that multiple independent symbols directly modulate a predefined set of orthogonal virtual patterns in the far-field, therefore allowing true MIMO transmission using a single RF chain and a compact antenna structure. More important in practice, the transmitted information can be retrieved using a conventional MIMO receiver. However, the transformation of this idea into reality entails dealing with various practical aspects that are commonly overlooked in theoretical and conceptual developments. This dissertation explores the beam-space MIMO concept from the perspective of the antenna engineering, and aims at addressing several key issues associated with the actual design and implementation of beam-space MIMO systems. The early developments of beam-space MIMO concerned switched parasitic arrays. However, the requirement of utilizing several physically-separate radiators is inconvenient for practicable implementation in compact portable devices. To solve this problem, a single-radiator load-modulated antenna solution is proposed in this dissertation. Another primary challenge consists in emulating high-order modulation schemes such as PSK with realistic hardware. Here, an efficient beam-space MIMO strategy is developed, which allows transmitting PSK data streams of any modulation order using only purely reactive reconfigurable loads, and without the need for a symbol-rate dynamic matching network. The approach is illustrated by the design and fabrication of a realistic antenna for QPSK signaling. The performance of a beam-space MIMO system which utilizes the fabricated antenna is then investigated through over-the-air experiments, and compared with conventional MIMO in realistic environments. Embedding information in the radiation patterns, beam-space MIMO systems are expected to be inherently prone to multiplexing performance degradation in the presence of external field perturbation. This makes the study of near-field interaction influence on beam-space MIMO distinct from those carried out for the case of conventional systems. This issue is considered for the first time in this dissertation. Moreover, like any reconfigurable system, a beam-space MIMO system may suffer from bandwidth expansion of the transmitted signals. The final part of the work is directed towards this important issue. To reduce out-of-band radiation effect, a solution based on shaping the time-domain response of the reconfigurable components is presented. The studies presented in this thesis constitute a crucial step towards MIMO with simpler and cheaper hardware for real-life terminals.