Theodoros N. Kaifas

On the design of a single-layer wideband Butler matrix for switched-beam UMTS system applications [Wireless Corner]

A Butler matrix (BM) for UMTS applications is presented. The Butler matrix was fabricated by conventional photolithography on a single laminate. The design is compact and complies with the required bandwidth. The design is based on the use of an elliptical coupler with certain geometrical characteristics, and a Lange coupler as a crossover. Furthermore, a design-scaling procedure that produces an N times N Butler matrix by using two (N/2)times(N/2) Butler matrices is given. The presented performance extrapolations clarify the potential and scalability of our approach

Analysis of Printed Antennas Mounted on a Coated Circular Cylinder of Arbitrary Size

An efficient numerical algorithm for the analysis of printed antennas on a dielectric coated circular cylinder is presented. The formulation employs the spectral domain approach. Greens function is evaluated via Olvers uniform asymptotic expansions. The Fourier summation is transformed into integration and the asymptotic part of the integral is evaluated in closed form. The whole procedure results in a numerical algorithm that is at least as accurate and efficient as the respective one for the planar structure. The present approach can be applied for any source-receiver position and any cylinder radius

Aperiodic Array Layout Optimization by the Constraint Relaxation Approach

An optimization procedure for the layout assessment of electrically large but finite planar arrays is presented. The synthesis takes into account the desired directivity pattern that is prescribed employing bound constraints. Moreover, the size of the radiators is taken into account, which results in a hard nonoverlapping, between the elements, constraint. The latter should not be violated if we want the attained solution not only to obey the far-field mask, but also to be physically realizable. As stated, the optimization problem is twofold. An antenna design is associated with a packing problem. In order to take the constraints on the layout into account and solve the whole problem, we propose the constraint relaxation approach, which is equipped with a packing algorithm. Our study is applied to various initial geometries, and the resulting arrays appear to comply with the desired pattern and the nonoverlapping constraint. Several examples for different cases including symmetric arrays and a study on maximally sparse arrays are presented, which show the applicability and merit of the method.

Direct Rational Function Fitting Method for Accurate Evaluation of Sommerfeld Integrals in Stratified Media

The current work proposes a direct rational function fitting method, employing cylindrical waves alone, for the accurate evaluation of Sommerfeld integrals for planar multilayered structures. Three are the key points of the effort. 1) Until now, relative works require the extraction of the quasi-static/asymptotic terms, and branch cut/continuous wave contribution explicitly. In the current one, the explicit treatment of those terms is avoided. The proposed methodology is based on the direct fitting of the spectrum of the Greens function by rational functions. Thus, it provides the spatial Greens function solely in terms of cylindrical waves. 2) The effectiveness, robustness and accuracy improvement of the rational function fit rely upon the proper sampling of the spectrum. This accurate fitting is possible because instead of avoiding large variations of the spectral kernel, we introduce proper paths to include more variation, and thus more spectrum information, before we apply the modified VECTFIT algorithm. 3) Furthermore, proper weighting of the VECTFIT is proposed in order to guide the algorithm in providing increased accuracy in specific desired areas of the horizontal distance between the source and observation points. Armed with the above the direct rational function fitting method provides accurate results both for the near and far-field regions. Various examples, among them the correct treatment of a two branch case, are given that prove the excellent performance and robustness of the proposed approach.

On the Geometry Synthesis of Arrays With a Given Excitation by the Orthogonal Method

A geometry synthesis procedure for arrays by the help of the orthogonal method (OM) is given in the work at hand. We start from an initial array with a given excitation and we perturb the element positions by combining an iterative technique with the OM. The final position of the elements is found from the last iteration where the desired approximation of the pattern is obtained. It is noticed that our formulation does not give always a successful outcome, since the search can be trapped in a local minimum. Thus, the successful issue comes from a suitable selection of the initial array. Cases with uniformly excited arrays or arrays with less number of excitations than the number of the elements are studied. Several examples for different cases will be presented and will show the applicability of the method.

A Stochastic Study of Large Arrays Related to the Number of Electrically Large Aperture Radiators

A study of the maximally sparse large planar arrays with electrically large elements is presented. The conditional probabilities of the element placements and their resulting auxiliary radiation integrals are derived. Through them the average and the directivity pattern formulas are also derived. Employing these formulas, we present a convex method that provides a solution to the maximally sparse problem when main lobe constraints are imposed on the directivity pattern. In particular, by taking the possible types of elements into account, we manage to obtain the lower bound of the directivity that an array should exhibit. This lower bound is analytically derived in the form of a Pareto (sub-) set that classifies the possible arrays into feasible and nonfeasible ones. This Pareto set can also enable tradeoff studies to be conducted without the need to consider the full range of every parameter. From the procedure, several acceptable combinations of elements are obtained. Simulation results, which confirm the methodology, are presented.

Design of Planar Arrays With Reduced Nonuniform Excitation Subject to Constraints on the Resulting Pattern and the Directivity

A design procedure for the synthesis of planar arrays is presented. Synthesis takes into account the desired pattern and the constraint on the directivity index. It makes use of the perturbation method in conjunction with the Lagrange multiplier theory. Starting from an initial array we perturb the element positions by employing an iterative technique. In the iteration, by setting the first variation of the Lagrangian (Cost function) equal to zero we derive the perturbed positions. Perturbation is applied simultaneously along two independent variables, (element coordinates x and y). The final position of the elements results from the last iteration where the stopping criteria are met. The choice of the initial array takes into account the number of elements and the reduction need of nonuniformity in the excitation. Our study is used for various initial geometries and the resulting arrays are shown to comply with the desired pattern and the directivity index.

Dual-Band Fractal Semi-Printed Element Antenna Arrays for MIMO Applications

Novel dual-element antenna arrays suitable for small devices of multiple-input-multiple-output (MIMO) applications have been designed and fabricated. The antenna elements have semi-printed structure, are fractally shaped, and operate in ISM (2.4-2.489 GHz) band as well as between 5 and 6 GHz. Although the size of the designed antenna prototypes is small and the elements are positioned too close to each other, correlation coefficients lower than 0.1 and satisfactory levels of mean effective gain have been obtained.

Direct radiating array design via convex aperture synthesis, pareto front theory, and deterministic sampling [antenna designer's notebook]

A deterministic method for the design of direct radiating arrays for multibeam satellite applications is presented in the current work. The method embodies the following steps: First, we consider the problem of continuous aperture synthesis, subject to directivity-pattern mask constraints. We succeed in stating the problem as a convex magnitude antenna design that exhibits a second-order cone program form. The solution provides the optimum continuous aperture current-density distribution. Second, exploiting the optimum solution, analytical formulas, in the form of the Pareto front, are presented through which the proposed method determines the minimum number of radiating elements that must be used in the process of transforming the continuous aperture to an antenna array. Third, an effective deterministic sampling approach is contributed to produce a discrete array with the desired characteristics. Armed with the above, the current work provides a generalized framework for the design of direct radiating arrays for multibeam satellite applications. The method is validated by the provided design examples.

Direction of Arrival (DoA) estimation for a Switched-Beam DS-CDMA System using Neural Networks

A new direction of arrival (DoA) estimation method using neural networks (NNs) is presented. The method is especially designed for a switched-beam system (SBS) that operates in an asynchronous direct sequence code division multiple access (DS-CDMA) scheme. The technique exploits the Multiple Access Interference (MAI) suppression done by the code-matched filters in DS-CDMA, and is simple and appropriate for real time applications. Simulations of DoA estimations tests show accurate results for a varying number of interferers.