A wideband switched beam antenna array system operating from 2 to 5 GHz is presented. It is comprised of a 4 × 1 Vivaldi antenna elements and a 4 × 4 Butler matrix beamformer driven by a digitally controlled double-pole four-throw RF switch. The Butler matrix is implemented on a multilayer structure, using 90° hybrid couplers and 45° phase shifters. For the design of the coupler and phase shifter, we propose a unified methodology applied, but not limited, to elliptically shaped geometries. The multilayer realization enables us to avoid microstrip crossing and supports wideband operation of the beamforming network. To realize the Butler matrix, we introduce a step-by-step and stage-by-stage design methodology that enables accurate balance of the output weights at the antenna ports to achieve a stable beamforming performance. In this paper, we use a Vivaldi antenna element in a linear four-element array, since such element supports wideband and wide-scan angle operation. A soft condition in the form of corrugations is implemented around the periphery of the array, in order to reduce the edge effects. This technique improved the gain, the sidelobes, and helped to obtain back radiation suppression. Finally, impedance loading was also utilized in the two edge elements of the array to improve the active impedance. The proposed system of the Butler matrix in conjunction with the constructed array can be utilized as a common RF front end in a wideband air interface for a small cell 5G application and beyond as it is capable to simultaneously cover all the commercial bands from 2 to 5 GHz.

The quality of small array antennas in airborne monopulse systems can be significantly reduced by the radome. We therefore present a convex optimization approach to minimize radome effects in monopulse arrays. This is achieved by using active element patterns in the optimization to determine the excitation weights. Simulation results for a BoR array with 48 elements and an extended hemispherical radome are presented. We demonstrate that it is possible to reduce the side-lobe level by 3.5 dB by taking radome effects into account in the optimization. This approach also results in an increased gain, particularly at large scan angles. Furthermore, the presented approach allows the monopulse slope to be indirectly specified as a design parameter. It is shown that the trade-off between the monopulse slope coefficient and the side-lobe level is approximately linear.

The four-quadrant monopulse array is widely used for direction of arrival (DOA) estimation. Errors in the angle estimate are introduced when installing the array on a platform, due to unwanted reflections in the platform, as well as reflection and refraction in the radome. These installation effects are captured in the installed element patterns, which can be computed using a number of computational electromagnetics methods. In this paper, we demonstrate that the error introduced in the DOA estimate can be determined from the installed element patterns. To illustrate how the method is used, we present results for two cases: (a) BoR-array without radome and (b) BoR-array with an extended hemispherical radome. The presented method can be applied for any installation configuration, as long as the installed element patterns can be computed.

A dual-band rectenna for radio frequency (RF) energy harvesting is presented in this letter. The proposed antenna has two concentric square patches electrically connected with a small microstrip line connection. Four ports are located in the inner patch. The configuration of the ports enables a differential field sampling scheme and dual polarization. The antenna operates for the WiFi frequency bands of 2.4 and 5.5 GHz with 7.52 and 7.26 dBi gain, respectively, for each frequency. A full-wave Greinacher voltage doubler rectifier for each polarization has been employed for RF-to-dc conversion. The proposed novel topology utilizes the differential field sampling for each polarization and quadruples the overall output voltage by the rectification process. The differential output voltage source from the rectenna can directly act as a power source as typically electronics require differential source for their operation.

This letter presents a method to evaluate the static electric polarizability of 2-D infinitely periodic metal patch arrays with dielectric substrate. Static polarizabilities are used in several design applications for periodic structures such as estimation of the bandwidth limitations for frequency-selective structures or prediction of the radiation properties for antenna arrays. The main features of the proposed method are its numerical efficiency and a deep insight into the physics of the fields interacting with the structure. We provide derivation and analysis of the method, and its verification against two another commercial solver-based approaches for various structure geometries. Additionally, we suggest the guidelines for applying the method to bandwidth optimization of frequency-selective structures and illustrate this with an example.

In this paper we determine the trade-off between the Q-factor and partial (super) directivity for linear arrays of dipoles. The dipoles are densely spaced in a one or two wavelength long array. We compare the bound for port-feed dipoles with the current optimization approach, with optimal currents on the whole dipole area, and also with an enclosing plate. It is interesting to note that port-feed dipoles are close to the optimal current dipoles in the endfire case, for sufficiently many dipoles in the array, for a wide range of directivities. We also show that the optimization problem for the 'optimal current' and 'the port-feed' cases are very similar and that the Pareto-front (trade-off curve) can be obtained and compared for the different cases.

The reaction theorem is applied to antenna coupling problems. It is shown that the reaction theorem can be used to calculate the mutual impedance between antennas, when the electromagnetic fields are known on a plane that separates the two antennas in two disjoint regions. We also show that coupling paths between the antennas can be visualized on the separation plane, by using intermediate results from the reaction theorem. The coupling paths are visualized based on the fields generated by each of the two antennas, and only take into account the energy that is actually transfered between the antennas. The visualization of coupling paths is useful for understanding how the coupling between the antennas is distributed in space.

The accuracy of two equivalent antenna representations, near-field sources and far-field sources (FFSs), is evaluated for an antenna installed on a simplified platform. We show that the accuracy of the installed far-field and surface current for the investigated weakly scattering platform depends strongly on the configurations associated with the equivalent antenna representation. The root-mean-square error for the installed far-field error varies between 4.4%-8.4% for the considered configurations of near-field equivalent representations installed on the investigated platform. When using FFSs, the design parameters have an even larger influence of the achieved accuracy. There is also a varying accuracy depending on the type of numerical method used. Based on the results, some recommendations on the choice of subdomain for the equivalent antenna representation are given associated with the platform. In industrial antenna applications, the accuracy in determining, e.g., installed far-fields and antenna isolation on large platforms is critical. Equivalent representations can reduce the fine-detail complexity of antennas and thus give an efficient numerical description to be used in large-scale simulations. The results, in this paper, highlight to antenna designers and system engineers the accuracy challenges associated with the use of equivalent antenna representations.

Polymer nanocomposite dielectrics are promising materials for electrical insulation in high voltage applications. However, the physics behind their performance is not yet fully understood. We use density functional theory to investigate the electronic properties of the interfacial area in magnesium oxide-polyethylene nanocomposite. Our results demonstrate polyethylene conduction band matching with conduction bands of different surfaces of magnesium oxide. Such band bending results in long range potential wells of up to 2.6 eV deep. Furthermore, the fundamental influence of silicon treatment on magnesium oxide surface properties is assessed. We report a reduction of the surface-induced states at the silicon-treated interface. The simulations provide information used to propose a new model for charge trapping in nanocomposite dielectrics.

This paper revisit and extend the interesting case of bounds on the Q-factor for a given directivity for a small antenna of arbitrary shape. A higher directivity in a small antenna is closely connected with a narrow impedance bandwidth. The relation between bandwidth and a desired directivity is still not fully understood, not even for small antennas. Initial investigations in this direction have related the radius of a circumscribing sphere to the directivity, and bounds on the Q-factor have also been derived for a partial directivity in a given direction. In this paper, we derive lower bounds on the Q-factor for a total desired directivity for an arbitrarily shaped antenna in a given direction as a convex problem using semidefinite relaxation (SDR) techniques. We also show that the relaxed solution is also a solution of the original problem of determining the lower Q-factor bound for a total desired directivity. SDR can also be used to relax a class of other interesting nonconvex constraints in antenna optimization, such as tuning, losses, and front-to-back ratio. We compare two different new methods to determine the lowest Q-factor for arbitrary-shaped antennas for a given total directivity. We also compare our results with full electromagnetic simulations of a parasitic element antenna with high directivity.