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[Abstract]: Frequency diverse arrays (FDAs) provide range-angle time-invariant transmit–receive beamforming, although a fine focus of their radiation patterns is costly since high-resolution mixers and phase shifters are required. An innovative approach to FDAs is proposed in which mixers and phase shifters are replaced by the application of periodic linear-phase time-modulating waveforms to the antenna excitations implemented with 2-bit variable phase shifters controlled with periodic on–off instants of binary sequences, which leads to a simple and precise method for FDA beam focusing.

[Abstract] Wireless sensor networks (WSN) are increasingly requiring directional antennas that not only provide higher capacity, security, transmission range or robustness against interference, but also contribute with smart antenna capabilities such as adaptive beamforming or multi beam radiation patterns. Standard phased arrays provide these features, but employing feeding networks based on digitally controlled variable phase shifters (VPSs) which have the disadvantage of high cost and limited angular resolution. Instead, time-modulated arrays (TMAs) use switched feeding networks governed by digital periodic sequences which allow harmonic patterns to be radiated and endows (TMAs) with attractive multifunctional capabilities. In this paper, we analyze and properly combine (TMA) switched feeding networks capable of time-modulating an antenna array with discretized amplitude modulation (AM) and phase modulation (PM) waveforms. The advantages of the proposed innovative dual-beam (TMA) with respect to the competing architectures are, on the one hand, its power efficiency and beamsteering (BS) phase sensitivity and, on the other, its hardware simplicity, which allows for an excellent relative cost advantage.

[Abstract]: Frequency diverse arrays (FDAs) allow the synthesis of dot-shaped radiated patterns in the range-angle space by applying frequency offsets to the antenna elements. Conventional FDAs are usually implemented with costly variable phase shifters and mixers and require an independent feeding network per beam for multibeam operation. In this letter, we propose an innovative approach to FDA that makes use of time-modulated array (TMA) building blocks modeled with inexpensive single-pole dual-throw switches. Our approach is able to simultaneously and independently steer two dot-shaped beam patterns, over the two most significant TMA harmonics, with an almost 90% time modulation efficiency.

[Abstract]: It is noted that the main innovation contained lies in the consideration of frequency diverse arrays (FDAs) feeding networks made up of time-modulating switches (hence, the term TMA-FDA) instead of phase shifters. This has the undoubted advantage of leveraging that RF switches have virtually zero insertion loss while offering high angle resolution and cost-efficiency.

[Abstract]: The application of frequency offsets to the elements of a conventional phased array with variable phase shifters (VPSs) leads to frequency diverse arrays (FDAs) with the ability to provide range-angle spatial focusing. In this letter, we propose an innovative approach to FDA using switched time-modulated arrays (TMAs) instead of VPSs. The time modulation efficiency of the combined FDA-TMA approach is over 94% when using Kaiser windows to determine the frequency offsets. Such windows have the advantage of flexibly handling the tradeoff between the beam collection efficiency and the half power beamwidth without the need of difficult optimizations.

[Abstract]: In this letter, sinusoidal pulsewidth modulation (PWM) is applied to the design of time-modulated arrays (TMAs). The array excitations are modulated using PWM-synthesized sine waveforms. This provides increased signal bandwidth, excellent rejection of the undesired harmonics, and modulation efficiencies aligned with the best ones in the literature. The simplicity of the technique allows for an easily implementable antenna array feeding network.

[Abstract]: The design of efficient time-modulated arrays (TMAs) for multi-beam steering purposes must be based on both a type of periodical sequences that guarantees an adequate windowing of exploited harmonics, and a feeding network capable of providing low hardware insertion losses. Compliance with these two requirements is not simple and is aggravated by the fact that, as the number of exploited harmonic beams increases, not only drastically decreases the antenna efficiency, but also the beam steering flexibility. In this work, we propose three different single sideband (SSB) feeding networks for multibeam TMAs constructed considering different combinations of single-pole multiple-throw (SPMT) basic modules. The proposed feeding networks simultaneously accomplish excellent levels of modulation efficiency, reasonable hardware insertion losses, and beam scanning flexibility. In addition, a comparison with standard phased arrays in terms of performance, phase sensitivity, complexity, and cost-effectiveness is carried out.

[Abstract]. Time-modulated arrays (TMAs) can effectively perform beamsteering over the first positive harmonic pattern by applying progressively delayed versions of stair-step approximations of a sine waveform to the antenna excitations. In this letter, we consider synthesizing such stair-step sine approximations by means of Haar wavelets. Haar functions constitute a complete orthonormal set of rectangular waveforms, which have the ability to represent a given function with a high degree of accuracy using few constituent terms. Hence, when they are applied to the TMA synthesis, employing single-pole double-throw switches, such a feature leads to an excellent rejection level of the undesired harmonics as well as a bandwidth greater than that supported by conventional TMAs with on-off switches.

[Abstract] In this work we make use of the frequency diverse array (FDA) concept, whose design is based upon a frequency increment across the antenna elements to generate a beam steering that is a function of angle, time and range. For a possible use of this technique in 5G detection systems, a 28-GHz FDA numerical model, designed with help of a software tool, is analyzed. Some practical conclusions are drawn from the presented results.

[Abstract]: Bipolar (±1) sequences with no zero state suit particularly well for safeguarding the switched feeding network efficiency when applied to time-modulated arrays. During the zero state of a conventional time-modulating sequence, if a given array element is switched off, a certain amount of energy of the transmitted/received signal is wasted. We propose a novel single sideband time-modulated phased array (TMPA) architecture governed by realistic bipolar squared sequences in which the rise/fall time of the switches is considered. By using single-pole dual-throw switches and nonreconfigurable passive devices, the TMPA exploits, exclusively, the first positive harmonic pattern while exhibiting an excellent performance in terms of efficiency and control level of the undesired harmonics without using synthesis optimization algorithms (software simplicity).

[Abstract]: Time-modulated arrays (TMAs) receiving beamforming provide a noteworthy hardware simplicity given the ability of this multi-antenna technique to transform spatial diversity into frequency diversity. However, the dual behavior at transmission seems to be as simple as limited: a given signal is simultaneously transmitted over all the different TMA harmonic patterns. We investigate the efficient and simultaneous transmission of two different signals over the same physical antenna using two independent harmonic beam-patterns of the TMA. For that purpose, we propose an innovative dual-signal TMA transmitter based on two complementary operations: the complex mixing of the baseband signals and the TMA processing with quadrature and time-delayed periodic pulses.

[Abstract]: We present a single sideband time-modulated multibeam phased array governed by periodic Nyquist pulsed signals. A Nyquist pulse is a physically realizable approach to the ideal sinc function. Hence, its low-pass spectrum suits particularly well for time-modulated arrays (TMAs) to perform harmonic beam steering. Contrarily to switched TMAs and standard solutions based on variable phase shifters, the performance and complexity of the proposed time modulation scheme is rather robust when increasing the number of multibeams.

[Abstract]: Conventional time-modulated arrays are based on the application of variable-width periodical rectangular pulses (easily implemented with radio frequency switches) to the individual antenna excitations. However, a serious bottleneck arises when the number of exploited harmonic beams increases. In this context, the modest windowing features of the rectangular pulses produce an inflexible and ineffective harmonic beamforming. The use of other pulses, such as sum of weighted cosines, partially solves these issues at the expense of introducing additional non-timing variables. We propose the discrete-time preprocessing of rectangular pulses before being applied to the antenna to accomplish an agile, efficient, and accurate harmonic beamforming, while keeping the simplicity of the hardware structure.

[Abstract]: Diversity occurs whenever several copies of the same transmitted signal arrive at a receiver. Such a situation allows for improving the performance of a wireless communication system transmitting over a radio channel without increasing the transmit power. Previous works have shown that time-modulated arrays are capable of faithfully acquiring digital signals while exploiting angular diversity through the adaptive beamforming of their harmonic patterns. In this work, we take a step further and consider a wireless communication system employing digital linear modulated signals and an innovative receiver that includes a time-modulated array and a maximum ratio combining subsystem. The maximum ratio combiner is adapted to optimally exploit the multipath channel angular diversity. The performance of the system is analyzed in terms of two metrics: the signal-to-noise ratio and the symbol error rate. The results are compared to those achieved with other receivers that include conventional antenna arrays, exhibiting the time-modulated array solution a good tradeoff between performance and hardware complexity.

[Abstract]: One of the primary features of time-modulated arrays (TMAs) is their ability to adapt their harmonic patterns to exploit the angular diversity of multipath wireless channels. In such a case, the TMA excitations must provide an adequate level of harmonic windowing in order to safeguard both the antenna efficiency and the signal-to-noise ratio at reception. In this work, we propose to time-modulate the array excitations with periodic sum-of-weighted-cosine (SWC) pulses rather than with the periodic rectangular pulses corresponding to the on-off switches of conventional TMAs. We show that how the larger flexibility of the SWC pulses allows for a more versatile design of the TMA harmonic radiation patterns without penalizing the array gain for each exploited harmonic.

[Resumo] Os selos de identidade das comunicacións móbiles e sen fíos de hoxe en día son a demanda continua e crecente de mobilidade, capacidade e fiabilidade, xunto cun firme e definitivo compromiso coa sustentabilidade. Baixo estas premisas, as antenas intelixentes –capaces de sensar a contorna electromagnética e adaptar de xeito eficaz as súas características de radiación– están chamadas a xogar un papel crucial nas ditas comunicacións. Neste senso, os estándares sen fíos actuais consideran técnicas multi-antena encamiñadas a explotar a diversidade espacial, o multiplexado espacial e o conformado de feixe, acadando así mellores niveis de fiabilidade e capacidade. Con todo, ditas vantaxes obtéñense a expensas dun incremento da complexidade do sistema, factor non sempre asumible en termos de tamaño e eficiencia enerxética. Consecuentemente, suscítanse unha serie de retos de cara a desenvolver tecnoloxías de antena axeitadas e capaces de dar resposta ás anteriores prestacións no espazo limitado que dita a mobilidade. O concepto de agrupación de antenas modulada temporalmente (TMAs, do inglés time-modulated arrays) é unha técnica multi-antena que achega unha simplificación hardware significativa: o seu diagrama de radiación contrólase mediante a sinxela aplicación de pulsos periódicos de duración variable ás excitacións individuais da agrupación. A natureza non lineal desta operación causa a aparición de diagramas de radiación nas frecuencias harmónicas dos pulsos periódicos aplicados. A técnica pode empregarse para mellorar a topoloxía do nivel dos lóbulos secundarios do diagrama de radiación na frecuencia central e/ou para explotar de xeito proveitoso os diagramas dos harmónicos, dotando á antena de capacidades de antena intelixente. Esta tese é o resultado dunha investigación das TMAs dende unha perspectiva interdisciplinaria, é dicir, non soamente dende a óptica do diagrama de radiación ou das agrupacións de antenas, senón tamén dende un punto de vista de procesad, [Resumen] Hoy en día, los sellos de identidad de las comunicaciones inalámbricas son la demanda continua y creciente de movilidad, capacidad y fiabilidad, junto con un firme y definitivo compromiso con la sostenibilidad. Bajo estas premisas, las antenas inteligentes –capaces de sensar el entorno electromagnético y adaptar de forma eficaz sus características de radiación– están llamadas a jugar un papel crucial en dichas comunicaciones. En este sentido, los estándares inalámbricos actuales consideran técnicas multiantena encaminadas a explotar la diversidad espacial, el multiplexado espacial o el conformado de haz, alcanzando así mejores niveles de fiabilidad y capacidad. Sin embargo, dichas ventajas se obtienen a expensas de un incremento de la complejidad del sistema, factor no siempre asumible en términos de tamaño y eficiencia energética. En consecuencia, se plantean una serie de retos en el desarrollo de tecnologías de antena adecuadas, capaces de dar respuesta a las anteriores prestaciones en el espacio limitado que dicta la movilidad. El concepto de agrupación de antenas modulada temporalmente (TMAs, del inglés time-modulated arrays) es una técnica multiantena que aporta una simplificación hardware significativa: su diagrama de radiación se controla mediante la sencilla aplicación de pulsos periódicos de duración variable a las excitaciones individuales de la agrupación. La naturaleza no lineal de esta operación causa la aparición de diagramas de radiación en las frecuencias armónicas de los pulsos periódicos aplicados. La técnica se puede utilizar para mejorar la topología del nivel de los lóbulos secundarios del diagrama de radiación en la frecuencia central y/o para explotar de forma beneficiosa los diagramas de armónicos, dotando a la antena de capacidades de antena inteligente. Esta tesis es el resultado de una investigación de las TMAs desde una perspectiva interdisciplinar, es decir, no solamente desde la óptica del diagrama de radiación o de las agrupa, [Abstract] An ever-increasing demand for higher mobility, capacity and reliability, together with a definitive compromise with sustainability, are the hallmarks of mobile and wireless communications systems nowadays. Under these premises, smart antenna devices –capable of sensing the electromagnetic environment and suitably adapting its radiation features– are correspondingly called to play a crucial role. In this sense, today’s wireless standards consider multiple-antenna techniques in order to exploit space diversity, spatial multiplexing and beamforming to achieve better levels of reliability and capacity. Such advantages, however, are obtained at the expense of increased system complexity which may be unaffordable in terms of size and energy efficiency. Consequently, some technical challenges remain to develop the adequate antenna technologies capable of supporting the aforementioned features in a limited physical space that the mobility demand dictates. The concept of time-modulated array (TMA) is a feasible multi-antenna technique that provides a significant hardware simplification: its radiated power pattern is controlled by the simple application of variable-width periodical pulses to the individual array excitations. The nonlinear nature of such an array operation causes the appearance of radiation patterns at the harmonic frequencies of the applied periodic pulses. The technique can be used for improving the side-lobe level topology of the radiation pattern at the central frequency and/or to profitably exploit the harmonic patterns in order to supply smart antenna capabilities. This thesis is the result of an investigation of TMAs from an interdisciplinary perspective, i.e., not only under a radiation pattern or an antenna array outlook but also from a signal processing point of view. More specifically, the thesis deals with an in-depth analysis of the application of TMAs in digital communications developed in four stages: 1) mathematical analysis of the f

[Abstract] Time-modulated arrays (TMAs) are electromagnetic systems whose radiated power pattern is controlled by the application of variable-width periodical pulses to the individual elements. The nonlinear nature of the array operation causes the appearance of radiation patterns at the harmonic frequencies of such periodic pulses. The technique can be used for improving the side-lobe level (SLL) topology of the radiation pattern at the central frequency and/or to profitably exploit the harmonic patterns in order to supply smart antenna capabilities. Among the latter features, the TMA harmonic beamforming takes on special importance due to its attractive trade-off performance-hardware complexity. From this perspective, TMAs are sensors capable of transforming the spatial diversity of a communication channel into frequency diversity, thus improving the performance of a wireless communication. In addition to a walk through the origins of the concept, and a brief analysis of the mathematical fundamentals, this paper organizes the prolific state of the art of TMAs in two major thematic blocks: (1) TMA design from an antenna perspective; and (2) TMA design from a signal processing perspective.

[EN] Electromagnetic laboratory teaching activities greatly benefit from utilizing Information and Communication Technologies (ICTs), due to the fact that the understanding of concepts that are difficult to be communicated using traditional approaches becomes easier. We have developed a graphical interface, designed with MATLAB®, that represents a very useful tool to convey concepts related to configuration and radiation of conformal antennasto students that are not experts in that particular field.

[Abstract] Time modulation provides a simple way to control the power radiation pattern of an antenna array. By appropriately selecting the parameters of a Time-Modulated Array (TMA) it is possible to obtain a reconfigurable pencil beam adequate for wireless communications. This work focuses on the impact of a TMA on the Bit Error Ratio (BER) performance of a wireless communication receiver with a linear digital modulation. We show how the BER of such a receiver is affected by the TMA synthesis variables.

[Abstract] Wireless communications are widely based on linear digital modulation methods, which introduce the digital baseband information in the amplitude and/or in the phase of a carrier sinusoid. Time-modulated arrays, on the other hand, constitute an interesting technique capable of adapting the power radiation pattern of an antenna array. In this work, we study from a theoretical point of view the impact of time-modulated arrays on the transmission of linearly modulated digital communication signals. The work focuses on the requirements to safeguard the integrity of the signal and on the quantification of the useful power radiated within the desired band.

[Abstract] This paper describes the implementation of a MATLAB® tool to plot the three-dimensional polar radiation diagram generated by a conformal antenna array. A related tool has also been developed for visualizing the geometrical arrangement (positions and orientations) of the elements, as well as the relative values of the amplitudes and phases of their excitations. Supporting M-files are available.

[Abstract] We describe an experimental set-up for exposure of small animals to radiofrequency standing waves that allows direct measurement of the power absorbed by the animal. Essentially, the set-up consists of a metallic box containing an antenna and experimental animal immobilized in a methacrylate holder; a signal generator feeding the antenna; and a power meter. In addition, the box can also contain a video camera to record the animal's behaviour, and a receiving antenna (connected externally to a power meter and a spectrum analyser) to detect alien radiation and harmonics. The absorbed power measurement trivially allows calculation of whole-body mean SAR from the animal's weight; and assuming local SARs to be proportional to whole-body mean SAR, the latter can be used to adjust organ-specific SAR predictions obtained by simulation using a commercial FDTD program with a numerical phantom. The use of the system is illustrated by application to rats given subconvulsive doses of picrotoxin to induce a seizure-prone state analogous to epilepsy: levels of the neuronal activity marker c-Fos in the frontal and piriform cortex of picrotoxin-treated rats exposed to 900 MHz GSM radiation were twice as high as those of unexposed animals.

[Abstract] The design of a number of new antennas that radiate linearly polarized toroidal beams is described. The developed procedures are based on the use of a method of moments commercial software tool. Several numerical examples, working at WLAN communication frequencies, are derived and analyzed. Two experimental prototypes validate the numerical results

[Abstract] A very fast technique that allows the synthesising of arbitrary footprint patterns by using conformal arrays with many radiating elements is described. This method is based on a combination of Woodward-Lawson and Elliott-Stern techniques, and it was applied to the synthesis of a triangular footprint generated by 657 axial dipoles placed on a cylindrical surface, obtaining acceptable ripple and sidelobe levels.

[Abstract] In this letter, the authors propose an optimization method based on the genetic algorithm (GA) to reconfigure a linear array of vertical half-wavelength dipole antennas to generate two patterns with minimum active impedance variation when the antenna switches from one pattern to other in the presence or absence of a ground plane behind the array. The problem is to find a fixed voltage amplitude distribution that generates two broadside symmetrical beams in the horizontal plane: a pencil beam with zero phases and a flat-top beam with phases in the range from -180deg to +180deg. Mutual coupling effect is taken into account via open circuit mutual impedance matrix

[Abstrac] An innovative method for linear arrays beam reconfiguration is presented. This pattern reconfigurability is achieved by a mechanical displacement of a parasitic array located in front of an active one. Two worked examples that use parallel dipoles are presented.

[Abstract] In this paper, time modulation is applied to a small number of elements of a linear array that radiates either sum or difference patterns, in order to take control over their sidelobe levels. The simulated annealing (SA) technique helps to obtain the optimum time pulses applied to such elements in terms of sideband minimization.

[Abstract] It is shown in this paper that when artificial neural networks are extended to be complex valued, they can be incorporated as a very powerful and effective tool in the analysis, synthesis, and diagnostics of antenna arrays.

[Abstract] Equispaced linear arrays that have uniformly excited central elements and monotonically decreasing flanking segments have been shown to yield low-sidelobe patterns with specified constraints on the effective radiated voltage (ERV). An analytical method for generating these distributions using a constrained least-squares (CLS) method, with restrictions on both the peak amplitude of the elements and the ERV, has been discussed in a previous work. Simulated annealing (SA) is a versatile global-optimization technique that can be effectively used for similar purposes, but can also provide additional control over some other design parameters. In this paper, we inspect both methods—indicating some of their advantages and drawbacks—through some numerical results.

The problem of synthesizing a linear array generating a shaped beam pattern with M filled s has 2M alternative solutions. In this study we examined their bandwidths as regards compliance with pattern quality or input impedance requirements in the presence and absence of a backing ground plane. Placing a ground plane behind the antenna almost doubles sidelobe level bandwidth.

[Abstract] It is shown, by means of numerical simulation, that the Moore-Penrose pseudoinverse of a matrix taken from an overdetermined system can be applied to retrieve the excitation distribution of a planar array of parallel dipoles with faulty elements, by measuring the complex radiated field in its near zone. Failures on voltage (considering mutual coupling) and current of several elements, and systematic or random measurement errors, are considered in the simulation.

[Abstract] C++ routines implementing the simple generalization of Simpson's rule to two dimensions for calculating antenna directivities compare well with methods currently available in common symbolic and numerical calculation packages.