Ing. Jan Kraček, Ph.D.

Contemporary beam-forming antenna arrays often use a large number of individual elements, sometimes hundreds or more, to achieve high gain for advanced applications like radar, space communication, and next-gen cellular networks. These arrays are complex and costly due to the need for precise amplitude and phase adjustments across the elements. The feeding network complexity leads to signal losses, reduced efficiency, and higher noise. Current research aims to simplify arrays, reduce active elements needing frontends, and streamline the feeding network, considering non-uniform, sparse, parasitic, or reflective arrays. Challenges arise from element coupling and imperfect models of high-frequency materials and electronic components. Therefore, in addition to simulations, practical experimentation remains vital. This paper focuses on designing a novel 3 × 3 element antenna array with digital quantized control. We explore the impact of quantized control on beamforming and plan to validate simplified orthogonal optimization methods with limited quantization depth. The proposed antenna array is applicable to 2400 GHz band research, including arrays with parasitic elements and switchable polarization for individual elements.

The synthesis and optimization of the radiation pattern of a monopole circular antenna array are described. The method is fast and effective since it employs impedance and radiation pattern matrices, which relate elements of the array, precalculated in a full-wave electromagnetic simulator. A simple excitation by only the central element is used, and the other elements are reactively loaded. The desired radiation pattern of the array is obtained by optimization of the reactive loads. A particular example is given for a circular array of 25 monopole elements.

The synthesis and optimization of the radiation pattern of a monopole circular antenna array are described. The method is fast and effective since it employs impedance and radiation pattern matrices, which relate elements of the array, precalculated in a full-wave electromagnetic simulator. A simple excitation by only the central element is used, and the other elements are reactively loaded. The desired radiation pattern of the array is obtained by optimization of the reactive loads. A particular example is given for a circular array of 25 monopole elements.

In this paper, we investigate the synthesis of antenna arrays capable of producing different radiation patterns, which can also be easily manufactured using planar Printed Circuit Board (PCB) technology. To this end, we employ algorithms using simultaneous placement techniques of antennas into non-regular arrays. We also inspect the effect of symmetries in the specification upon the symmetries of the array’s geometry. The arrays are simulated via the Finite Element Method (FEM). Through numerical examples, we illustrate the scope and convergence of the method. Applications of interest include circularly polarized patterns, since the algorithm can handle the rotation as well as translation of the antenna elements, in addition to broadband systems, as the method is capable of characterizing the performance of arrays at multiple frequencies.

Scattering analysis of a thin, single-layer radiator with parity, time-reversal, duality symmetry is discussed. The analysis is based on the network description of the radiator by a scattering matrix in conjunction with the transformation of its electromagnetic field by parity, time-reversal, and duality operators. The full-wave simulation of the radiator is also presented.

Beamforming for an antenna array using a modal technique is presented. The approach is suitable for the antenna array consisting of arbitrary elements in any mutual arrangement since its full-wave characterization is employed. The technique is demonstrated on an example of a planar antenna array with linear polarization.

A design of a compact three-element waveguide antenna array for the microwave frequency band 24–28 GHz is presented. The array is intended for 5G telecommunication systems where beamforming is necessary. Careful attention is paid on the design of its feeding lines to be of equal physical lengths. Therefore, beamforming phasing is significantly simplified. The proposed array is made using CNC machining and measured.

A method for the construction of an antenna array based on the iterative addition of its elements is presented. Mutual interactions of the elements are taken into account through a full-wave simulation using the Method of Moments. After adding the desired number of elements, the algorithm of the method can continue by iterative repositioning the already added elements. Some numerical examples are shown to illustrate the scope and convergence of the method.

A compact three-element shared-aperture waveguide antenna array for the 24–28 GHz microwave-frequency band is presented as a proof-of-concept of an array with steerable directional beam suitable for 5G telecommunication systems. The array is intended for use in a microwave photonic link and is sufficiently steerable only with the progressively phased excitation signals of equal magnitudes. The mutual interactions between the array elements are minimized to maintain the properties of the individual elements, even if they are embedded and closely spaced in the array. The proposed concept could be simply extended by adding more elements to further increase the directivity and enhance the steering properties of the array.

In this article, we investigate a novel iterative antenna array synthesis method. The method is based on the iterative addition of antenna array elements. After defining the synthesis algorithm, we prove that the discrepancy between the goal and the synthesized pattern converges to the theoretical lower bound in the sense of a certain norm. The algorithm is also extended to iteratively replace already placed elements and for the synthesis of multiple goal patterns with the same array geometry but with different excitations. Some numerical examples are shown to illustrate the convergence properties of the proposed method. Possible applications include circularly polarized patterns, since the algorithm can handle the rotation as well as translation of the antenna elements.

The modal approach was used to maximize the directivity of the linear dipole array above a perfect electric conductor ground plane. The directivity was maximized for the direction perpendicular to the ground in which this array can produce maximal directivity from all directions by a proper excitation. The modal approach was also connected with the optimization of geometrical parameters of the array to obtain not only optimal excitation currents for the array with fixed geometry but also optimal geometry of the array. It was observed that, thanks to the presence of the ground, the currents are equiphase and can be set to be real.

The theory of superdirective antenna arrays was discussed. The procedure for finding the excitation currents of fixed-geometry arrays, which maximizes the directivity, termed as superdirectivity, for a given direction and polarization, was described. The procedure was based on the solution of an eigenvalue problem including the directivity and matrices of normalized self- and mutual radiation intensities and normalized self- and mutual radiated powers of array elements. For examples of a linear dipole array located above a perfect electric conductor ground plane, this procedure was also combined with the optimization of the parameters of geometry of the array as a spacing of dipoles, or their height above the ground, for various number of dipoles in the array.

The analysis of capacitive wireless power transfer was conducted in a general manner. The circuit model of a capacitive wireless power transfer chain was presented. The derivation of the power transfer efficiency through the chain in question as well as the active power delivered to the appliance terminating this chain was shown. Both the case of the maximal efficiency and the one of the maximal appliance power were treated and conditions for these optima were found in both cases. The appliance power corresponding to the maximal efficiency and the efficiency corresponding to the maximal appliance power were also expressed. The total admittance of the capacitive wireless power transfer chain was calculated. For both optimal conditions, the appliance power and total admittance were written in the normalized form, which enabled to express them as functions of single variable in the same way as the efficiency.

In this paper, we investigate a novel iterative antenna array synthesis method. After defining the synthesis algorithm, we prove that the discrepancy between the target and the synthesized pattern in a certain norm converges to the theoretical lower bound. Some numerical examples are shown to illustrate the convergence properties of the proposed method. Possible applications of the method include circularly polarized patterns, since it can handle the rotation as well as translation of the antenna elements.

Antenna arrays are widely used due to their properties in comparison to single element antennas. The possible achievable gain can be much higher. Also, one of the biggest advantages is the possibility to steer the main radiation beam and cover a wider area. The problematic part of obtaining maximum achievable directivity for antenna arrays is the corresponding feeding. In this paper, we use the one term sinusoidal current density excitation and more precise King’s three-term excitation for optimization of an array of four dipoles for maximum directivity. The goal of this paper is to present a new fast method for antenna array design and to compare its exactness. As an example, a four-element Yagi-Uda antenna is optimized.

The concept of source currents of a radiating source can be employed to express its directivity in some particular cases analytically. For an antenna array, this concept can be extended to the concept of a generalised directivity based on self- and mutual radiation intensities, self- and mutual radiated powers and excitation currents of array elements. These approaches were applied to examples of the array of two elementary dipoles and the array of two isotropic radiators. Particularly, the evaluation of the directivity of the end-fire arrays, which are of special attention due to superdirective properties, was treated. Novel closed-form expressions for the directivity of these arrays with out-of-phase excitation were derived. It was observed that the end-fire directivity can be further enhanced by optimising the excitation currents of the arrays. Their optimal phase difference and corresponding increased directivity were also found analytically. The results were validated by a full-wave simulator.

The power balance analysis of the capacitive wireless power transfer was conducted in a general manner. The circuit model of a capacitive wireless power transfer chain was presented. The derivation of the power transfer efficiency through the chain in question as well as the active power delivered to the appliance terminating this chain was shown. The case of the maximal efficiency was treated and conditions for this optimum were found. The appliance power corresponding to the maximal efficiency was also expressed. For the optimal conditions, the appliance power was written in the normalized form, which enabled to express it as a function of single variable in the same way as the efficiency.

In this paper, we propose a novel frequency domain reading method for the chipless RFID tags. It is based on a single scalar measurement of transmission coefficient magnitude between two antennas by the presence of a tag. The method enables to reliably read the chipless RFID tags based on a dipole resonator array, which is backed by a limited ground plane, in a real environment outside the anechoic chamber. Moreover, it suits simple, yet not necessarily well impedance matched, reader antennas represented by an open end of the rectangular waveguide. The method was verified experimentally in the frequency band of 7–11 GHz.

In this paper, we employ the concept of source currents to express the directivity of an array in generalized matrix form. It allows finding the excitation currents that maximize directivity for given array. First, we demonstrate this method on an array of two isotropic radiators and express the resulting superdirective currents in closed-form for limiting separation going to zero. Then, using the proposed approach, a superdirective closely-spaced array of two loaded dipoles is designed using full-wave simulator CST Microwave Studio.

In this paper we develop and utilize theory for finding excitation currents that maximize the directivity of arbitrary dipole arrays. Such excitation is known as superdirective. It is shown that for arrays above perfectly electric ground, the corresponding optimal currents are purely real. It is also observed that for horizontally oriented dipoles, there exist optimum for spacing and height producing maximum obtainable directivity.

The concept of source currents of a radiating source can be employed to express its directivity in some particular cases analytically. For an antenna array, this concept can be extended to the concept of a generalized directivity based on self- and mutual radiation intensities, self- and mutual radiated powers and excitation currents of array elements. These concepts were applied to example of the array of two isotropic radiators above perfect electric conductor (PEC) ground, where the PEC ground is simulated by mirror image of the original image of isotropic radiators.

The theory of characteristic modes is used to analyze antenna arrays. In particular, to study arrays of thin-wire dipoles backed by an infinite electric ground plane. The proposed theory is based on an analytically prescribed current distribution on the dipoles where, when compared with the full method of moments solution, only a self- and mutual impedance matrix relating excitation currents and voltages of array elements is the subject for modal decomposition. Therefore, the impedance matrix of interest has dimension N x N, where N is number of the array elements resulting in an extremely fast approach. The results of the proposed method are validated by a full-wave simulator and show good agreement.

In this paper, the radiation efficiency modes of an array of lossy dipoles above infinite electric ground are evaluated. A simple theory, involving prescribed sinusoidal currents on dipoles, is presented. In turn, the impedance matrix of interest has dimension N × N where N is a number of the array elements resulting in an extremely fast approach. The ohmic losses are represented by a diagonal matrix and a particular eigenvalue equation is constructed. The modal efficiencies are studied as a function of the height of the array above the ground. The results are in good agreement with full-wave simulation.

The paper presents the novel 20-bit chipless RFID transponder (tag) based on spiral capacitively loaded dipole scatterers of electrical size ka = 0.47, radar cross section σ = 29.3 dBsm, and bandwidth BW3dB = 20.9 MHz. It outperforms U-folded dipole scatterer (ka = 0.79, σ = 34.3 dBsm, BW3dB = 18.1 MHz) in the first two parameters in exchange for a slight increase in bandwidth. The total tag size (17 × 68 mm2) is roughly half the size of a credit card. The frequency and amplitude stability of its RCS response, when the zero bit information is coded via removal of individual scatterers, is improved by reordering scatterers in the array. The simulated results were verified by the monostatic measurement of tag RCS.

The Method of Moment, used for computation of the impedance matrix of the dipole array, is substituted here by faster method using simple approximation by the sinusoidal current. This method is verified by the theory of characteristic modes that provide a set of eigenvectors and eigenvalues, which are determined entirely by a structure's geometry at a certain frequency. When computing over a range of frequencies, the methods for eigenvalue tracking are essential. Proposed eigenvalue perturbation method is a powerful tool in this phenomena.

This article presents recent European-based contributions for wireless power transmission (WPT), related to applications ranging from future Internet of Things (IoT) and fifth-generation (5G) systems to highpower electric vehicle charging. The contributors are all members of a European consortium on WPT, COST Action IC1301 (Table 1). WPT is the driving technology that will enable the next stage in the current consumer electronics revolution, including batteryless sensors, passive RF identification (RFID), passive wireless sensors, the IoT, and machine-to-machine solutions.

The Characteristic Mode Analysis (CMA) provides a set of eigenvalues and eigenvectors that are determined entirely by the antenna array structure at a particular frequency. The presented method solely relies on the mutual impedance matrix of elements of the array. In turn, the proposed approach is very fast. Various modal decompositions are performed on an array of parallel dipoles placed horizontally above an infinite electric ground plane. The dipoles have prescribed sinusoidal currents. The results are validated by a full-wave simulator FEKO, showing good agreement.

Various modal decompositions are performed on an array of dipoles backed by electric ground plane. The theory is based on analytically prescribed current on dipoles so the decomposition returns directly excitation coefficients of an array with respect to given quantity.

The analysis of magnetic field of a thin-wall air induction coil with a shape of finite cylinder of arbitrary cross section is described. The result of the analysis is a method for finding the scalar magnetic potential of the coil with the help of mutual inductance of the coil and the elementary loop. The magnetic flux density of the coil is found as the gradient of the scalar magnetic potential. The proposed analysis is verified using two examples, which are thin-wall air coils with a shape of finite cylinder of circular and rectangular cross sections. The magnetic flux density calculated by the presented method is compared with other approaches for both examples with excellent agreement.

The objective of this paper was to design a compact, battery-less system for an implantable semi-active UHF RFID tag with inductive wireless power transfer for the human body. An RFID chip of the tag, combining powering from a reader by communication and from another source through inductive wireless power transfer, was used. Tag sensitivity for communication was increased by about 21 dB with the help of wireless inductive powering when compared to tags which did not employ this system. Communication and powering circuits were integrated within compact structures on the sides of the reader and the tag. The structure for communication and powering on the side of the reader consists of a centre-excised Archimedes spiral antenna and a circular loop respectively. Similarly, the structure on the side of the tag consists of a folded dipole antenna for communication and a rectangular loop for powering.

This contribution deals with a design of a system with RFID communication and inductive wireless powering for a human body implantable tag. A chip of the tag employs powering from an RFID reader by communication and from another source with the help of inductive wireless power transfer. Circuits for communication and powering on the reader and tag sides were designed as compact planar structures. The reader side structure comprises a centre-excised Archimedes spiral antenna for communication and a circular loop for powering. The tag side structure integrates a folded dipole antenna for communication and a rectangular loop for powering. The communication sensitivity of the tag was increased by about 21 dB when the additional inductive wireless powering was used.

The paper presents a compact battery-less semi-active UHF RFID tag powered by an inductive wireless power transfer designed to be implanted into the human body. Communication runs at frequency of 866 MHz whereas powering is performed at frequency of 6.78 MHz to reduce losses. Tag sensitivity for communication was increased by about 21 dB with the help of inductive wireless powering when compared to a tag which did not employ this powering. Communication and powering circuits were integrated within compact structures on the sides of the reader and the tag. The reader side consists of a center-excised Archimedes spiral antenna for communication and a circular loop for powering. The tag side consists of a folded dipole antenna for communication and a rectangular loop for powering.

Wireless power transmission (WPT) is an emerging technology that is gaining increased visibility in recent years. Efficient WPT circuits, systems and strategies can address a large group of applications spanning from batteryless systems, battery free sensors, passive RF identification, near-field communications, and many others. WPT is a fundamental enabling technology of the Internet of Things concept, as well as machine-to-machine communications, since it minimizes the use of batteries and eliminates wired power connections.WPT technology brings together RF and dc circuit and system designers with different backgrounds on circuit design, novel materials and applications, and regulatory issues, forming a cross disciplinary team in order to achieve an efficient transmission of power over the air interface. This paper aims to present WPT technology in an integrated way, addressing state-of-the-art and challenges, and to discuss future R&D perspectives summarizing recent activities in Europe.

This contribution presents a measurement of harmonically changing magnetic flux density in the free space. The components of magnetic flux density vector are measured separately as effective value in the time. The measuring probe and its calibration are described in detail.

This paper describes the method by which it is possible to construct simple algorithms for the calculation of the axial and transverse components of the intensity of the magnetic field of current loops and thin-wall air coils (solenoids). These are planar loops and tightly wound coils with a cross-section of an arbitrary shape and parallel walls in an axial direction. Computational algorithms show the same formal structure for every shape of coil; the shape of the coils is defined by the special shape function. The method is based on the superposition of the magnetic field of the equivalent magnetic dipoles. Utilization of this method can be observed, for example, in the calculation of the magnetic field of a cylindrical current loop and a thin cylindrical coil, for which the resulting relations are compared with a familiar method of calculation, on which solving equations for the vector potential is based. The method of calculation described can also be equally effectively used for coils of a non-circular shape. In this study, for the purpose of illustration, simple algorithms are created for the calculation of the magnetic field of a rectangular coil and the calculated values of the magnetic field are compared with the actual measurements.

The analysis of power balance for concepts of wireless power transmission based on electromagnetic induction is performed in general way. The derivation of power transmission efficiency is shown. The conditions for maximal efficiency and maximal active power delivered to the appliance are found. The maximal efficiency is expressed as a function of single variable. The power balance for maximal efficiency and maximal active power delivered to the appliance is derived in terms of normalized active powers as functions of the same variable as maximal efficiency. The system input impedance is calculated.

The wireless power supply is motivated by simple and comfortable use of many small electric appliances with low power input. This paper reviews the concepts that are suitable for wireless power transmission with respect to power supply of such appliances in small areas. The categorization of the concepts is made. The efficiency of the concepts is discussed on general base. The reference levels for exposure to electric and magnetic fileds are mentioned, and maximal power delivered to an appliance by fulfillment of these levels is considered.

This paper presents an overview of the principles suitable for wireless power supply of devices with a small power input in picocells. This means predominantly different types of small electric devices in the space of rooms. Basic principles, namely electromagnetic induction and electromagnetic wave, are explained using examples of developed systems. Different types of wireless power systems are compared with respect to efficiency, frequency, power, and transmission distance.

This paper presents a part of a system for wireless power supply with the use of guided wave in the coupling surface. This system type is suitable for the supply of different devices with small power input in picocells. The main elements of the RF section of the system are slab dielectric waveguide which creates a coupling surface and adaptor to this waveguide. The text especially focuses on this adaptor which ensures the coupling of RF power to and from the slab dielectric waveguide.

This paper presents set of software tool for the design and optimization of fractal patches describes by the IFS.

This paper analyses the application of higher order mode of a rectangular dielectric resonator for the construction of a transistor oscillator in the microstrip line structure. A scheme of the microstrip line and the rectangular dielectric resonator was found experimentally which enabled the excitation of its higher order modes with suitable characteristics for these purposes. This found scheme was analysed by a simulator of electromagnetic field and selected higher order modes were identified as and . Using the or mode, a functional sample of a reflection type transistor oscillator with working frequency 21,3 GHz or 25,4 GHz was proposed and implemented by small signal design.