Antenna currents for optimal Q, superdirectivity, and radiation patterns using convex optimization

The high Q-factor (low bandwidth) and low efficiency make the design of small antennas challenging. Here, convex optimization is used to determine current distributions that give upper bounds on the antenna performance. Optimization formulations for maximal gain Q-factor quotient, minimal Q-factor for superdirectivity, and minimum Q for given far-field are presented. The effects of antennas embedded in structures are also discussed. The results are illustrated for planar geometries

Analysis and synthesis of antenna arrays with respect to mutual coupling and beamforming

ato práce se zabývá analýzou a syntézou (zejména liniových) anténních řad umístěných ve volném prostoru nebo nad nekonečnou zemní rovinou. K charakterizaci problému byla odvozena teorie a algoritmus implementován v programu MATLAB. Pro liniové anténní řady se vyvinutá metoda se vyznačuje velkou rychlostí z důvodu použití vhodné aproximace proudového obložení na jednotlivých prvcích řady. Pro analýzu řad jsou využity modální techniky, tj. řada je charakterizována maticemi o rozměru NxN (kde N je počet prvků v řadě) popisujícími její impedační a vyzařovací vlastnosti. Tyto matice jsou následně podrobeny modálním rozkladům, jejichž výsledek poskytuje optimální buzení elementů pro dosažení daných vlastností – rezonance řady, činitel jakosti, směrovost. Kromě semi-analytických metod aplikovaných na liniové řady byl rovněž vyvinut algoritmus využívající simulátor elektromagnetického pole CST MWS, jež je pomocí maker propojen s programem MATLAB. Takto je možné syntetizovat vyzařovací diagram řady s libovolným typem elementů, tj. nikoli jen s dipóly. Výše zmíněné metody jsou aplikovány a ověřeny na několika příkladech: Optimalizace Yagi-Uda antény s různou délkou elementů, Optimalizace šířky pásma a směrovosti řady nad zemní rovinou, Řízení směrovosti kruhové řady, Syntéza supersměrového buzení řady, Syntéza daného vyzařovacího diagramu řady včetně zahrnutí vzájemných vazeb. V neposlední řadě jsou tyto příklady a techniky inspirací pro návrh a výrobu anténní řady na frekvenci 26 GHz. Tato řada byla vyrobena, změřena a bude implementována spolu s optickým systémem, který bude tvořit napájecí a přenosovou část pro systém 5G.This work deals with the analysis and synthesis of (especially linear) antenna arrays located in free space or above the infinite ground plane. The theory and algorithm implemented in MATLAB were derived to characterize the problem. For linear antenna array the developed method is characterized by a high computational speed due to the use of suitable current distribution approximation on individual elements of the array. Modal techniques are used to analyze the array, i.e., the array is characterized by matrices of N x N dimension (where N is the number of elements) describing its impedance and radiation properties. These matrices are then subject to modal decomposition, the result which provides optimal excitation of the elements to achieve given properties - resonance, quality factor, directivity. In addition to semi-analytical methods applied to linear arrays, an algorithm using electromagnetic field simulator CST MWS, which is connected to MATLAB by macros, was also developed. In this way, it is possible to synthesize a radiation pattern of an array with any type of element, i.e., not just dipoles. The above methods are tested and validated on several examples: Optimization of Yagi-Uda antenna with different element lengths, Optimization of bandwidth and directivity of an array above ground plane, Directivity control of circular array, Synthesis of super-directivity excitation of an array, Synthesis of a given radiation pattern of an array, including the mutual coupling. Last but not least, these examples and techniques are an inspiration for the design and manufacture of the 26 GHz antenna array. This array has been manufactured, measured and will be implemented together with an optical system that will form the power and transmission part to the 5G system

Mixtures of Multipoles - Should They Be in Your EM Toolbox?

Multipole expansions are an essential analysis tool in the foundations of the descriptions of the electromagnetic fields radiated by electric and magnetic sources. Nevertheless, practical antenna systems generally rely on them as an academic explanation, not as a fundamental building block. An overview of the recent surge in interest in multipole sources and their fields to achieve useful radiated and scattered fields with, for example, high directivities in preferred directions is given. Topics include Huygens sources, dielectric-based Mie-tronics, edge-singularity multipoles, and exotic metamaterial-inspired superdirective lenses and radiators. While there has been a never-ending stream of physics publications, little has happened in the engineering electromagnetics community. I will try to answer the title with examples that may stimulate interest in the field

The design, simulation, and pattern synthesis of novel reflectarrays

The main focus of this thesis is on the development of both a comprehensive understanding and a thorough computational routine of the reflectarray metasurfaces designs, with focuses on a liquid crystal-based reconfigurable reflectarray metasurface and on the phase-retrieval/optimisation techniques for reflectarray-based pattern synthesis. A dielectric-based polarisation converting reflectarray metasurface is also presented, with the advantage of having a thinner profile over the traditional quartz-based half-wave plates. In the introductory sections, a thorough review of the state-of-the-art metasurfaces is presented, with a focus on applications to high-frequency wireless communications, as the motivation of this PhD is on the development of technologies that would facilitate the wireless communication challenges for the 5G-and-beyond frequency spectrum. In this section, a review of array antennas, including phased arrays, reflectarrays, transmitarrays, as well as metasurfaces utilising Mie-resonance, plasmonic resonance, geometric phase (Pancharatnam-Berry phase) and photoconductive material is presented. The following chapter on the theoretical background ensures the understanding of the fundamental mechanisms that will be applied to the study of reflectarray metasurfaces and optimisation routines. The high-frequency propagation associated with beyond-5G wireless communications brings many challenges to the current standards: some of the biggest problems are the much greater path loss and heavy non-line-of-sight signal attenuation. Traditionally, this has been dealt with in phased arrays. However, in the introduction part of this thesis, I show that this becomes impractical due to the requirements of enormous array sizes and expensive high-frequency phase shifters. Therefore, in our research, we have focused on reconfigurable reflectarrays as an intermediate solution to alleviate the tough propagation challenges faced by beyond-5G wireless communications. The reconfigurable reflectarray can either be designed to reflect off from a nearby mobile cell site to enhance the signal strength for non-line-of-sight areas, or it can include an integrated source to function independently, reducing the losses associated with power amplifiers and complex circuitries associated with the enormous array sizes. This thesis aims to produce a high-frequency tailored reconfigurable reflectarray design, which combines the conceptual advantages from state of-the-art lumped-element-based and liquid crystal-based reflectarrays. As shown in the literature review section, most recent researches on lumpedelement- based reconfigurable reflectarrays are designed for the sub 40 GHz frequencies; with higher frequencies, the intrinsic losses associated with lumped-elements such as PIN diodes make them unsuitable choices. On the other hand, liquid crystals have been used as a tunable material for different radio-frequency applications; however, most state-of-the-art designs of liquid crystal-based reflectarrays do not incorporate individual biasing control for maximum beam-control. There are also challenges faced with individually-biased reconfigurable reflectarrays. Traditionally, phased arrays can perform single beam-scanning or multiple beam-scanning with the control of multiple sub-arrays. We intend to achieve more complex beam-functionalities (such as vortex, null, and magnitude-specific beams) within the domain of manipulating one individual array. This is already possible with optimisation algorithms such as the genetic algorithm. However, traditional optimisers such as the genetic algorithm and particle swarm optimisation are far too slow to be implemented in an “online” mode, where the algorithm runs onboard the reflectarray to give low-latency solutions. The “online” optimisation mode would be very beneficial as it would reduce the channel occupation from the transmission of configuration information and thus increase channel capacity. In this thesis, I aim to develop an individually biased liquid crystal-based reconfigurable reflectarray for >100 GHz frequencies. I also aim to develop an algorithm that is sufficiently quick to have the potential to be practically utilised as an onboard pattern synthesis optimisation method. Additionally, using the same design principles, I have designed an all-dielectric-based reflectarray metasurface that acts as a polarisation-converting quarter-wave plate, which is much thinner than traditional quartz-based quarter-wave plates. In the Research Results and Publications chapter, a complete procedure for the design of LC-based reconfigurable and dielectric-based nonreconfigurable reflectarray metasurfaces is presented, where much of the content comes from the author’s own publications[52, 78, 50, 51]. This thesis provides details on the computational tools/programs used, cross-platform routines development with CST Studio Suite, MATLAB and VBA, and the pattern synthesis algorithm, whereby a genetic algorithm is employed for the global optimisation, and an improved Gerchberg-Saxton algorithm is developed and adapted to the application of faster local optimisation for the pattern synthesis. For the all-dielectric reflectarray metasurface, the further functionality of polarisation conversion (linear to circular and circular to linear) is demonstrated on top of the beam-manipulation capabilities of the reflectarrays. The reflectarray metasurfaces can be designed to beamform, beamsteer, beamsplit/multibeam, as well as achieve novel beam profiles such as the vortex profile. Originally, the idea was to completely focus on the liquid crystal-based study and to develop a down-scaled 28 GHz proof-of-concept, for which partial work had already begun (the simulation, optimisation and initial planning on the construction with collaborators from other departments); however, due to the pandemic and numerous other uncontrollable factors, this was later discarded and replaced by remaining on and extending upon the computational studies, to further understand and improve the pattern synthesis algorithms and the other types of phase-change metasurfaces

Reconfigurable pixel antennas for communications

The explosive growth of wireless communications has brought new requirements in terms of compactness, mobility and multi-functionality that pushes antenna research. In this context, recon gurable antennas have gained a lot of attention due to their ability to adjust dynamically their frequency and radiation properties, providing multiple functionalities and being able to adapt themselves to a changing environment. A pixel antenna is a particular type of recon gurable antenna composed of a grid of metallic patches interconnected by RF-switches which can dynamically reshape its active surface. This capability provides pixel antennas with a recon guration level much higher than in other recon gurable architectures. Despite the outstanding recon guration capabilities of pixel antennas, there are important practical issues related to the performance-complexity balance that must be addressed before they can be implemented in commercial systems. This doctoral work focuses on the minimization of the pixel antenna complexity while maximizing its recon guration capabilities, contributing to the development of pixel antennas from a conceptual structure towards a practical recon gurable antenna architecture. First, the conceptualization of novel pixel geometries is addressed. It is shown that antenna complexity can be signi cantly reduced by using multiple-sized pixels. This multi-size technique allows to design pixel antennas with a number of switches one order of magnitude lower than in common pixel structures, while preserving high multiparameter recon gurability. A new conceptual architecture where the pixel surface acts as a parasitic layer is also proposed. The parasitic nature of the pixel layer leads to important advantages regarding the switch biasing and integration possibilities. Secondly, new pixel recon guration technologies are explored. After investigating the capabilities of semiconductors and RF-MEMS switches, micro uidic technology is proposed as a new technology to create and remove liquid metal pixels rather than interconnecting them. Thirdly, the full multi-parameter recon guration capabilities of pixel antennas is explored, which contrasts with the partial explorations available in the literature. The maximum achievable recon guration ranges (frequency range, beam-steering angular range and polarization modes) as well as the linkage between the di erent parameter under recon guration are studied. Finally, the performance of recon gurable antennas in beam-steering applications is analyzed. Figures-of-merit are derived to quantify radiation pattern recon gurability, enabling the evaluation of the performance of recon gurable antennas, pixel antennas and recon guration algorithms

Miniaturization and evaluation methods of mobile terminal antenna structures

In recent years, the trend in the mobile communications market has been towards thinner and mechanically more complex terminal devices, which are able to operate in several wireless systems. Due to this development, miniaturization and performance enhancement of internal mobile terminal antennas have become major challenges for the industry of the field. Not only should an internal antenna be small in size and broadband, it must also be able to ensure reliable power transmission in a real multipath propagation environment. A well-known way to improve the reliability of a radio connection is to use multi-antenna reception in the mobile terminal. In multiple-input multiple-output (MIMO) systems, multiple antennas are additionally used at the base station. In both cases, fast and reliable methods are needed for the performance evaluation of multi-antenna terminals. In the first part of this thesis, the main benefits, drawbacks and applications of coupling element based mobile terminal antenna structures are studied, with special emphasis on antenna miniaturization. Optimum shaping and placement of capacitive coupling elements are first studied in this work. Next, it is demonstrated by simulations and measurements that the bandwidth-to-volume ratio of a mobile terminal antenna structure can be improved significantly by using optimized coupling elements instead of traditional self-resonant antenna elements. To facilitate the implementation of coupling element based multi-resonant antenna structures, a theoretical study on the dual-resonant impedance matching of non-resonant coupling elements is also presented. The feasibility of coupling elements for multi-band terminals is demonstrated with a novel quad-band GSM antenna, which consumes a total volume of only 0.7 cm3. Finally, a novel frequency tunable matching circuitry designed for non-resonant coupling elements is introduced. In this work, also the bandwidth, efficiency in talk position, and SAR (specific absorption rate) of mobile phone antennas are studied as a function of frequency over wide frequency band (0.6 GHz - 6 GHz) by applying the idea of coupling elements. The results show that below 3 GHz the three parameters are strongly affected by the resonant wavemodes of the chassis, whereas above 3 GHz, the wavemodes of the coupling element dominate. In addition to the above, the resonant wavemodes of the chassis of a clamshell phone are investigated in this work by using coupling elements. The results bring out several challenges, such as a non-radiating resonance, that an antenna designer may face with clamshell phones. The second part of this thesis concentrates on the performance evaluation of mobile terminal multi-antenna configurations. First, the accuracy of a novel measurement based antenna test bed (MEBAT) is thoroughly studied. After this, the performance of several mobile terminal multi-antenna configurations is systematically investigated using the MEBAT. The emphasis is kept on the power reception properties (effective array gain or EAG) of the multi-antenna configurations. An accurate and fast theoretical way of predicting the median EAG of an antenna configuration is proposed in the work. Based on a comprehensive analysis of the theoretical and empirical EAG results, guidelines for optimum radiation pattern characteristics of a multi-antenna configuration are given. Based on the eigenvalue dispersion and capacity results obtained in the studied MIMO environments, it is concluded that it may be difficult to affect the spatial multiplexing properties of a MIMO system by means of handset antenna design. The presented results indicate that a handset antenna designer should mainly focus on maximizing the EAG.Viime vuosina yleinen kehityssuunta matkaviestinmarkkinoilla on ollut kohti ohuempia ja mekaanisesti monimutkaisempia päätelaitteita, jotka kykenevät toimimaan useissa langattomissa järjestelmissä. Tämän kehityksen johdosta matkapuhelinten sisäisten antennien koon pienentäminen ja suorituskyvyn parantaminen ovat muodostuneet merkittäviksi haasteiksi alan teollisuudelle. Sen lisäksi, että sisäisen antennin tulee olla pienikokoinen ja laajakaistainen, tulee sen myös kyetä takaamaan luotettava tehonsiirto todellisessa monitie-etenemisympäristössä. Hyvin tunnettu tapa radioyhteyden luotettavuuden parantamiseksi on hyödyntää moniantennivastaanottoa matkapuhelimessa. Multiple-input multiple-output (MIMO)-järjestelmissä useita antenneja hyödynnetään lisäksi tukiasemassa. Kummassakin tapauksessa tarvitaan nopeita ja luotettavia menetelmiä matkapuhelinten moniantennirakenteiden suorituskyvyn arvioimiseksi. Työn ensimmäisessä osassa tutkitaan kytkentäelementteihin perustuvien antennirakenteiden tärkeimpiä hyötyjä, haittoja ja sovelluksia painottaen antennien koon pienentämistä. Työssä tutkitaan ensin kapasitiivisten kytkentäelementtien optimaalista muotoilua ja sijoittamista. Seuraavaksi osoitetaan simulaatioin ja mittauksin, että matkapuhelimen antennirakenteen kaistanleveys-tilavuus-suhdetta saadaan parannettua huomattavasti käyttämällä optimoituja kytkentäelementtejä perinteisten resonanssityyppisten antennielementtien sijaan. Kytkentäelementteihin perustuvien moniresonanssi antennirakenteiden toteuttamisen helpottamiseksi työssä esitetään myös teoreettinen tutkimus ei-resonoivien kytkentäelementtien sovittamisesta kaksoisresonanssiin. Kytkentäelementtien soveltuvuutta monitaajuuspäätelaitteisiin havainnollistetaan uudentyyppisellä nelitaajuus-GSM-antennilla, jonka kokonaistilavuus on ainoastaan 0.7 cm3. Lopuksi esitellään uudentyyppinen kytkentäelementeille suunniteltu taajuussäädettävä sovituspiiri. Tässä työssä tutkitaan myös matkapuhelinantennien kaistanleveyttä, hyötysuhdetta puheasennossa ja SAR- (specific absorption rate) arvoja taajuuden funktiona laajalla taajuuskaistalla (0.6 GHz - 6 GHz) hyödyntäen kytkentäelementtejä. Tulokset osoittavat, että 3 GHz:n alapuolella näihin kolmeen parametriin vaikuttavat vahvasti rungon resonoivat aaltomuodot, kun taas 3 GHz:n yläpuolella kytkentäelementin aaltomuodot dominoivat. Yllä olevan lisäksi työssä tutkitaan taitettavan puhelimen rungon resonoivia aaltomuotoja kytkentäelementtien avulla. Tulokset tuovat esille useita haasteita, kuten säteilemättömän resonanssin, joita antennisuunnittelija saattaa kohdata taitettavien puhelinten kanssa. Työn toinen osa keskittyy matkapuhelinten moniantennirakenteiden suorituskyvyn arviointiin. Ensin uudentyyppisen mittauksiin perustuvan antennien testialustan (MEBAT) tarkkuutta arvioidaan perusteellisesti. Tämän jälkeen useiden matkapuhelinten moniantennirakenteiden suorituskykyä tutkitaan järjestelmällisesti MEBATin avulla. Painotus on moniantennirakenteiden tehovastaanotto-ominaisuuksissa (effective array gain tai EAG). Työssä ehdotetaan tarkkaa ja nopeaa teoreettista tapaa antennirakenteen EAG:n mediaanin ennustamiseksi. Teoreettisten ja empiiristen EAG-tulosten perusteellisen analyysin pohjalta annetaan suosituksia moniantennijärjestelmän säteilykuvion optimaalisille ominaisuuksille. Tutkituissa MIMO-järjestelmissä saatujen ominaisarvohaje- ja kapasiteettitulosten perusteella tehdään johtopäätös, että matkapuhelimen antennisuunnittelun keinoin voi olla vaikeaa vaikuttaa MIMO-järjestelmän kykyyn muodostaa rinnakkaisia kanavia. Esitetyt tulokset ehdottavat, että matkapuhelimen antennisuunnittelijan tulisi pääasiassa keskittyä EAG:n maksimointiin.reviewe

Physical Knowledge Based Scalable Phased Array Antenna Modeling for Radar Systems

The development of a large-scale phased array radar system such as the future MPAR will need a cost-effective tool for predicting electromagnetic characteristics of antennas. Simulating and optimizing of large finite phased array antennas using commercially available solvers are time-consuming and memory-extensive even though they are highly capable of solving general electromagnetic problems with acceptable accuracy. In this work, a full-wave electromagnetic solver based on finite-difference time-domain (FDTD) method has been developed for simulating phased array antennas. The planar array or array element can be simulated, optimized, or analyzed using FDTD theory based on an orthogonal, regular Cartesian lattice. The FDTD updating equation for diagonally anisotropic material was obtained for periodic structure based on the cylindrical coordinate system. This FDTD algorithm can be used to simulate active element patterns of conformally cylindrical array antennas. The simulation of active element patterns in an infinite faceted-cylindrical array was accomplished with a nonorthogonal and unstructured grid. The derivation of FDTD theory and periodic boundary condition for a structure based on the nonorthogonal and unstructured grid is presented. In this work, two simulation schemes, which are based on computed near-field current density information and the physical knowledge of finite array antennas, were presented for predicting broadside array radiation characteristics with the consumption of relatively low computational resources. The validation of the simulation program and schemes was fulfilled by comparing simulation results with measurements taken by near-field and far-field techniques

Mutual Coupling Reduction Techniques for Multi-band Base Station Antennas

This dissertation proposes antenna design techniques which suppress mutual coupling in densely populated dual-polarized broadband multi-band base station antenna (BSA) arrays for improved radiation characteristics. Array face densification without the proposed techniques leads to undesirable interactions between arrays and radome cover. This research is supported by extensive full-wave electromagnetic simulations, characteristic mode analysis, equivalent circuit models, and array theory. The results are validated through measurements of the radiation patterns and scattering parameters of fabricated prototypes.Upon excitation of a mid-band (MB) array, an electrically long low-band (LB) parasitic, necessary for broadband matching of the baseline LB dipole, exhibits dipole-like induced currents which degrade the MB radiation patterns. The currents are suppressed through parallel plate series capacitance integrated along the length of the LB dipole arms to lower the dipole self-impedance, enabling broadband matching using an electrically short LB parasitic. The proposed technique improves MB radiation pattern symmetry, cross-polar radiation (CPR), sidelobe level, and gain. Upon excitation of the proposed LB dipole, the baseline MB dipole balun feed stems exhibit induced common mode currents, which radiate as electrically short monopoles over a conducting ground plane, that degrade the LB radiation characteristics. These currents are suppressed by integrating parallel plate series capacitance along the length of the balanced ports of the MB balun. The proposed balun improves the LB beamwidth stability, CPR, and gain stability. The proposed serially loaded LB dipole and MB dipole balun feed stem are implemented simultaneously in a six-foot BSA in which the system performance improvements are verified. Upon excitation of a high-band (HB) array in a tri-band array environment, the dielectric radome behaves as a partially reflecting surface. The reflected waves interfere with the HB excitation, resulting in significant boresight radiation attenuation in the upper part of the HB operating band. A Fabry-Perot cavity antenna (FPCA) operating in the second resonance mode (N=1) is proposed to improve this attenuation through increasing the boresight directivity. Upon HB excitation of the proposed FPCA, circular-disk MB parasitics, necessary for broadband matching of the neighboring MB array, exhibit dipole-like induced currents. The induced currents radiate as dipoles which reduce the HB half power beamwidth (HPBW) in the lower and middle part of the HB operating band. These currents are suppressed by introducing electrically thin radial slots in the baseline MB parasitic. The radial slots change the characteristic modes and lower their modal significance, which eliminates the secondary radiation sources. The proposed technique increases the HPBW in the lower and middle part of the HB operating band. These proposed design techniques provide new tools to antenna designers which allow for the densification of BSA arrays for use in next generation cellular networks, while minimizing radiation pattern degradation otherwise present in baseline BSA designs using existing techniques