This thesis presents an efficient self-consistent Hybrid Spectral Ray Tracing (HSRT) technique for analysis and design of multi-scale sub-millimeter wave problems, where sub-wavelength features are modeled using rigorous methods, and complex structures with dimensions in the order of tens or even hundreds of wavelengths are modeled by asymptotic methods. Quasi-optical devices are used in imaging arrays for sub-millimeter and terahertz applications, THz time-domain spectroscopy (THz-TDS), high-speed wireless communications, and space applications to couple terahertz radiation from space to a hot electron bolometer. These devices and structures, as physically small they have become, are very large in terms of the wavelength of the driving quasi-optical sources and may have dimension in the tens or even hundreds of wavelengths. Simulation and design optimization of these devices and structures is an extremely challenging electromagnetic problem. The analysis of complex electrically large unbounded wave structures using rigorous methods such as method of moments (MoM), finite element method (FEM), and finite difference time domain (FDTD) method can become almost impossible due to the need for large computational resources. Asymptotic high-frequency techniques are used for analysis of electrically large quasi-optical systems and hybrid methods for solving multi-scale problems. Spectral Ray Tracing (SRT) has a number of unique advantages as a candidate for hybridization. The SRT method has the advantages of Spectral Theory of Diffraction (STD). STD can model reflection, refraction and diffraction of an arbitrary wave incident on the complex structure, which is not the case for diffraction theories such as Geometrical Theory of Diffraction (GTD), Uniform theory of Diffraction (UTD) and Uniform Asymptotic Theory (UAT). By including complex rays, SRT can effectively analyze both near-fields and far-fields accurately with minimal approximations. In this thesis, a novel matrix representation of SRT is presented that uses only one spectral integration per observation point and applied to modeling a hemispherical and hyper-hemispherical lens. The hybridization of SRT with commercially available FEM and MoM software is proposed in this work to solve the complexity of multi-scale analysis. This yields a computationally efficient self-consistent HSRT algorithm. Various arrangements of the Hybrid SRT method such as FEM-SRT, and MoM-SRT, are investigated and validated through comparison of radiation patterns with Ansoft HFSS for the FEM method, FEKO for MoM, Multi-level Fast Multipole Method (MLFMM) and physical optics. For that a bow-tie terahertz antenna backed by hyper-hemispherical silicon lens, an on-chip planar dipole fabricated in SiGe:C BiCMOS technology and attached to a hyper-hemispherical silicon lens and a double-slot antenna backed by silica lens will be used as sample structures to be analyzed using the HSRT. Computational performance (memory requirement, CPU/GPU time) of developed algorithm is compared to other methods in commercially available software. It is shown that the MoM-SRT, in its present implementation, is more accurate than MoM-PO but comparable in speed. However, as shown in this thesis, MoM-SRT can take advantage of parallel processing and GPU. The HSRT algorithm is applied to simulation of on-chip dipole antenna backed by Silicon lens and integrated with a 180-GHz VCO and radiation pattern compared with measurements. The radiation pattern is measured in a quasi-optical configuration using a power detector. In addition, it is shown that the matrix formulation of SRT and HSRT are promising approaches for solving complex electrically large problems with high accuracy. This thesis also expounds on new measurement setup specifically developed for measuring integrated antennas, radiation pattern and gain of the embedded on-chip antenna in the mmW/ terahertz range. In this method, the radiation pattern is first measured in a quasi-optical configuration using a power detector. Subsequently, the radiated power is estimated form the integration over the radiation pattern. Finally, the antenna gain is obtained from the measurement of a two-antenna system.

This dissertation presents the research on several different projects. The first project is a via-less CPW RF probe pad to microstrip transition; The second, the third, and the fourth one are reconfigurable microwave circuits using RF MEMS switches: an X-band reconfigurable bandstop filter for wireless RF frontends, an X-band reconfigurable impedance tuner for a class-E high efficiency power amplifier using RF MEMS switches, and a reconfigurable self-similar antenna using RF MEMS switches. The first project was developed in order to facilitate the on-wafer measurement for the second and the third project, since both of them are microstrip transmission line based microwave circuits. A thorough study of the via-less CPW RF probe pad to microstrip transition on silicon substrates was performed and general design rules are derived to provide design guidelines. This research work is then expanded to W-band via-less transition up to 110 GHz. The second project is to develop a low power reconfigurable monolithic bandstop filter operating at 8, 10, 13, and 15 GHz with cantilever beam capacitive MEMS switches. The filter contains microstrip lines and radial stubs that provide different reactances at different frequencies. By electrically actuating different MEMS switches, the different reactances from different radial stubs connecting to these switches will be selected, thus, the filter will resonate at different frequencies. The third project is to develop a monolithic reconfigurable impedance tuner at 10 GHz with the cantilever DC contact MEMS switch. The impedance tuner is a two port network based on a 3bit-3bit digital design, and uses 6 radial shunt stubs that can be selected via integrated DC contact MEMS switches. By selecting different states of the switches, there will be a total of 2^6 = 64 states, which means 64 different impedances will be generated at the output port of the tuner. This will provide a sufficient tuning range for the output port of the power amplifier to maximize the power efficiency. The last project is to integrate the DC contact RF MEMS switches with self-similar planar antennas, to provide a reconfigurable antenna system that radiates with similar patterns over a wide range of frequencies.

Multi-hop wireless networks or ad-hoc networks face several limiting characteristics that make it difficult to support a multitude of applications. It is in this context that we find smart antennas to find significant applications in these networks, owing to their ability to alleviate most of these limitations. The focus of my research is thus to investigate the use of smart antennas in ad-hoc networks and hence efficiently design network protocols that best leverage their capabilities in communication. There are two parts to the proposed objective of designing efficient network protocols that pertain to the nature of the smart antenna network considered, namely, homogeneous and heterogeneous smart antenna networks. Unlike heterogeneous smart antenna networks, where different devices in the network employ different antenna technologies, homogeneous smart antenna networks consist of devices employing the same antenna technology. Further, in homogeneous smart antenna networks, different antenna technologies operating in different strategies tend to perform the best in different network architectures, conditions and application requirements. This motivates the need for developing a {em unified} framework for designing efficient communication (medium access control and routing) protocols for homogeneous smart antenna networks in general. With the objective of designing such a unified framework, we start by designing efficient MAC and routing protocols for the most sophisticated of the smart antenna technologies, namely multiple-input multiple-output (MIMO) links. The capabilities of MIMO links form a super-set of those possible with other antenna technologies. Hence, the insights gained from the design of communication protocols for MIMO links are then used to develop unified MAC and routing frameworks for smart antennas in general. For heterogeneous smart antenna networks, we develop theoretical performance bounds by studying the impact of increasing degree of heterogeneity on network throughput performance. Given that the antenna technologies are already unified in the network, unified solutions are not required. However, we do develop efficient MAC and routing protocols to best leverage the available heterogeneous capabilities present in the network. We also design efficient cooperation strategies that will further help the communication protocols in exploiting the available heterogeneous capabilities in the network to the best possible extent.

Bandwidth enhancement of the LTE/WWAN handset antenna achieved by improving the excited surface current distribution in the system ground plane is presented. To achieve this goal, the system ground plane is shaped to be of a C-shape in this thesis. In this case, the longitudinal excited surface current in the system ground plane can have a smooth distribution. The proposed antenna is disposed at one edge of the C-shaped system ground plane, and the antenna comprises a driven monopole strip, a shorted monopole strip and a distributed parallel resonance (PR) circuit. By controlling the resonant frequency of the distributed PR circuit, it can result in dual-resonance excitation of the lowest resonant mode contributed by the shorted monopole strip to cover the LTE700/GSM850/900 operation. The antenna also occupies a small volume of 44 ¡Ñ 10 ¡Ñ 5 mm3 (about 2.2 cm3). Good radiation characteristics of the antenna are also obtained. For practical applications, effects of the electrical connection between the system ground plane and the surrounding metal elements such as the battery¡¦s metal casing and the handset metal midplate are studied. The simulated SAR (Specific Absorption Rate) and HAC (Hearing Aid Compatibility) results are also analyzed.

Fractal is a recursively-generated object describing a family of complex shapes that possess an inherent self-similarity in their geometrical structure. When used in antenna engineering, fractal geometries provide multi-band characteristics and lowering resonance frequencies by enhancing the space filling property. Moreover, utilizing fractal arrays, controlling side lobe-levels and radiation patterns can be realized. In this thesis, the performance of Koch curve as antenna is investigated. Since fractals are complex shapes, there is no well&ndash / established for mathematical formulation to obtain the radiation properties and frequency response of Koch Curve antennas directly. The Koch curve antennas became famous since they exhibit better frequency response than their Euclidean counterparts. The effect of the parameters of Koch geometry to antenna performance is studied in this thesis. Moreover, modified Koch geometries are generated to obtain the relation between fractal properties and antenna radiation and frequency characteristics.

In this thesis, an analysis technique for the slot coupled patch antennas using MoM in conjunction with the closed form Green&rsquo / s functions is presented. Slot coupled patch antennas are fed by a microstrip open stub which is coupled to the patch through an electrically small slot. Current distributions over the microstrip line, slot line and the patch are represented by rooftop basis functions. First, a relatively simple structure, microstrip coupled slot line is investigated using the proposed technique. Then the method is extended to the slot coupled patch antenna geometry. By using the method, current distributions on the feedline and the patch are calculated for a generic slot coupled patch antenna. Then by using the distributions, return scattering parameters of the antenna is approximated with complex exponentials using Prony&rsquo / s method. A parametric study is carried out to observe the effect of each antenna component on the antenna performance. Current distributions and return loss calculations are repeated for modified antennas to observe and demonstrate the performance differences. All simulations are verified using HFSS&reg / software and the results available in the literature.

Dual band (K and Ka) electronically scanning reflectarray with RF MEMS switches is designed, implemented and measured. Unit cell of the reflect array is composed of conductor backed split-ring elements. In order to steer the beam, the phase of the incident circularly polarized wave is controlled by RF MEMS switches that modify the angular orientation of split-rings individually. Reflectarray is designed using unit cell approach with periodic boundary conditions. The antenna is fabricated by using surface micromachining process developed in METU MEMS Center. Radiation patterns of the antenna are measured and compared with the simulations. It has been shown that the reflectarray is capable of beam switching to 35&deg / in Ka band, 24&deg / in K band.

Antenna is an indispensable part of a radar or free space communication system. Antenna requires different stringent specifications for different applications. Designed and fabricated for an intended application, antenna or antenna array has to be evaluated for its far-field characteristics in real free space environment which requires setting up of far-field test site. Maintenance of the site to keep the stray reflections levels low, the cost of the real estate are some of the disadvantages. Nearfield measurements are compact and can be used to test the antennas by exploiting the relationship between near-field and far-field. It is shown that the far-field patterns of an antenna can be sufficiently accurately predicted provided the near-field measurements are accurate. Due to limitation in the near-field measurement systems, errors creep in corrupting the nearfield-measured data thus making error in prediction of the far field. All these errors ultimately corrupt the phase and amplitude data. In this thesis, one such near-field measurement facility, the Planar Near Field Measurement facility is discussed. The limitations of the facility and the errors that occur due to their limitations are discussed. Various errors that occur in measurements ultimately corrupt the near-field phase and amplitude. Investigations carried out aim at a detailed study of these phase and amplitude errors and their effect on the far-field patterns of the antenna. Depending on the source of error, the errors are classified as spike, pulse and random errors. The location of occurrence of these types of errors in the measurement plane, their effects on the far-field of the antenna is studied both for phase and amplitude errors. The studies conducted for various phase and amplitude errors show that the near-field phase and amplitude data are more tolerant to random errors as the far-field patterns do not get affected even for low sidelobe cases. The spike errors, though occur as a wedge at a single point in the measurement plane, have more pronounced effect on the far-field patterns. Lower the taper value of the antenna, more pronounced is the error. It is also noticed that the far-field pattern gets affected only in the plane where the error has occurred and has no effect in the orthogonal plane. Pulse type of errors which occur even for a short length in the measurement affect both the principle plane far-field patterns. This study can be used extensively as a tool to determine to the level to which various error such as mechanical, RF etc need to be controlled to make useful and correct pattern predictions on a particular facility. Thereby, the study can be used as a tool to economise the budget of the facility wherein the parameters required for building the facility need not be over specified beyond the requirement. In general, though this is a limited study, it is certainly a trendsetter in this direction.

Στην παρούσα διδακτορική διατριβή παρουσιάζεται µια εκτενής µελέτη συστηµάτων απόκλισης τυπωµένων κεραιών για ασύρµατες συσκευές. Η εξέλιξη των ασύρµατων επικοινωνιών και ειδικότερα η κατανόηση των φαινοµένων που λαµβάνουν χώρα στο ασύρµατο κανάλι µετάδοσης έδωσε ώθηση στην ανάπτυξη τεχνικών αντιµετώπισης φαινοµένων όπως η πολυοδευτική εξασθένηση και η διασυµβολική παρεµβολή. Τέτοιες τεχνικές είναι οι τεχνικές απόκλισης και ειδικότερα οι τεχνικές απόκλισης κεραιών (χώρου, πόλωσης και διαγράµµατος ακτινοβολίας) οι οποίες συνδυάζουν τα λαµβανόµενα από διαφορετικές κεραίες σήµατα µε σκοπό την δηµιουργία ενός σήµατος υψηλότερης στάθµης. Η µεθοδολογία αξιολόγησης της ικανότητας απόκλισης ενός συστήµατος κεραιών περιλαµβάνει τα κριτήρια του συντελεστή συσχέτισης των λαµβανοµένων σηµάτων και του λόγου των µέσων ενεργών κερδών των κεραιών που αποτελούν το σύστηµα. Καθοριστικό ρόλο στην τεχνική απόκλισης διαδραµατίζει ο τρόπος που συνδυάζονται τα λαµβανόµενα σήµατα στον δέκτη, µε την τεχνική του µεγίστου λόγου να αποτελεί την βέλτιστη λύση από πλευράς απόδοσης, αλλά ταυτοχρόνως την πιο δύσκολα υλοποιήσιµη. Η ποσοτικοποίηση της ικανότητας απόκλισης ενός συστήµατος πραγµατοποιείται µε το κέρδος απόκλισης το οποίο λαµβάνει διαφορετικές µορφές και ουσιαστικά παρέχει την πληροφορία του ποσοστού βελτίωσης του συνδυασµένου σήµατος σε σχέση µε µία κεραία αναφοράς. Στα πλαίσια της διατριβής αναπτύχθηκαν συστήµατα απόκλισης τυπωµένων κεραιών δύο, τριών, τεσσάρων έως και έξι στοιχείων. Χρησιµοποιήθηκαν κεραίες γεωµετρίας fractal λόγω της σπουδαίας ικανότητας σµίκρυνσης που παρέχει η εν λόγω τεχνική καθώς και το µονόπολο γεωµετρίας ανεστραµµένου F. Ακολουθήθηκε η µεθοδολογία αξιολόγησης της ικανότητας απόκλισης για όλα τα προτεινόµενα συστήµατα και έγινε σύγκριση µεταξύ των συστηµάτων. Μελετήθηκε αφ’ ενός η επίδραση της θέσης της κεραίας σε µία ασύρµατη συσκευή και αφ’ ετέρου η επιλογή της καταλληλότερης προς χρήση κεραίας στην µείωση του φαινοµένου της αµοιβαίας σύζευξης, φαινόµενο το οποίο αποτελεί τον κυριότερο παράγοντα µείωσης της συνολικής ικανότητας απόκλισης σε ένα σύστηµα πολλαπλών κεραιών. Ακόµη, µελετήθηκαν συστήµατα απόκλισης κεραιών τα οποία λειτουργούν σε διαφορετικές µπάντες συχνοτήτων. Τέλος, έγινε µια συγκριτική µελέτη για τον ρόλο που διαδραµατίζει το περιβάλλον και ο τρόπος που κατανέµεται η προσπίπτουσα ισχύς στην συµπεριφορά της απόκλισης των µελετηθέντων συστηµάτων. / This thesis presents a comprehensive study on printed antenna diversity systems for wireless devices. The current upsurge in wireless communications systems and, in particular, the realization of the immanent propagation mechanisms that take place in the transmission medium led to the development of special techniques in order to mitigate the undesired phenomena such as multipath fading and intersymbol interference. Antenna diversity (space, polarization and pattern) is one of these techniques and is based on the assumption that the received signals of two or more antennas could be efficiently combined in order to produce a stronger signal. The evaluation of the diversity performance involves the correlation coefficient of the received signals and the mean effective gain ratio of the diversity antennas. A key role in diversity action plays the combining technique used with the maximum ratio technique producing the best results. Diversity performance is “quantified” by means of diversity gain, which gives the amount of improvement of the combined signal relative to a signal received from a reference antenna. In this thesis, printed antenna diversity systems comprising up to six elements were developed. Antennas of fractal geometry such as the Koch, Minkowski, Sierpinski and FRC monopoles, were utilized due to their inherent miniaturization ability as well as the printed inverted F antenna. All the proposed systems were evaluated according to the methodology mentioned earlier and a comparison of the diversity performance between the systems was carried out. The effect of the antenna placement as well as the proper antenna selection for each system on the reduction of mutual coupling was addressed, which is an issue of primal importance in multi element antenna systems. Multi band antenna diversity systems were also studied. Finally, the impact of the environment’s power distribution on the diversity performance of the antenna systems was considered.

This thesis presents an efficient self-consistent Hybrid Spectral Ray Tracing (HSRT) technique for analysis and design of multi-scale sub-millimeter wave problems, where sub-wavelength features are modeled using rigorous methods, and complex structures with dimensions in the order of tens or even hundreds of wavelengths are modeled by asymptotic methods. Quasi-optical devices are used in imaging arrays for sub-millimeter and terahertz applications, THz time-domain spectroscopy (THz-TDS), high-speed wireless communications, and space applications to couple terahertz radiation from space to a hot electron bolometer. These devices and structures, as physically small they have become, are very large in terms of the wavelength of the driving quasi-optical sources and may have dimension in the tens or even hundreds of wavelengths. Simulation and design optimization of these devices and structures is an extremely challenging electromagnetic problem. The analysis of complex electrically large unbounded wave structures using rigorous methods such as method of moments (MoM), finite element method (FEM), and finite difference time domain (FDTD) method can become almost impossible due to the need for large computational resources. Asymptotic high-frequency techniques are used for analysis of electrically large quasi-optical systems and hybrid methods for solving multi-scale problems. Spectral Ray Tracing (SRT) has a number of unique advantages as a candidate for hybridization. The SRT method has the advantages of Spectral Theory of Diffraction (STD). STD can model reflection, refraction and diffraction of an arbitrary wave incident on the complex structure, which is not the case for diffraction theories such as Geometrical Theory of Diffraction (GTD), Uniform theory of Diffraction (UTD) and Uniform Asymptotic Theory (UAT). By including complex rays, SRT can effectively analyze both near-fields and far-fields accurately with minimal approximations. In this thesis, a novel matrix representation of SRT is presented that uses only one spectral integration per observation point and applied to modeling a hemispherical and hyper-hemispherical lens. The hybridization of SRT with commercially available FEM and MoM software is proposed in this work to solve the complexity of multi-scale analysis. This yields a computationally efficient self-consistent HSRT algorithm. Various arrangements of the Hybrid SRT method such as FEM-SRT, and MoM-SRT, are investigated and validated through comparison of radiation patterns with Ansoft HFSS for the FEM method, FEKO for MoM, Multi-level Fast Multipole Method (MLFMM) and physical optics. For that a bow-tie terahertz antenna backed by hyper-hemispherical silicon lens, an on-chip planar dipole fabricated in SiGe:C BiCMOS technology and attached to a hyper-hemispherical silicon lens and a double-slot antenna backed by silica lens will be used as sample structures to be analyzed using the HSRT. Computational performance (memory requirement, CPU/GPU time) of developed algorithm is compared to other methods in commercially available software. It is shown that the MoM-SRT, in its present implementation, is more accurate than MoM-PO but comparable in speed. However, as shown in this thesis, MoM-SRT can take advantage of parallel processing and GPU. The HSRT algorithm is applied to simulation of on-chip dipole antenna backed by Silicon lens and integrated with a 180-GHz VCO and radiation pattern compared with measurements. The radiation pattern is measured in a quasi-optical configuration using a power detector. In addition, it is shown that the matrix formulation of SRT and HSRT are promising approaches for solving complex electrically large problems with high accuracy. This thesis also expounds on new measurement setup specifically developed for measuring integrated antennas, radiation pattern and gain of the embedded on-chip antenna in the mmW/ terahertz range. In this method, the radiation pattern is first measured in a quasi-optical configuration using a power detector. Subsequently, the radiated power is estimated form the integration over the radiation pattern. Finally, the antenna gain is obtained from the measurement of a two-antenna system.