Thesis (M.Eng.)--Memorial University of Newfoundland, 1999. / Bibliography: leaves [97]-101.

Thesis (M.S.E.E.)--University of Puerto Rico, Mayagüez Campus, 2005. / Tables. Printout. Includes bibliographical references (leaves146-148).

In order to establish a secure connections in the wireless environment, cryptographic methods may require an exchange of a key or secret. Fortunately, the environment provides randomness due to multi-path fading that can be exploited by physical-layer security algorithms to help establish this shared secret. However, in some cases, multi-path fading might be absent or negligible; therefore, we look for artificial ways to increase randomness. In this thesis, we explore antenna radiation variation by altering the phase between two antennas as a means of creating artificial fading. We construct a model of the antenna gain variation by analyzing the radiation pattern and run Monte-Carlo simulations to compare our approach to a base case with only multi-path fading. We then empirically collect data in order to confirm our analysis. Finally, we incorporate this model in a prominent security algorithm to demonstrate the improvements in security possible through such an approach.

A novel microstrip patch antenna topology is presented for achieving a dual-band response with arbitrarily closely spaced resonances. This topology is based on a coupled transmission line structure in order to take advantage of the separation in propagation constants for parallel (even-mode) and anti-parallel (odd-mode) current modes. Applying a metamaterials inspired design approach, periodic reactive load­ings are used to design the underlying transmission line to have specific propagation constants necessary to realize a desired separation between two resonant frequencies. Using a single probe feed for a finite coupled line segment, both even-and odd-mode resonances can be excited to radiate efficiently at their respective design frequencies. The efficiency of the odd-mode radiation is enhanced by separating the two lines, while strong coupling is maintained by inserting a series of narrowly-separated thin loops between them. Several example resonant antenna designs, in the 2.45 GHz band, are presented. The directivities of these microstrip patch antennas are enhanced by optimizing the physical length of the resonant structure. For a resonant antenna obtained by cas­cading several unit cells of reactively loaded microstrip segments, dispersion analysis is employed for the unit-cell design. Maximum directivity is achieved by choosing the overall physical length to be slightly less than a half wavelength in free space at the design frequency. This gain optimization is applied to three coupled-line antennas, as well as a single resonance patch. Excellent agreement is observed between simulated and measured responses across all designs. The potential of loading the coupled line structure with active components is also explored. Varactor diodes are placed on coupled-line structures in two configurations. In one configuration, both resonant frequencies are affected. In the other configura­tion, only the odd-mode characteristics are reconfigured. In this way, the resonant frequency of either one or both modes can be adjusted by applying a DC bias voltage to the varactor diode loading elements. Two antennas, one employing each of these topologies, were designed and fabricated. Control of the resonant frequency over the predicted range through applying a bias voltage is observed with the fabricated prototypes.

Tapered slot antennas (TSAs) have seen considerable application in the millimeter-wave portion of the spectrum. Desirable characteristics of TSAs include symmetric E- and H-plane antenna patterns, and broad non-resonant bandwidths. We investigate extension of TSA operation toward higher frequencies in the thermal infrared (IR), using a metal-oxide-metal diode as the detector. Several different infrared TSA design forms are fabricated using electronbeam lithography and specially developed thin-film processes. The angular antenna patterns of TSA-coupled diodes are measured at 10.6 micrometer wavelength in both E- and H-planes, and are compared to results of finite-element electromagnetic modeling using Ansoft HFSS. Parameter studies are carried out, correlating the geometric and material properties of several TSA design forms to numerical-model results and to measurements. A significant increase in antenna gain is noted for a dielectric-overcoat design. The traveling-wave behavior of the IR TSA structure is investigated using scattering near-field microscopy. The measured near-field data is compared to HFSS results. Suggestions for future research are included

No / Noise amplitude distribution measurements relevant to%satellite-mobile radio systems are reported. The rationale for the%measurements is outlined and the choice of measurement parameters%justified. The measurement equipment and measurement methodology are%described in detail. Results characterizing the elevation angle%distribution of impulsive noise are presented for rural, suburban and%urban environments and also for an arterial road (U.K. motorway)%carrying high density, fast moving traffic. Measurements of the levels%of impulsive noise to be expected in each environment for high- and%low-elevation satellite scenarios using appropriate antenna%configurations are also presented

As ubiquitous wireless communication becomes part of life, the demand on antenna miniaturization and interference reduction becomes more extreme. However, antenna size and performance are limited by radiation physics, not technology. In order to understand antenna radiation and energy storage mechanisms, classical and alternative viewpoints of radiation are discussed. Unlike the common sense of classical antenna radiation, it is shown that the entire antenna fields contribute to both radiation and energy storage with varying total energy velocity during the radiation process. These observations were obtained through investigating impedance, power, the Poynting vector, and energy velocity of a radiating antenna. Antenna transfer functions were investigated to understand the real-world challenges in antenna design and overall performance. An extended model, using both the singularity expansion method and spherical mode decomposition, is introduced to analyze the characteristics of various antenna types including resonant, frequency-independent, and ultra-wideband antennas. It is shown that the extended model is useful to understand real-world antennas. Observations from antenna radiation physics and transfer function modeling lead to both corrections and extension of the classical fundamental-limit theory on antenna size. Both field and circuit viewpoints of the corrected limit theory are presented. The corrected theory is extended for multi-mode excitation cases and also for ultra-wideband and frequency-independent antennas. Further investigation on the fundamental-limit theory provides new innovations, including a low-Q antenna design approach that reduces antenna interference issues and a generalized approach for designing an antenna close to the theoretical-size limit. Design examples applying these new approaches with simulations and measurements are presented. The extended limit theory and developed antenna design approaches will find many applications to optimize compact antenna solutions with reduced near-field interactions. / Ph. D.

Tο αντικείμενο της διδακτορικής διατριβής είναι η μελέτη και σε βάθος ανάλυση και μοντελοποίηση της ηλεκτρομαγνητικής αλληλεπίδρασης του ανθρώπινου σώματος και των κεραιών που χρησιμοποιούν οι φορητές τερματικές συσκευές των σύγχρονων συστημάτων κινητής τηλεφωνίας. Ο στόχος είναι διπλός: αφενός μεν να υπολογιστεί η υποβάθμιση της απόδοσης της κεραίας, που προκαλείται από την παρουσία του σώματος του χρήστη σε μικρή απόσταση από αυτή και αφετέρου να εξεταστεί και να προσδιοριστεί επακριβώς το ποσό της ηλεκτρομαγνητικής ακτινοβολίας που απορροφάται από το ανθρώπινο σώμα και ειδικότερα από τον ανθρώπινο εγκέφαλο κατά τη χρήση του κινητού τηλεφώνου. Ο απώτερος σκοπός είναι η συμβολή στην ανάπτυξη ασύρματων τερματικών (πχ. κινητά τηλέφωνα) που θα είναι πιο αποδοτικά στη λειτουργία τους και ταυτόχρονα περισσότερο ασφαλή για το χρήστη τους. Στην παρούσα διδακτορική διατριβή πραγματοποιήθηκε εκτενής μελέτη και ανάλυση των παραμέτρων που σχετίζονται με την αλληλεπίδραση μεταξύ της κεραίας ασύρματων τερματικών συσκευών και του σώματος του χρήστη. Σχεδιάστηκαν, μοντελοποιήθηκαν, υλοποιήθηκαν και μετρήθηκαν πειραματικά πρωτότυπα τερματικών συσκευών παρουσία ομοιωμάτων του ανθρώπινου κεφαλιού με σκοπό τον υπολογισμό της απορρόφησης ακτινοβολίας από το κεφάλι και της μεταβολής της απόδοσης της κεραίας του τερματικού. Αναπτύχθηκε κατάλληλη μεθοδολογία μετρήσεων στο μακρινό πεδίο για την αξιόπιστη και ακριβή μέτρηση των χαρακτηριστικών των κεραιών του τερματικού. Τα αποτελέσματα έδειξαν έντονη αλλαγή των χαρακτηριστικών της κεραίας και του διαγράμματος ακτινοβολίας της, παρουσία του κεφαλιού του χρήστη. Επίσης, υπολογίστηκε η απότομη πτώση της απορρόφησης ακτινοβολίας και η αύξηση της απόδοσης με την απομάκρυνση του τερματικού από το κεφάλι. Επιτεύχθηκε σημαντική βελτίωση της λειτουργίας μέσω της μορφοποίησης του διαγράμματος ακτινοβολίας που κατορθώνεται με τη χρήση πολλαπλών κεραιών (συστοιχία) στο τερματικό. Τέλος εξετάστηκε η επίδραση του χεριού του χρήστη στην απόδοση ενός ΜΙΜΟ τερματικού και βρέθηκε μείωση της μέσης χωρητικότητας του καναλιού με την παρουσία του χεριού. / Τhe object of this doctoral thesis is the study, in depth analysis and modelling of the electromagnetic interaction between the human body and the antennas used in the handsets of modern wireless telecommunication systems. The aim is twofold. On one hand is the estimation of the antenna efficiency reduction that is caused by the presence of the user’s body in small distance and on the other hand is the study and precise determination of the electromagnetic radiation absorbed by the human body (especially the human head) at the use of a wireless terminal. The final aim is the contribution in the design of wireless terminals (e.g. mobile telephones) that will be more efficient in their operation and simultaneously safer for their user. In this thesis, an extensive study and analysis of the parameters related with the interaction between the wireless terminal antenna and the user’s body were realized. Experimental terminal prototypes were designed, modelled, constructed and measured in the presence of human head models in order to estimate the radiation absorption from the head and the degradation of the antenna efficiency. An appropriate measurements methodology at the far field was developed for the precise measurement of the terminal antenna characteristics. According to the results, an intense change of the antenna characteristics and of its radiation diagram in the presence of the user’s head was observed. Also, the rapid decrease of absorbed power and the increase of the efficiency were calculated after moving the handset away from the head. An important operation improvement was achieved with beamforming, which is realized using multiple antennas at the terminal. Finally, the effect of the user’s hand at MIMO terminal performance was examined and a reduction of the mean capacity of the channel in the presence of the hand was found.

The objective of this research is to realize compact and reconfigurable antennas for next generation Ultra Wide Band (UWB) and Wireless Local Area Network (WLAN) applications. The contributions of this research are, a methodology for designing compact UWB antennas, a compact WLAN prototype antenna with reconfigurable characteristics in both radiation pattern and frequency of operation, and compact UWB antennas with reconfigurable WLAN band rejection characteristics. For the completion of this dissertation, five research projects have been studied. First, a double exponentially tapered slot antenna with conformal shape, high gain, and consistent radiation patterns is implemented. The radiation pattern consistency results in minimum distortion for any transmitted pulse. The second and third projects involve an elliptical slot with a tuning uneven U-shaped stub and two cactus-shaped monopoles. The elliptical slot demonstrates omni-directional radiation patterns and compact size. As an improved iteration of the elliptical slot antenna, two cactus-shaped monopoles are implemented. The two prototypes occupy only 60% and 40%, respectively, of the area that the original elliptical slot occupies resulting in a significant size reduction, while maintaining omni-directional radiation patterns. Through the cactus-shaped monopoles some general design methodologies for UWB antennas are introduced and successfully applied. The fourth research topic introduced, concerns the study of compact elliptical UWB monopoles. Several prototypes of different geometrical characteristics were designed and tested. Broadband matching techniques and the integration of reconfigurable features on the elliptical radiator are investigated. For the reconfigurable UWB antenna, resonating elements are used to create a rejection band in the frequency range that is occupied by WLAN applications. The performance of several of the introduced slot and monopole antennas are tested when the antennas under detection are mounted and operate on non-planar surfaces. Finally, a reconfigurable annular slot antenna operating at the wireless local area network (WLAN) band is implemented. The proposed antenna demonstrates reconfigurable characteristics in both radiation pattern and return loss. All of the UWB antennas are fabricated on liquid crystal polymer (LCP) and can be easily integrated with active components on the same module using system on package (SoP) technology.

M. Tech. Electrical Engineering. / Objectives of this research is to use the node orientation, coupled with antenna radiation pattern of each node, to improve the Received Signal Strength (RSS) range measurement technique. As energy eciency is critical to WSNs, it is necessary to minimize both computation and communication costs in any operation involving WSNs,including during the localisation process. To achieve that, accelerometer measurements are used to reduce the number of iteration of the optimisation process during the refinement phase by computing more accurately an initial position for the optimisation using dead reckoning and approach the localisation in a distributed manner. The contribution of this is the investigation and development of an ecient localisation algorithm that can be used on a low cost wireless sensor board developed using existing technology. A review of the existing methods is conducted to highlight the key aspect to consider when developing an ecient localisation algorithms. A mathematical modelling of the proposed algorithm is developed and simulation is conducted to analyse the performance of the algorithm. An exhaustive test bed hardware has been designed on which the algorithm can to be validated.