Search

Antennas on multiple platforms with the capability to be reused is a novel concept in antenna technologies. This thesis explores the idea of such a flexible antenna that can be stuck on any surface using certain adhesives and be peeled off for reuse for a different purpose. This thesis specifically focuses on the application of being used on small Unmanned Aircraft Systems (sUAVs). Three basic designs are fabricated and investigated. One of them is a reconfigurable meandering monopole, consisting of a PIN diode which resonates at two different frequencies depending on the applied external voltage. A challenge that is addressed is the need for the antenna to be miniaturized, since the frequencies used by sUAVs traditionally require large antennas. To tackle this problem a high bandwidth Log Periodic dipole array (LPDA) is designed and tested. The elements of the LPDA are folded, and the effect this has on the antenna performance is studied. The Q factor for such an antenna is evaluated and correlated as a function of how well the antenna was miniaturized. All the fabricated antennas are tested on multiple surfaces, some of them even conformal, to see how their properties change when stuck to different surfaces.

This thesis focuses on two main subjects; mm-wave and THz on-chip antennasfor imaging, signal generation and radiometry applications and tunable high performancecircuits for cellular communications.First, three on-chip antennas designed at 360 GHz for imaging, designed at 375GHz for THz signal generation and designed at 160 GHz for radiometry are presented.The antennas are implemented in commercially available integrated circuit (IC) processesfollowing an approach which does not required post processing on the siliconchip after fabrication. For all three antennas the radiating structures are isolated fromthe lossy silicon by a ground and loaded by a superstrate for enhanced gain and efficiency.In commercial IC processes metal fill structures are included in the designs to comply with the strict fabrication rules and a new approach has been proposed onplacing the metal fills to enhance the gain and efficiency further.Second, two different tunable circuits have been designed for cellular systems.First circuit is a tunable phase shifter designed incorporating a tunable coupler whichallows operation over a wide frequency range to meet the requirements of future communicationsystems. Second circuit is a tunable notch filter designed to be planar forsmall volume and very low-loss for high performance. RF MEMS devices are usedas tuning elements in both circuits to get the low-loss performance and high linearityrequired by modern communication systems.

The thesis presents three dual-band frequency tunable antennas for carrier aggregation systems and two new feeding networks for reducing the number of phase shifters in limited-scan arrays. First, single- and dual-feed, dual-frequency, low-profile antennas with independent frequency tuning using varactor diodes are presented. The dual-feed planar inverted F-antenna (PIFA) has two operating frequencies which are independently tuned at 0.7--1.1 GHz and at 1.7--2.3 GHz with better than −10 dB impedance match. The isolation between the high-band and the low-band ports is $>$ 13 dB; hence, one resonant frequency can be tuned without affecting the other. The single-feed contiguous-dual-band antenna has two resonant frequencies, which are independently tuned at 1.2--1.6 GHz at 1.6--2.3 GHz with better than −10 dB impedance match for most of the tuning range. And the single-feed dual-band antenna has two resonant frequencies, which are independently tuned at 0.7--1.0 GHz at 1.7--2.3 GHz with better than −10 dB impedance match for most of the tuning range. The tuning is done using varactor diodes with a capacitance range from 0.8 to 3.8 pF, which is compatible with RF MEMS devices. The antenna volumes are $63 \times 100 \times 3.15$ $mm^3$ on $\epsilon_r = 3.55$ substrates and the measured antenna efficiencies vary between 25\% and 50\% over the tuning range. The application areas are in carrier aggregation systems for fourth generation (4G) wireless systems. Next, a new phased array feeding network that employs random sequences of non-uniform sub-arrays (and a single phase shifter for each sub-array) is presented. When these sequences are optimized, the resulting phased arrays can scan over a wide region with low sidelobe levels. Equations for analyzing the random arrays and an algorithm for optimizing the array sequences are presented. Multiple random-solutions with different number of phase shifters and different set of sub-array groups are analyzed and design guidelines are presented. The performance of the random array feeding scheme is compared to the conventional uniform sub-arraying for multiple cases. It is shown that with the random feeding networks, the number of phase shifters can be reduced up to 30\% while preserving the system performance. This results in more affordable and more reliable systems. The proposed feeding network is demonstrated for a 30 element array of slot- fed patch antennas at 7.9 GHz. The fabricated array uses 12 phase shifters, has a half power beamwidth (HPBW) of $4^o$ and can scan up to $\pm 14^o$ with sidelobe levels less than -15 dB.Another phase shifter reducing method, the interwoven feeding networks, is investigated. These passive feeding networks are composed of power dividers, couplers and resistive attenuators. In this configuration, each phase shifter feeds all of the antennas and creates a sinc-like current distribution over the array elements which results in a boxcar function-like element pattern. This element pattern is used to cancel the grating lobes. By changing the inter-element spacing and the coupling and attenuation coefficients of the feed network, it is possible to adjust the width of the scan region. Different network configurations along with theoretical limitations are investigated to determine the scanable region, side-lobe level and power loss. For the demonstration, two prototype linear arrays with 28 elements are fabricated. Both of the arrays operate at 7.9 GHz. The first array employs 14 phase shifters, has a half power beamwidth (HPBW) of $4^o$ and can scan up to $\pm 24^o$ with sidelobe levels less than -15 dB. The second array uses 7 phase shifters, has a half power beamwidth (HPBW) of $4^o$ and can scan up to $\pm 11^o$ with sidelobe levels less than -15 dB. Both of these arrays show state-of-the-art performance in terms of reducing the number of phase shifters while still keeping a low sidelobe level and reducing the effect of the grating lobes.

The main aim of this thesis is the investigation and optimisation of microstrip antennas and microwave techniques for wireless power transmission in human tissue at 2.45 GHz (i.e., in the Industrial, Scientific, and Medical band). For this purpose, microstrip antennas of different geometries and sizes and a microwave two-antenna setup that includes an implantable encased antenna and an external antenna, are designed and investigated computationally and experimentally. An implantable antenna module was designed by enclosing a microstrip rectangular patch antenna inside a protective dielectric housing. The simulation results showed that, by optimising the dimensions and the dielectric properties of the protective housing, the influence of the surrounding material on the performance of the implanted antenna can be reduced and the efficiency of wireless power transmission can be increased. The performance of wireless power transmission in different materials including tissue mimicking gel and fresh minced meat was experimentally investigated using a measurement system with the two-antenna setup. A parametric study was performed using the measured and simulated transmission coefficients to determine the unknown electrical properties of the materials. The measurement and simulation results are in good agreement. An implantable miniature encased microstrip ring disk antenna was designed and optimised for wireless power transmission in human tissue, and a computational model of a measurement system was proposed and used to characterise microwave two-antenna setup techniques. It was found that, by matching the resonant frequencies, aligning the directions of propagation of the electromagnetic waves and electric field polarisation vectors of the external and implanted antennas, maximum wireless power transmission in human tissue can be achieved even if the external and implanted antennas have different geometries and sizes. It was also found that reflections of the electromagnetic waves at the air-tissue interface can be reduced and the efficiency of microwave wireless power transmission techniques can be significantly enhanced by adding a matching dielectric layer between the external antenna and human tissue. A novel implantable open cylindrical-rectangular microstrip patch antenna was proposed and investigated. This antenna has a geometry and size that facilitates its direct implantation on nerves for the purpose of nerve stimulation. The simulation results showed that the two-antenna setup with the proposed antenna and a rectifier is capable of delivering a DC stimulus that can be used for nerve regeneration with a specific absorption rate that conforms to the IEEE safety standards. The results in this thesis showed that wireless power transmission in human tissue using microwave techniques with implantable microstrip antennas can provide sufficient power levels to be used for biomedical applications such as nerve stimulation. The use of microwave wireless power transmission with miniature implanted antennas can result in significant miniaturisation of implantable medical devices by eliminating the need for the implanted battery.

Abstract: Recently, antenna designs with multi-frequency operation and multi-polarization characteristics are in great demand for future wireless systems. In the thesis, several integrated microstrip antenna designs have been proposed for wireless applications. Integrated semi-circular and triangular patch antennas have been designed for various wireless data communication applications exhibiting linear polarization characteristics. In these designs, the patch has been integrated with slots and monopoles for multi-resonance operation. Another multi-band antenna realizing annular ring patch configuration has been designed for similar wireless applications. A technique to independently tune the frequencies of operation has also been introduced. Furthermore, a single layer, multi-band rectangular patch antenna has been proposed for data transmission and localization applications with different polarization requirements. The patch has been integrated with four monopoles to achieve circularly and linearly polarized radiation characteristics. The design procedures of the antennas have been described and their performances have been validated using electromagnetic solvers and measurements.

The design of an S-band aperture coupled microstrip antenna array for use on airborne telemetry platforms is presented. The High Sensitivity Diverse Tracking Antenna (HSDTA) is capable of receiving both left and right hand circular polarizations and has RF auto-tracking capability. Novel placement of RF terminations throughout the design allows for HSDTA to be installed in multiple configurations aboard different airborne platforms while maintaining consistent mechanical mobility and high sensitivity. The HSDTA was successfully simulated, fabricated, and deployed. Measured RF performance, including the operational bandwidth, gain, and sensitivity, are discussed and compared to the simulation. The HSDTA was designed to withstand the environmental conditions present while flying at altitude. The system has been successfully used to collect telemetry while deployed on an airborne platform flying over the South Pacific at 35000 ft.

As the trend continues towards miniaturizing integrated electronic circuits, the need to reduce the size of the antenna continues to be a challenge particularly for the microstrip antenna. Several techniques that have been successfully implemented for size reduction up to 75%, by a variety of methods such as cutting slots in the patch or reconfiguring its size using MEMS, etc. in conjunction with optimization techniques such as genetic algorithms or particle swarm. However, antenna size reduction comes at the expense of a reduced gain by a few dBs. Recently, materials engineered to have negative permittivity and permeability (DNG) or left handed materials (LHM) have been used as a superstrate to focus the radiated field. While most of the work to date has used metallic DNG material, implementing dielectric DNG materials has been limited to optical applications. Dielectric DNG materials have advantages over metallic DNG materials primarily for low losses and wider bandwidths since they are non-resonance structures. The present research uses a flat lens available in literature and developed in 2005 that has a negative refractive index in a certain frequency range as a superstrate for the microstrip patch antenna. It is a high dielectric material with a triangular lattice of holes drilled along its length. At first the superstrate has been analyzed using a finite difference time domain software tool (EM Photonics) for its left handed properties by exciting it with a point source and comparing the magnitude and phase of the electrical field in the transmission plane with that in the incident plane. It is shown that an image is formed at a distance about half the thickness of the slab. By comparing its behavior to that of a similar size slab without holes; it is conclusively shown that the slab with holes behaves as a DNG material. In order to use this DNG slab as a superstrate, the rectangular microstrip patch antenna has been designed to resonate at 31.5 GHz which lies in the same frequency range where the superstrate has a negative refractive index. At first the DNG has been analyzed for its lens behavior properties with respect to focusing the electric field. In order to optimize the field focusing, the edges of the DNG slab had to be scalloped. Another optimization that is very critical is the height at which the superstrate is placed over the antenna. This is done by calculating and maximizing the gain using a 3D finite element full wave solver (Ansoft's HFSS). A gain enhancement of 3 dB to 4 dB for a single element has been achieved. The aperture field distribution for the optimized configuration has been plotted at selected heights. In addition, the E field distributions for the optimized configuration have been compared with E field distributions for the patch antenna without the DNG superstrate configuration. This comparison reveals that the field strength above the DNG superstrate is twice the value of the field for the antenna without a superstrate configuration. This further proves that the superstrate behaves as a DNG material. Finally, the proposed superstrate has been scaled to exhibit a negative refractive index at 10 GHz and integrated with an X band (10 GHz) patch antenna, and the same type of characterization steps described have been repeated. Simulation results of the scaled configuration are consistent with the original configuration; where a gain enhancement of 3dB has been achieved. This ensures the scalability and robustness of the design.

At millimeter wavelength, spatial power combining techniques offer a viable approach to realize compact, reliable, lightweight, robust, higher-power and economical systems This work focus on the tray based perpendicularly fed array systems, several design issues such as bandwidth improvement, power combining efficiency, dual polarity and beam steering were addressed in this thesis. A broadband tray based spatial power amplifier that employs dielectrically filled miniature horn arrays was investigated. Bandwidth improvement was achieved by replacing the patch arrays of an earlier design with the broadband dielectrically filled miniature horn arrays. This 5x5 spatial power amplifier with miniature horn arrays has a 3dB bandwidth of 1.32 GHz (13%), which is more than 4.5 times the 3 dB bandwidth of the perpendicularly fed patch array spatial power amplifier. This work presents experimental and numerical investigations on a 49-element Ka-band amplifier array. This study is aimed at determining the origin of various losses in the amplifier array. Passive simulation data confirms that the load seen by the active devices is well matched and that most of the power is coupled to the LSE10 mode. This means that coherent power combining should take place if there is no phase and amplitude variation due to the active devices and phase correcting dielectric lenses. This thesis presents a single aperture multibeam spatial power combining system that can support two separate polarizations. A Rotman lens is used as the power dividing network, which also has the ability to scan the beam over discrete angles. 1x9 dual polarized array was designed to demonstrate the beam steering in azimuth plane. The entire system is designed for the Ka band, with a center frequency of 32.6GHz. Through simulations and measurements, a ±30 degree scan range was achieved for the horizontal and vertical polarizations. The cross polarization for both the vertical and horizontal polarization ports was better than –16dB.

The purpose of this thesis is to investigate a novel method to develop broadband microstrip (patch) antennas using substrates containing photonic crystals. Photonic crystals are a class of periodic dielectric, metallic, or composite structures that when introduced to an electromagnetic signal can exhibit a forbidden band of frequencies (or bandgap) in which the incident signal destructively interferes and thus is unable to propagate. It is proposed that such photonic crystals will reduce surface waves and prohibit the formation of substrate modes, which are commonly known inhibitors of patch antenna designs. By reducing or eliminating the effects of these electromagnetic inhibitors with photonic crystals, a broadband response can be obtained from inherently narrowband antennas. In addition, it is also proposed that the behavior of the photonic crystals will lead to a reduction in pattern sidelobes resulting in improvements in radiation pattern front-to-back ratio and overall antenna efficiency. This research is verified through analytical simulations and experimental investigations in the Virginia Tech anaechoic chamber.

This thesis demonstrates a comprehensive study of optimization algorithms combined with finite element-boundary integral simulations on microstrip antennas and periodic structures. Among the various optimization methods considered, the sequential quadratic programming (a class of gradient-based algorithms) is applied to several size optimization problems; and two gradient-free methods (genetic algorithms and simulated annealing) are employed for shape and topology optimization. Recent developments in fast algorithms now offer the possibility of designs and moreover allow for full flexibility in material specification across three dimensions. The first part of the thesis considers various antenna designs already investigated but not optimized before. The examples include ferrite antennas, stacked patches, a patch with a folded-slot feed, and irregular-shaped broadband/dual-band patches. These designs serve to validate the optimization process and develop confidence prior to proceeding with more complex design problems. The second part of the thesis deals with the design of novel antennas and periodic structures. Frequency selective surface (FSS) elements are designed for a high-pass filter application, as well as a flat-phase element which is demonstrated to achieve much higher bandwidth for a dipole array placed on top of it. Finally, shape and material optimization are combined to design a full 3-D photonic bandgap (PBG) structure which substantially increases the bandwidth of a patch residing on the PBG substrate.

This thesis investigates the integration of planar antennas with planar circuit technologies (cpw and microstrip) using standard IC fabrication techniques to achieve high performance front-end receivers at millimeter-wave frequencies. The focus is to eliminate substrate losses from surface waves which greatly limit the radiation performance and bandwidth at millimeter-wave frequencies. In the first part, millimeter-wave receivers integrated on a silicon substrate backed with a dielectric hyperhemispherical lens are described. The lens eliminates the propagation of surface modes. A 94 GHz double-slot Schottky-diode receiver is shown. A commercial Shottky-diode is placed at the center of a double-slot antenna and acts as a receiver. The LO pump signal is quasi-optically injected after being combined with the RF signal through a Martin-Pupplett diplexer. The total measured SSB conversion loss is 9.3 dB over the 86--90 GHz band. In order to avoid the drawbacks of a heavy quasi-optical diplexer, a 150 GHz subharmonic mixer where the LO is injected on-wafer using W-band probes is demonstrated. The RF signal is received by a double folded-slot antenna, which has a low impedance of 20 W at 150 GHz. In the second part, micromachining fabrication techniques are applied to microstrip antennas on silicon substrates to synthesize by removing dielectric a low dielectric constant region around the microstrip antenna, resulting in improved radiation efficiency and radiation patterns. A larger bandwidth can also be achieved. Two different approaches are investigated: first closely spaced holes are drilled around and below the microstrip patch at 13 GHz. The radiation efficiency is improved from 48% for a high er substrate antenna to 70% for the synthesized antenna. The 10-dB bandwidth is also improved from 0.8% to 8% resulting from a resonance effect in the drilled region. Second a cavity is chemically etched locally below an aperture coupled microstrip antenna at 94 GHz. The measured radiation efficiency is 58% compared to 28% for an antenna built on a full silicon substrate, while the front-to-back ratio is better than -12 dB. Also, in an array configuration, the mutual coupling between adjacent elements can be lowered to better than -20 dB, allowing the integration of this particular design in phased arrays. Finally, low-loss W-band interconnections based on electromagnetic coupling are explored. Uniplanar and vertical transitions that combine cpw and microstrip technologies show losses better than -1 dB and are very wideband, over close to the entire W-band region (70--120 GHz). These techniques are attractive approaches for low-cost, high-performance 3-dimensional monolithic millimeter-wave front-end receivers, compatible with standard IC fabrication techniques in the perspective of mass production. Applications are in personal and commercial communications, automotive radars, detection and guidance.

The Finite-Difference Time-Domain (FDTD) method has gained tremendous popularity in the past decade as a tool for solving Maxwell's equations. Phased Array Antennas find several applications including mobile communications ( cellular, personal communication systems and networks), satellite communications, global positioning system (GPS), aeronautical and radar systems. This dissertation describes the application of the FDTD method for calculating broadband characteristics of finite-sized phased array antennas consisting of microstrip elements fed with coaxial probes. The characterization of such antennas is dependent upon the development of simulation tools that can accurately model general topologies including wires, dielectrics, conductors lumped elements and metallic strips. The use of these simulation tools reduces the cost and effort associated with fabricating and testing phased array antennas. The FDTD formulation is inherently broadband, very general, and easily accorrunodates arbitrary conductor geometry and dielectric configurations. The FDTD method is implemented and applied to determine the input impedance, radiation-patterns and gain of microstrip antennas. Next, the main contributions of this work are described which include the full time-domain characterization of broadband characteristics of finite-sized phased array antennas for different scan conditions. Active reflection coeffici nt gain scan-element patterns and scanning-array radiation patterns are calculated.