Wide Band Embedded Slot Antennas for Biomedical, Harsh Environment, and Rescue Applications

For many designers, embedded antenna design is a very challenging task when designing embedded systems. Designing Antennas to given set of specifications is typically tailored to efficiently radiate the energy to free space with a certain radiation pattern and operating frequency range, but its design becomes even harder when embedded in multi-layer environment, being conformal to a surface, or matched to a wide range of loads (environments). In an effort to clarify the design process, we took a closer look at the key considerations for designing an embedded antenna. The design could be geared towards wireless/mobile platforms, wearable antennas, or body area network. Our group at UT has been involved in developing portable and embedded systems for multi-band operation for cell phones or laptops. The design of these antennas addressed single band/narrowband to multiband/wideband operation and provided over 7 bands within the cellular bands (850 MHz to 2 GHz). Typically the challenge is: many applications require ultra wide band operation, or operate at low frequency. Low frequency operation is very challenging if size is a constraint, and there is a need for demonstrating positive antenna gain

Optically transparent UWB antenna for wireless application & energy harvesting

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Transparent UWB antennas have been the focus of this PhD research. The use of transparent UWB antennas for stealth and energy harvesting has been the underlying applications that have given impetus to this research. Such transparent antennas being built on materials that are discreet, flexible, conformal, conductive and having the ability to provide good antenna performance on glass to serve as the ‘last mile’ link in subsequent generation communications after 4G have been the basis for this contention. UWB in this regard is able to provide the transmission and reception of high data rates and fast video transmission that is an elementary demand of even a 4G wireless communications system. The integration of UWB antennas with photovoltaic to provide integral energy harvesting solutions that will further enhance the value of the UWB system in terms of cost effectiveness and performance are thus the basis of this work. This work hence starts with the study of a transparent conductive oxide polymer, AgHT and its properties, and culminates in the development of a transparent UWB antenna, which can be integrated with photovoltaic for window glass applications on homes and buildings. Other applications such transparent antennas can find use for like on-body wireless communications in healthcare monitoring was also analysed and presented. The radar absorbing material (RAM) property of the AgHT was investigated and highlighted using CST simulation software, as no measurement facilities were available. The transparent UWB antenna in lieu of the inherent absorbent property of the AgHT material is thus able to exhibit stealth characteristics, a feature that would be much desired in military communications. Introduction of a novel method of connecting the co-axial connector to the feed of the antenna to improve gain and efficiency of transparent polymer based antennas and the development of a UWB antenna that maintains its Omni-directional characteristic instead of becoming directional on an amorphous silicon solar cell are presented as some of the contributions for this research work. Some preliminary analysis on the impact of glass on UWB antennas for video transmission and how to improve transmission is presented. The ability of the conductive part of the antenna radiator to be used as a RF and microwave harvester and how it can further add value to a transparent UWB antenna is presented by way of experimental data. Finally yet importantly, this thesis presents some insight into how transparent antennas may be used in Green Technology Buildings to provide an integrated solution for both wireless communications and energy harvesting as part of the future work. Improvement to the aesthetics of the external appearance of residential buildings through the integration of transparent satellite dish onto solar panels on rooftops is also discussed and illustrated as part of this future work

Microwave antennas for biomedical application

Medical diagnosis is one of the key steps to determine the problem of the human body. The current diagnostic tools are expensive, bulky and long exposure to some of these diagnostic tools can be injurious to the human body. Hence, researchers are now exploring through different possibilities to replace current diagnostic tools. Microwave regime is one of the potential candidate to replace current diagnostic system providing with a chip, portable system suitable for the human body. One of the fundamental tool for a microwave diagnostic system is microwave antenna. The current findings on designing microwave antennas for biomedical diagnosis lacks due to low microwave power penetration inside the human body, high specific absorption rate (SAR), low directivity and compactness. This thesis aims on improving the microwave penetration inside the human body and develop antennas that can perform efficiently for biomedical diagnosis application. A multi-layer reflection model is investigated for evaluation of the combined material characteristics of different lossy human tissues, along with the enhanced antenna designs, suitable for biomedical application, operating on-body and as an implant, have been presented within this thesis. The rationale behind this work relates to the early detection of cancerous tissues, internal injuries and other characteristic changes inside the human body with the primary goals being to improve microwave power penetration inside the human body and to provide low SAR and compact microwave antenna system for biomedical diagnosis. The penetration of microwave power inside a human head model is improved by employing calculated permittivity inside a rectangular waveguide used as the microwave transmission source. Firstly, a multi-layer reflection model is created from various human tissue material. The wave impedance of the multi-layer is then extracted from the overall reflection coefficient found at the edge of the multi-layer tissue model. Furthermore, a rectangular waveguide is constructed and an L-probe rectangular waveguide feeding technique is presented. The measured results validate the approach with an increment in power penetration inside the human head 1.33 dB at 2.45 GHz.  Antennas are characterized in-front of homogeneous and a frequency-dependent inhomogeneous human head and shown that inhomogeneous phantom provides with real-life scenario for the measuring antenna whereas the homogeneous phantom only resembles the scenario. The effect of superstrate at the boresight of an on-head matched antenna for biomedical applications is analysed and shown that superstrate layer at the boresight direction of the antenna provides with ~8 dB increased directivity towards the human head with 0.0731 W/kg reduction of SAR compared to the antenna without the superstrate. The design of a 3-D on-body antenna and a coplanar waveguide (CPW) fade antenna matched with an inhomogeneous human head provides the second investigation area. Specific focus has been given to make the designs compact, increase the front to back ratio (FBR) of the radiation pattern and decrease the SAR of the antenna. The 3-D antenna is realized combining a folded inverted F-like structure and a slot-loaded ground plane and backed by a rectangular cavity to minimize side and back lobe radiation. An FBR of 17 dB with SAR less than 0.0147 W/kg is achieved throughout the operating frequency ranging from 1 - 1.7 GHz by the designed antenna while acquiring a compact dimension of 0.23 × 0.23 × 0.04 λ in size with respect to the lowest operating frequency.  An inhomogeneous human head phantom is constructed and used to analyze the antennas performance in real-life scenario. Moreover, the choice of operating frequency for on-head antennas and effect of a superstrate on on-head matched antennas is investigated. An FBR of 20 dB with SAR less than 0.037 W/kg is achieved throughout the operating frequency ranging from 0.788 - 2.5 GHz by the designed antenna while acquiring a compact dimension of 0.1 × 0.1 × 0.008 λ in size with respect to the lowest operating frequency. Finally, the design of an implantable coil antenna is investigated for wireless power transmission inside the human body. The biocompatibility of the building material is analyzed. Polydimethylsiloxane (PDMS) and gold (Au) is utilized as the biocompatible building material to realize the designed implantable antenna. Furthermore, the antenna is characterized in a "complete medium" composed of fetal bovine serum (FBS), penicillin-streptomycin and dulbecco's modified eagle's medium (DMEM) which is used as the cell culture media to resemble designed antennas operation environment. The antenna is impedance matched at 5 MHz frequency with a maximum received voltage of 35 mV is recorded by utilizing the designed implantable antenna

Performance Enhancement of Radiation and Scattering Properties of Circularly Polarized Antennas Using Frequency Selective Surface

At millimetre-wave (MMW) frequencies, losses associated with wireless link and system are critical issues that need to be overcome in designing high-performance wireless systems. To compensate the overall loss in a wireless communication system, a high-gain antenna is required. Circularly polarized (CP) antennas are among preferred choices to design because they offer many advantages due to their good resistance to polarization mismatch, mitigation of multipath effects, and some phasing issues and immunity to Faraday rotation. On the other hand, frequency selective surface (FSS) technology is recently employed to enhance the performance of radiation and scattering properties of antennas used in different sectors such as aerospace, medical, and microwave industry. Therefore, it is appropriate and attractive to propose the use of FSS technology to design practical and efficient CP antennas. CP Fabry-Perot cavity (FPC) antennas based on FSS are investigated in this thesis to fulfil the growing demand for broadband high-gain antennas with low radar cross section (RCS). The thesis investigates both characteristic improvement of CP antennas and RCS reduction issues employing FSS structures. Initially, a high gain CP dielectric resonator (DR) antenna is proposed. Using an FSS superstrate layer, a gain enhancement of 8.5 dB is achieved. A detailed theoretical analysis along with different models are presented and used to optimize the superstrate size and the air gap height between the antenna and superstrate layer. The second research theme focusses on developing an effective approach for mitigating the near-field coupling between four-port CP antennas in a Multiple-Input, Multiple-Output (MIMO) system. This is obtained by incorporating a two-layer transmission-type FSS superstrate based on planar crossed-dipole metal strips. Another technique for suppressing the spatially coupling between DR antennas using a new FSS polarization-rotator wall is studied as well. The coupling reduction is achieved by embedding an FSS wall between two DRAs, which are placed in the H-plane. Utilizing this FSS wall, the TE modes of the antennas become orthogonal, which reduces the spatially coupling between the two DRAs. The third research theme of this thesis is to enhance the purity and bandwidth of CP with the least amount of insertion loss by the use of an LP-to-CP-polarizer which is based on multilayer FSS slab. This polarizer is approximately robust under oblique illuminations. To have a high-gain CP antenna, an 8-element LP array antenna with Chebyshev tapered distribution is designed and integrated with the polarizer. Eventually, in order to enhance the scattering property, the fourth research theme investigates on RCS reduction by the use of two different approaches which are based on FSS. Initially, a wideband FSS metasurface for RCS reduction based on a polarization conversion is proposed. To distribute the scattered EM waves and suppress the maximum bistatic RCS of the metasurface over a broad band of incident angles at both polarizations, the elements are arranged using the binary coding matrix achieved by group search optimization (GSO) algorithm. The reflective two-layer metasurface is designed in such a way to generate reflection phase difference of 180° between two elements “0” and “1” on a broad frequency band. A theoretical analysis is performed on the ratio of the “0” and “1” elements using Least Square Error (LSE) method to find the best ratio value. As the second activity of this research theme, wideband CP antenna with low RCS and high gain properties is presented. The proposed antenna is based on a combination of the FPC and sequential feeding technique

A High Gain Dual Notch Compact UWB Antenna with Minimal Dispersion for Ground Penetrating Radar Application

A compact (27.5×16.5×0.8 mm3) co-planar waveguide fed printed ultra-wideband antenna operating in the impedance band of 1.75-10.3 GHz with two wide frequency notch bands at 2.2–3.9 GHz and 5.1–6 GHz, is introduced. Dual notch is achieved by inserting U-slot on the radiator and with inverted patch shaped downscaled parasitic load at the opposite end of feed line. Maximum antenna gain augmentation by about 5 dBi is achieved without changing the bandwidth, by incorporating a dual layer reflective frequency selective surface (FSS) of dimension 33×33×1.6 mm3 below the antenna. The antenna-FSS composite structure exhibits maximum radiation in the broadside direction with a peak gain of 9 dBi and an average radiation efficiency of more than 80% in the operating band. Antenna transfer function and group delay are experimentally studied in ground coupling mode of ground penetrating radar (GPR). Linear magnitude response of transfer function and consistent, flat group delay are achieved, that ensure minimal antenna dispersion and its ability for GPR application

Wideband Linear and Circularly Polarised Transmitarray Antennas

The millimetre-wave (mm-wave) band is expected to be one of the solutions for future wireless communication systems, as it provides wide bandwidth, enhancing the data transmission rate. Therefore, high-directive antennas, which are principal elements for mm-wave wireless communications, are used since they can overcome the negative effects of high path losses. These high-gain antennas can be used in various applications, including point-to-point communication, automotive radar, and imaging. The main goal of this project is to design transmit-array (TA) antennas that operate at the mm-wave band (around 28 GHz) with high aperture efficiency, high gain, wide bandwidth, and a low-profile TA surface. Basically, the aperture efficiency of the TA antenna depends on the unit cell (UC) characteristics and the feed antenna radiation pattern. The UC characteristics, including the transmission coefficient, bandwidth, and phase range, all play an essential role in designing efficient TA antennas. Therefore, the unit cells of the TA surface are designed to have a wide bandwidth with a stable broadside radiation pattern and a full phase range (0 ͦ-360 ͦ). Such unit cells are designed using two techniques: 1) Capacitive feeding to extend bandwidth and 2) differential feeding to minimize cross-polarization and enforce a broadside radiation pattern that, enhancing the antenna gain. Simulated results show that the unit cells have achieved a 360 ͦ phase range and a maximum element loss of 0.5 dB. In addition to the design’s high performance UC, a wideband feeder antenna is required to illuminate the TA surface. Thus, a wideband patch antenna which is a part of the UC is used to illuminate the TA surface. Also, a hybrid antenna with a high data rate has been designed and fabricated to excite the TA aperture. The designed hybrid antenna has a 14.5 dBi maximum realized gain at 30 GHz, and its return loss bandwidth is 34.48%. Results show that the TA’s aperture efficiency depends on the illuminating source’s radiation pattern, such as its amplitude and phase distribution over the TA surface. Experiments show efficient illumination of TA surface requires a specific radiation pattern; therefore, a circular horn antenna has been designed with high acceptable amplitude tapering, uniform phase distribution, symmetric radiation pattern, and low sidelobe level. This utilized horn antenna has a particular radiation magnitude distribution, and the radiation pattern model is sec⁡θ, which makes it capable of dealing with the relative difference in path loss and resulting in a good tapering efficiency. The TA antenna’s surface and the feeding antennas have been designed, fabricated and measured. The TA antenna measured results show a maximum gain of 31.15 dBi, with a 1-dB gain bandwidth of 12.7%, while the 3-dB gain bandwidth is 21%, around 28 GHz. On the frequency range from 25 to 31.5 GHz, the aperture efficiency is better than 50%, and the cross-polarization level is less than -37 dB. Furthermore, factors that affect the TA antenna’s performance are studied and summarized in this work. These factors are mutual coupling, phase errors in TA design, quantization error, phase range error, feed antenna, TA antenna shape, and incident angle approximation. Circular polarization antennas have advantages over linear polarization antennas because rapid alignment is not required between transmitter and receiver antennas, reducing polarization mismatching error. The Faraday rotation effect also harms the linear polarized waves. Moreover, circularly polarized wave energy is on both planes, suppressing interference. A circular polarizer is used to convert the LP incident signal to CP signals, in which the incident electric field is resolved to its orthogonal components, introducing a 90 ͦ phase shift. The proposed polarizer contains two layers of the Jerusalem cross (JC). The JC UC simulated results show equal orthogonal retransmitted electric fields with 90 ͦ phases. The designed CP transmit-array results show a maximum realized gain of 30.5 dBi, with an AR bandwidth of 23%. Finally, a general TA antenna design methodology is also provided

Wideband Low Side Lobe Aperture Coupled Patch Phased Array Antennas

Low profile printed antenna arrays with wide bandwidth, high gain, and low Side Lobe Level (SLL) are in great demand for current and future commercial and military communication systems and radar. Aperture coupled patch antennas have been proposed to obtain wide impedance bandwidths in the past. Aperture coupling is preferred particularly for phased arrays because of their advantage of integration to other active devices and circuits, e.g. phase shifters, power amplifiers, low noise amplifiers, mixers etc. However, when designing such arrays, the interplay between array performance characteristics, such as gain, side lobe level, back lobe level, mutual coupling etc. must be understood and optimized under multiple design constraints, e.g. substrate material properties and thicknesses, element to element spacing, and feed lines and their orientation and arrangements with respect to the antenna elements. The focus of this thesis is to investigate, design, and develop an aperture coupled patch array with wide operating bandwidth (30%), high gain (17.5 dBi), low side lobe level (20 dB), and high Forward to Backward (F/B) ratio (21.8 dB). The target frequency range is 2.4 to 3 GHz given its wide application in WLAN, LTE (Long Term Evolution) and other communication systems. Notwithstanding that the design concept can very well be adapted at other frequencies. Specifically, a 16 element, 4 by 4 planar microstrip patch array is designed using HFSS and experimentally developed and tested. Starting from mutual coupling minimization a corporate feeding scheme is designed to achieve the needed performance.To reduce the SLL the corporate feeding network is redesigned to obtain a specific amplitude taper. Studies are conducted to determine the optimum location for a metallic reflector under the feed line to improve the F/B. An experimental prototype of the antenna was built and tested validating and demonstrating the performance levels expected from simulation predictions. Finally, simulated beam scanning in several angles of the array is shown considering specific phases for each antenna element in the array

Simultaneous Transmit and Receive (Star) Antennas for Geo-Satellites and Shared-Antenna Platforms

This thesis presents the analysis, design, and experimental characterization of antenna systems considered for shipborne, airborne, and space platforms. These antennas are innovated to enable Simultaneous Transmit and Receive (STAR) at same time and polarization, either at the same, or duplex frequencies. In airborne and shipborne platforms, developed antenna architectures may enhance the capabilities of modern electronic warfare systems by enabling concurrent electronic attack and electronic support operations. In space, and more precisely at geostationary orbit, designed antennas aim to decrease the complexity of conventional phased array systems, thereby increasing their capabilities and attractiveness. All antennas researched are first designed as a standalone radiator, then as entity of a platform having multiple different antennas.An ultrawideband, lossless cavity-backed Vivaldi antenna array for flush-mounting applications is first investigated. Eigen-mode analysis is used to analyze antenna-cavity interaction and to show that the entire structure may resonate within the band of interest resulting in a significant degradation of antenna performance. A simple approach based on connecting the array’s edge elements in E-plane to the cavity walls is proposed to eliminate the deleterious impact of these cavity resonances. The designed antenna is a 3 × 4 array with 3 elements in E-plane and 4 elements in H-plane, fabricated using stacked all-metal printed circuit board technique. Scan performance of the proposed cavity-backed antenna is investigated in two principal planes and is shown to have similar performance compared to its free-standing counterpart. A simplified version of this single-polarized antenna, when used for broadside only applications is developed. This antenna, excited with a single coaxial feed is shown to have a smaller aperture than the 3 × 4 array. Isolations between two of these antennas when mounted on a compact shared-antenna platform are investigated through computation and experiments.To extend the capability of systems relying on these designed antennas, frequency reuse is enabled through dual-polarized functionality. A dual-polarized, flush mounted, Vivaldi antenna, directly integrated with an all-metal cavity is introduced as an alternative to coax-fed quad-ridge horns. An approach based on shaping the side walls of the cavity is used to eliminate the occurrence of resonances. The proposed dual-polarized resonant-free antenna has two orthogonal 2 × 1 arrays with two elements in the E-plane, one element in the H-plane. It is fed using two 2-way power dividers that can be easily designed to maintain low amplitude and phase imbalances. The antenna is fabricated as a single piece and experimentally shows a monotonic gain increase with low cross-polarization over 4:1 bandwidth.Phased array antennas operating at geostationary orbit are required to scan within Earth’s field of view, without any grating lobe appearance. For dual-polarized applications, this requirement has limited the widespread and attractiveness of these systems at frequencies such as X-band. The narrow 150 MHz guard range between transmit and receive bands, leads to impractical diplexers in conventional dual-polarized systems. This research introduces a dual-polarized subarray architecture for X-band phased array systems which enables high isolation between closely separated TX and RX bands. The proposed approach either eliminates the need for diplexers, or significantly decreases their required complexity