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

MIMO antenna systems for next generation wireless communications

Multiple Input Multiple Output wireless communications systems require as the name implies multiple antennas at the transmit and receive side of a link, as all multiple elements operationally occupy the same spectrum, the capacity of carrying information is increased with no increase in the transmission bandwidth or power. Antennas destined for MIMO systems need to address the issue of adequate isolation between elements and the issue of the diversity performance of the array, these issues become challenging for mobile terminals. In this thesis dual band arrays for the mobile and the access point are proposed along with dual band mutual coupling reduction and radiation pattern improvement methods. First a dual band two element printed inverted F stacked monopole array is proposed for the mobile terminal. The single elements in the array are easily tuneable and achieve impedance matching from an open stub. The configuration is compact, with radiators distanced at 0.13λ0. By use of a grid of parasitically coupled printed lines mutual coupling is reduced by 9dB, where at the lower band at 2.4GHz, S12 = −18dB. Then a dual band two element printed dipole array is proposed for a pico–micro cell access point. The dipoles are fed by a printed balun which provides wide impedance bandwidth at two bands. To improve the radiation pattern at both frequencies the array is positioned above a dual band frequency selective surface, acting as an artificial magnetic conductor, thus allowing the screen to be placed 0.03λ0 from the array while maintaining good radiation efficiency. Finally a brief discussion of dual band surface wave suppression for printed antennas is presented. Here it is suggested that the surface waves can be eliminated by a superstrate at one band and by an EBG lattice at the second band. Initial experiments with different size superstrates and three periods of mushroom type EBG, show that mutual coupling can be reduced and the radiation pattern can be modified.EThOS - Electronic Theses Online ServiceEngineering and Physical Sciences Research Council (EPSRC)GBUnited Kingdo

Miniature MEMS-Based Adaptive Antennas on Flexible Substrates

Current trends in technology are moving to increased use of wireless communication with rapidly increasing data transmission rates and higher frequencies. Miniaturization is essential to allow electronics of increasing complexity to fit into smaller devices. Adaptive technologies allow a single system to operate across multiple wireless protocols, adjusting to changing conditions to minimize interference and enhance performance. Flexibility is essential as the use of wireless technology increases and spreads to new industries. The objective of this research is twofold: to develop novel reconfigurable electromagnetic structures and a novel process to fabricate microelectromechanical systems (MEMS) devices on flexible substrates. The novel electromagnetic structures are passive frequency-switchable parasitic antennas, conformal MEMS-tunable frequency selective surfaces (FSS) and MEMS-tunable electromagnetic bandgap (EBG) structures. Fabricating the reconfigurable conformal FSS and EBG structures requires the development of a new fabrication process to produce MEMS devices monolithically integrated onto a flexible substrate. Novel frequency-switchable parasitic antenna arrays are developed, fabricated and measured. The structure radiates efficiently when placed over metal and absorbing material, improving the range of conventional RFID systems, as well as minimizing blind spots to provide continuous coverage in a hemisphere. A novel analysis method is developed to characterize frequency-switchable parasitic patch arrays. The purpose of the analysis is to provide an approximation of the input impedance and variation of the radiation pattern with frequency. The analysis combines models based on electromagnetic theory and circuit theory to provide a fast and yet reasonable approximation of the parasitic array characteristics. The analysis also provides a good deal of physical insight into the operation of multi-mode parasitic patch arrays. The end result is an initial array design which provides a good starting point for full EM simulation and optimization. The new analysis method is validated alongside measured and simulated results, with good correlation for both impedance characteristics and far-field radiation patterns. A MEMS-based switched parasitic antenna array is designed, fabricated and measured with good correlation between simulated and measured results. The structure is a direct-coupled parasitic patch array which is capable of frequency steering and has additional MEMS-enabled beam-steering capabilities at each frequency. An EBG-based multi-mode radiating structure design is presented, which is capable of frequency-switchable beam steering. The antenna area is significantly reduced compared to the parasitic patch array structure, but at a considerable cost in terms of gain and efficiency. A novel MEMS process is developed to fabricate large numbers of high-performance MEMS devices monolithically integrated onto a rigid-flex organic substrate using low-temperature processes. The rigid-flex substrate is all dielectric, which is amenable to low-loss electromagnetic structures. The substrate provides mechanical support to the MEMS devices while maintaining overall flexibility. The adaptation of each fabrication process step to handle flexible substrates is analyzed and documented in detail. The newly-developed MEMS process is used to fabricate a MEMS reconfigurable frequency-selective surface. A practical bias network is incorporated into the structure design to ensure that all devices are actuated simultaneously. FSS structures operating in the Ku and Ka bands are fabricated and tested, with good correlation between simulated and measured results for individual devices as well as the entire FSS structures. The newly-developed MEMS process is also used to fabricate a MEMS reconfigurable electromagnetic bandgap structure. An EBG structure operating in the Ka band is fabricated and tested to verify the validity of the proposed concept

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium