Broadband high-gain planar leaky-wave antennas

High-gain, low-cost, planar antennas have attracted a lot of interest in recent years, with regard to applications as fixed wireless access, satellite reception and various point-to-point radio links. Microstrip patch arrays have primarily been good candidates, but the complex feeding mechanisms degrade the antenna performance. A method of producing a high gain planar antenna with a simple feed has been proposed in an earlier study. This technique utilises a partially reflective surface (PRS) to introduce a leaky wave and beamforming effect when placed in front of a waveguide aperture in a ground plane. The partial reflection can be obtained from periodic arrays, also referred to as Frequency Selective Surfaces (FSSs) when used for their filtering properties. The research effort in this thesis focuses on the theory underpinning the beamforming effect of single and double-layer PRSs in a leaky-wave antenna configuration and subsequently on novel leaky-wave antenna designs. [Continues.

Near-field analysis of frequency selective surfaces and applications in directive antennas

A near-field characterisation of two dimensional metallo-dielectric frequency selective surfaces either in a single or double layer configuration is presented in this thesis. Motivated by the current attention of the electromagnetic properties of near-fields, an in-house periodic MoM-based computational tool is developed for the efficient and rigorous estimation of the near-fields in frequency selective surfaces (FSS) illuminated by a plane wave. For this purpose a thorough convergence study related to the calculation of the near fields is initially presented. The near-field estimation allows us to calculate the power stored in an FSS at resonance which, in turn, can be used in the calculation of the loaded quality factor of the FSS. Based on the characterisation of various topologies, new techniques for the analysis of highly-directive and broadband leaky wave antennas are proposed. An initial design based on a perturbed FSS results in a structure with multiband response and near-fields enhanced by more than 70 fold, which can be relevant to sensor applications. Subsequently, the near-field technique is used in combination with reciprocity for the extraction of the radiation patterns in Fabry-Perot cavity antennas formed between a FSS and a metamaterial ground plane. In combination with traditional array theory the complex dispersion characteristics of high-gain sub-wavelength 2-D Fabry-Perot leaky-wave antennas (LWA) consisting of two periodic metallodielectric arrays over a ground plane are extracted. This yields a fast and rigorous tool for the characterisation of this type of antennas. Design guidelines are given throughout to synthesize a highly-directive antenna and a broadband leakywave antenna. This thesis was fully funded by the Joint Research Institute for Integrated Systems in Edinburgh, Scotland

A simulation tool for the analysis and design of leaky wave antennas in laterally shielded planar technology with application to metamaterials

Leaky-waves have been a topic of increasing interest in the last years, with diverse practical applications in many different engineering fields. From periodic, FSS, EBG or even metamaterial leaky-wave based antennas to waveguide filters and higher efficiency energy guiding, they all share a common base structure: a travelling-wave propagating within a metal encapsulation, that can be open or closed, and altered by a planar metallization of periodic nature, from which the energy may radiate. Due to the fact that these antennas are usually electrically large and the periodic printed circuit requires a certain grade of complexity, 3D commercial software is prohibitively time consuming. Also, the homebrew methods developed up to this day are either not rigorous and accurate enough or unable to deal with complex periodic geometries. At this point, the evolution of leaky-wave antennas needs a solid, efficient and versatile tool where to base the future design research on. In this work a novel simulation tool for waveguide embedded leaky-wave antennas is presented. It is based on a full-wave Method of Moments applied to the spectral domain Green Functions for a rigorous modal analysis of the finite structure. The use of Subdomain basis functions allows the software to model complex periodic geometries, overcoming a main limitation, and the analytical nature of the method combined with its 2.5D approach, results in a significant computing time reduction. It is built on a modular coding philosophy and provided with a user-friendly graphical interface, and an intuitive working procedure, making the program not only fast and accurate, but also easy to use and extend to new geometries. Finally, it is remarkable the educational potential of this new analysis software, since it identifies higher order effects as bandgaps and multi-harmonic radiation from a complete and simple modal approach

Applications of Floquet Analysis to Modern Phased Array Antennas

Next generation radar technology is based on phased array technology and provides remarkable scanning flexibility and spatial search capability for the multifunction weather and air surveillance radar systems. The future weather radar is comprised of thousands of antenna elements and requires strict polarization purity, grating lobe free system, low sidelobe levels, suppressed surface waves, low cross-polarization, with beam shape requirements. To address these demands is a serious challenge. Over the past few decades, phased array radar technology has been a tremendous advancement in search for future radar technology. With the blessing of modern computational electromagnetic tools, the theory behind the electromagnetic and circuit-level behavior of large-scale phased array system opened the door to analyze the wide variety of multi-layered, complex system of large arrays. However, numerous challenges still remained unsolved for large scale development. One such challenge in integrating a large phased array is the threat of grating lobes that are introduced by unavoidable disturbances to the periodic structure at the seams between mechanical sub-array modules. In particular, gaps in the ground plane may interrupt the natural currents between elements, leading to radiation from periodic sources that are spaced at regular distances that are typically many wavelengths apart. In order to quantify these grating lobe effects, an appropriate analysis framework and accurate model are of utmost importance. The model must capture all surface wave and mutual coupling between elements, and the analysis must have a clear formulation that allows for the calculation of worst-case grating lobe levels as well as differences in active reflection as a function of location within a sub-array. To accurately predict those effects, this dissertation work applied a modern method called Floquet framework, coupling with full wave solver to explore the grating lobe effects in infinite arrays of sub-arrays, with each physical sub-array potentially separated from the others by a gap or discontinuity in the ground plane. Calculations are then performed to extract active reflection coefficients and grating lobe levels from the resulting Floquet mode scattering parameters. Additionally, this Floquet framework is expanded from broadside to any scan angles in space. In the mathematical framework, the surface equivalence theorem based on Huygens’s equivalence principle is applied to authenticate its findings. From the simulation results, it is evident that the grating lobe amplitude level emerged to around 30 dB in the E-plane scan and E- plane grating lobes for a patch array. This is due to natural current disruption in between sub-arrays in the ground plane gap and it is very strong in the E-plane, leading to the potential for low-level grating lobe effects. The other planes and scan angles show less significant effects. It was found that the measurements qualitatively follow the simulated results. The Floquet-based method may therefore be used as a good approximation for a worst-case scenario where all gap-based perturbation effects are identical on each sub- array. This can be used for system-level planning to inform a mechanical solution to the electrical connection between sub-arrays. Another fundamental and paramount challenge for phased array antenna is scan blindness. Scan range of the printed phased arrays is limited by the phenomenon known as scan blindness, which is induced by coherent coupling between the substrate waves/surface waves and the array’s space harmonic fields. Near the scan blindness angle, a phased array system fails to function as a radiator or receiver because of strong excitation of substrate Transverse Electric (TE) and Transverse Magnetic (TM) waves and coupling of desired radiating energy to these unwanted substrate waves. Moreover, this dissertation work, with the aid of Floquet framework, accurately and more precisely captures the surface wave phenomena and its behavior using Electromagnetic Bandgap (EBG) structures to aim to reduce the surface wave excitation in an intelligent way. The reduction of surface waves can be beneficial in several ways to the next generation of digital phased arrays. First, the radiation efficiency will increase due to reduced surface wave excitation. Second, due to decreased surface waves the diffraction from the edges will also be decreased, leading to decreased back radiation and interference with the main pattern in the forward region. Finally, reduction of surface wave excitation ultimately reduces coupling between adjacent antenna elements. Furthermore, cylindrical radiating phased array radars have a unique challenge. Due to their conformal nature, they support cylindrical surface waves and cylindrical creeping waves. These modes have detrimental effects on the overall pattern quality and lead to “phase mode blindness” like as planar equivalent “scan blindness”. This dissertation seeks to explain, address, and mitigate these surface and creeping wave effects and ultimately suppress “phase mode blindness” using cylindrical EBG structure

Microwave antenna system for passive discrimination

A novel passive antenna system, capable of discriminating specific electromagnetic signals is addressed. This antenna system will be able to detect signals of certain bandwidths, amplitudes and propagation directions. The philosophy behind this design was to maximise the signal discrimination at a stage prior to reception. The development of such systems could relieve the work involved in post detection discrimination, which may be time consuming and expensive. A major motivation of these studies lies in the difficulties inherent in signal detection for mobile radio communication systems operating at microwave frequencies. Such an antenna system consists of two components. They are the filter section and the detector array. The filter is designed in such a way that only the near normal signal to the locally flat area will be admitted and the rest reflected. The detector array will be at an appropriate position below the filter. Two types of filter structures have been studied for this angular filtering property. They are the Dielectric Multilayers (DML) and periodic arrays of slots as Frequency Selective Surfaces (FSS). DML are constructed by stacking layers of dielectric material whose permittivities vary in a near sinusoidal manner. Such a structure is known to have the ability to admit certain frequency bands of signals. The conventional transmission/reflection matrix method is used for its analysis. Also an optimisation procedure is carried out to minimise the loss of the signal in the DML. The characteristics of the DML as a beam-director and Beam-shaper have also been investigated. FSS exhibit the characteristics of band pass and band stop filters, depending upon the nature of the surface (periodic arrays of elements or slots) . Here the band pass nature is utilised by using arrays of slotted elements. These surfaces are tuned to admit narrow band signals. The well-known modal analysis method has been employed to study the FSS characteristics. The FSS have been studied in the context of frequency scanning, beam shaping, beam directing as well as angular scanning. A prototype has been constructed to simulate a multi signal environment in which the above structures have been experimentally assessed

Reconfigurable Transmitarray with Near Field Coupling to Gap Waveguide Array Antenna for Efficient 2D Beam Steering

A novel array antenna architecture is proposed that can enable 2D (full-space) radiation pattern control and efficient beam steering. This solution is based on a fixed-beam gap wave-guide (GWG) array antenna and a reconfigurable transmitarray (TA) that are coupled in the radiative near field. An equivalent two-port network model of the coupling mechanism is presented and validated numerically. The desired TA reconfiguration capability is realized by an 8 78 array of cavity-backed patch resonator elements, where two AlGaAs PIN-diodes are integrated inside each element providing a 1-bit phase shift. The TA is implemented in an 8-layer PCB, which includes radiating elements, fixed phase-shifting inner-stripline sections, impedance matching and biasing circuitry. The combined antenna design is low-profile (~ 1.7 wavelength) owing to the small separation between two arrays (~ 0.5 wavelength), as opposed to conventional TAs illuminated by a focal source. The design procedure of the proposed architecture is outlined, and the measured and simulated results are shown to be in good agreement. These results demonstrate 23.5 — 25.2 GHz –10-dB impedance bandwidth and 23.3 — 25.3 GHz 3-dB gain bandwidth, a beam-steering range of \ub130\ub0 and \ub140\ub0 in the E- and the H-plane with the gain peak of 17.5 dBi, scan loss ≤ 3.5 dB and TA unit cell insertion loss ≤ 1.8 dB

Beam-Steerable and Reconfigurable Reflectarray Antennas for High Gain Space Applications

Reflectarray antennas uniquely combine the advantages of parabolic reflectors and phased array antennas. Comprised of planar structures similar to phased arrays and utilizing quasi-optical excitation similar to parabolic reflectors, reflectarray antennas provide beam steering without the need of complex and lossy feed networks. Chapter 1 discusses the basic theory of reflectarray and its design. A brief summary of previous work and current research status is also presented. The inherent advantages and drawbacks of the reflectarray are discussed. In chapter 2, a novel theoretical approach to extract the reflection coefficient of reflectarray unit cells is developed. The approach is applied to single-resonance unit cell elements under normal and waveguide incidences. The developed theory is also utilized to understand the difference between the TEM and TE10 mode of excitation. Using this theory, effects of different physical parameters on reflection properties of unit cells are studied without the need of full-wave simulations. Detailed analysis is performed for Ka-band reflectarray unit cells and verified by full-wave simulations. In addition, an approach to extract the Q factors using full-wave simulations is also presented. Lastly, a detailed study on the effects of inter-element spacing is discussed. Q factor theory discussed in chapter 2 is extended to account for the varying incidence angles and polarizations in chapter 3 utilizing Floquet modes. Emphasis is laid on elements located on planes where extremities in performance tend to occur. The antenna element properties are assessed in terms of maximum reflection loss and slope of the reflection phase. A thorough analysis is performed at Ka band and the results obtained are verified using full-wave simulations. Reflection coefficients over a 749-element reflectarray aperture for a broadside radiation pattern are presented for a couple of cases and the effects of coupling conditions in conjunction with incidence angles are demonstrated. The presented theory provides explicit physical intuition and guidelines for efficient and accurate reflectarray design. In chapter 4, tunable reflectarray elements capacitively loaded with Barium Strontium Titanate (BST) thin film are shown. The effects of substrate thickness, operating frequency and deposition pressure are shown utilizing coupling conditions and the performance is optimized. To ensure minimum affects from biasing, optimized biasing schemes are discussed. The proposed unit cells are fabricated and measured, demonstrating the reconfigurability by varying the applied E-field. To demonstrate the concept, a 45 element array is also designed and fabricated. Using anechoic chamber measurements, far-field patterns are obtained and a beam scan up to 25o is shown on the E-plane. Overall, novel theoretical approaches to analyze the reflection properties of the reflectarray elements using Q factors are developed. The proposed theoretical models provide valuable physical insight utilizing coupling conditions and aid in efficient reflectarray design. In addition, for the first time a continuously tunable reflectarray operating at Ka-band is presented using BST technology. Due to monolithic integration, the technique can be extended to higher frequencies such as V-band and above

Impedance Transformers

Non

Propagation characteristics of cylindrical frequency selective guides

Recent experimental investigation on FSS arrays forming waveguides (FSGs) and horns showed that incident electromagnetic energy can be guided and radiated at specific frequencies. This thesis aims to provide the theoretical understanding of the waves propagating inside a cylindrical FSS waveguide. With immediate applications on horn antennas the research deals with cylindrical guides, made entirely from double periodic arrays. The theoretical analysis begins as a standard electromagnetic boundary value problem. The formulated system of algebraic equations is solved either for the complex propagation constant, by an iterative procedure or, for the fields. The analysis makes use of the Floquet modal expansion, the current representation as a set of sub-domain basis functions and the Method of Moments. Initially, the thesis is concerned with single periodic structures, which is a special case to the analysis. The efficiency of the model to provide stable and valid results is examined. Next the elements are finite dipoles. The effects of the dipole resonance to the propagating and radiating characteristics of the FSS is closely investigated. Other aspects include the effects of the periodicity and the element size. The investigation concludes with an FSG with square loop elements. Validation of the results for some designs is made by comparison with measured data