Surface Wave Transmission Line Theory for Single and Many Wire Systems

Examining cables using many conductor transmission line theory has shed light on the modes supported by various cable types. However, so far the theory disregards the fundamental surface wave mode whose lateral confinement increases with frequency and hence is expected to play an important role in high frequency applications. To address this issue, we propose an extension to the theory which incorporates surface waves on uncoated, cylindrical wires. Crucially, this requires new definitions of the per unit length transmission line parameters which are derived using the single wire surface wave solution. By closely examining a two wire and three wire system, we show that these new parameters can predict surface waves as well as modes found using conventional many conductor transmission line theory. Furthermore, all calculated modes are validated experimentally by diagonalization of a measured channel transfer matrix. Additionally, the theoretically predicted propagation constants for the modes are validated against full numerical simulation for the two wire case and good agreement is observed when proximity effects can be neglected.Comment: 22 pages, 5 Figures, data and supplementary material will be made available at a later stag

Dielectric-induced surface wave radiation loss.

We investigate a model which shows how the introduction of a perturbing dielectric close to an electromagnetic surface wave leads to radiation away from the surface through the dielectric. This resembles a surface waveguide passing through a wall or being deployed underground. Our theory, which is based on the mode-matching technique, allows quantitative determination of losses from a bound surface wave mode up to the point of its complete extinction. For a surface wave supported by a coated, conducting sheet the attenuation due to the perturbing dielectric is calculated for a number of frequencies, permittivities of the perturbation and separations between the sheet and the perturbing dielectric. The accuracy of our results is verified by simulation of the system with a full-wave numerical solution. Finally, we report experimental data of perturbed surface waves on a cable, which are in qualitative agreement with our model

Interaction between surface waves on wire lines.

This paper investigates coupling between electromagnetic surface waves on parallel wires. Finite-element method (FEM)-based and analytic models are developed for single- and double-wire Sommerfeld and Goubau lines. Models are validated via measurements for Goubau lines and a comparison between the analytic and the FEM-based computations for coupled Sommerfeld- and Goubau-type lines is carried out. The measurements and calculations show remarkable agreement. The FEM-based and analytic models match remarkably well too. The results exhibit new favourable effects for surface waves propagation over multiple conductors. The short-range behaviour of the coupled wires and, consequently, the existence of an optimum separation of coupled wires is one of the most significant findings of this paper. We comment on the relevance of our results, particularly in relation to applications of high bandwidth demands and cross-coupling effects

Analytic modelling of a planar Goubau line with circular conductor.

Planar Goubau lines show promise as high frequency, low-loss waveguides on a substrate. However, to date only numerical simulations and experimental measurements have been performed. This paper analytically investigates the surface wave mode propagating along a planar Goubau line consisting of a perfectly conducting circular wire on top of a dielectric substrate of finite thickness but infinite width. An approximate equation for the propagation constant is derived and solved through numerical integration. The dependence of the propagation constant on various system parameters is calculated and the results agree well with full numerical simulations. In addition, the spatial distribution of the longitudinal electric field is reported and excellent agreement with a numerical simulation and previous studies is found. Moreover, validation against experimental phase velocity measurements is also reported. Finally, insights gained from the model are considered for a Goubau line with a rectangular conductor. The analytic model reveals that the propagating mode of a planar Goubau line is hybrid in contrast to the transverse magnetic mode of a classic Goubau line

Modelling of Electromagnetic Surface Wave Propagation for High-Speed Communications

Surface waves have been known as a low-loss electromagnetic mode supported by a single conductor for well over a century but little research has been dedicated to surface wave interactions with its surroundings and surface wave propagation on printed circuit boards and on multiple wires. This thesis establishes novel analytic models, numerical simulations and experimental results in these areas and uses these insights to assess the application of surface waves with particular focus on telecommunications. To begin with, well-known surface wave solutions are reviewed. Radiation induced by dielectric material in the vicinity of the binding structure is discussed as a potential loss mechanism of propagating surface waves. In addition, a model is developed for the planar Goubau line consisting of a single conducting trace on a dielectric substrate. The model assumes lossless materials and replaces the conducting trace by a current filament. A boundary condition is imposed on the electromagnetic field to determine the phase constant of the supported surface wave. The results of the model are consistent with numerical solutions using the COMSOL software package and with experiments. Next, surface waves on uniform two-wire and multi-conductor systems are studied. Two-wire systems are modelled perturbatively by assuming that the two-wire modes are a superposition of single-wire surface waves. By neglecting all but the fundamental surface wave mode, approximate expressions for the propagation constant and loss are derived and solved. The results are compared to experimental and numerical analysis and show high consistency when the wire separation is much larger than their radius. The methodology is generalised to an arbitrary number of conductors but quickly increases in computational complexity. Hence, an alternative approach is developed where surface waves are represented by a transmission line segment. Per-unit-length transmission line parameters are derived under the assumption that the coupled electromagnetic field is approximated by a superposition of fundamental surface waves on each wire. The system can then be treated as a coupled many-conductor transmission line system. Phase and attenuation constant of modes supported by the coupled system are compared to experimental results and to numerical simulations. Results agree well when the wire separation compared to the radius is large. The final part of the thesis investigates non-uniform systems supporting surface waves. In particular, twisted pair cables are studied. Experimentally observed stop bands are explained through contradirectional mode coupling which can occur in systems without helical symmetry. The many-conductor transmission line model for surface waves on uniform systems is adapted to simulate the non-uniform twisted pair cables. The stop band of systems containing a single twisted pair together with one or two straight wires are analysed experimentally, numerically and with the transmission line model. Further investigations into systems containing multiple twisted pairs with different twist rates show stop bands associated with the twist rate of each pair. Stop bands are predicted by the transmission line model but the simulated shape and frequency are only approximate because all systems considered have closely bundled conductors. The simulation tool CST Microwave Studio shows better accuracy in predicting the stop bands. However, code runtime for simulations are much greater than for the transmission line model developed in this thesis. The results of this thesis play an import part in assessing the applicability of surface waves as a high-speed communications channel. The research into surface waves supported by two or more conductors including the analysis of twisted pair cables is especially valuable because these are common cables found in the deployed telecom infrastructure. When stop bands are avoided on these cables, surface waves can be used to transmit broadband data signals over large distances subject to deployment scenarios

Land-use legacies in the forest structure of silvopastoral oak woodlands in the Eastern Mediterranean

Abstract Eastern Mediterranean silvopastoral oak woodlands have been greatly damaged through forest conversion, illegal lumbering, overgrazing, and forest fires. The aim of this study was to assess land-use changes and the legacies that they have imprinted on the forest structure of Quercus macrolepis and accompanying Quercus pubescens and Quercus cerris woodlands on Lesvos Island, Greece. The size structures of adult oak populations were analyzed as indicators of long-term oak regeneration, while short-term recruitment was determined by counting oak seedlings and saplings. The size structure of the adult Q. macrolepis population was similar to the inverse J-shaped distribution typical for natural Mediterranean oak forests, indicating continuous recruitment with a constant mortality rate of mature individuals. Seedling and sapling densities were highly variable, but generally low in relation to adult oak densities. Recruitment of oak seedlings and saplings was positively related to determinants such as forest cover, adult oak density and basal area, woody plant richness, and litter cover. Both seedling and sapling occurrence were negatively associated with dung frequency, which suggests that sheep grazing imposes a barrier to oak recruitment. The study outlines a comprehensive land-use transition from the 1950 to 1970s, during which a complex and multifunctional agrosilvopastoral land-use system was simplified to an intensive grazing system. The discrepancy between the successful long-term regeneration and the less successful short-term recruitment of oaks illustrates that intensified livestock grazing has been a major driver of vegetation change. Grazing impact is likely to interact with increasing drought conditions, which may trigger a negative feedback cycle that undermines the capacity of woodlands to sustain ecosystem services

Linking ecosystem services with cultural landscape research

ecosystem services approach has become prominent in conservation science and practice. There is an abundance of data, indicators, and models for assessing provisioning and regulating ecosystem services. However, the concept of ecosystem services has not been successful in capturing cultural ecosystem services in any detail. Research in the two fields “cultural landscape ” and “ecosystem services ” should be conducted jointly to enhance the understanding of cultural ecosystem services in social-ecological systems and to develop methods of assessment