Symmetrically Truncated Right-Angle Corners in Parallel-Plate and Rectangular Waveguides

In this work, a comparative study of the susceptibility chart in a partially dielectric-loaded rectangular waveguide and in its equivalent partially dielectric-loaded parallel-plate waveguide with the same height is performed. This study shows how the inhomogeneity of the electric field inside the rectangular waveguide modifies the edges of the multipactor susceptibility chart with respect to that predicted by the parallel-plate waveguide case. Moreover, comparisons of the evolution of the population and the trajectory of the effective electron and the DC field appearing in the waveguide in a point well inside the multipactor region obtained with both models are performed, showing some quantitative differences in the multipactor evolution, but a similar qualitative behaviour is observed.

For of applications of waveguide-slot antennas, it is important to know the radiation characteristics of the slot in a rectangular waveguide wall radiating into a parallel-plate waveguide (PPW). In this work the properties of a longitudinal slot in the narrow wall of a rectangular waveguide radiating into a PPW are investigated. A model of a waveguide-slot radiator is presented and an accurate mathematical analysis provided. It is shown how to improve and optimize the matching of the considered radiators.

In this paper, waveguide mode conversions in a parallel plate waveguide, rectangular waveguide, and circular waveguide are all demonstrated at terahertz frequencies. Different from the traditional ways to realize mode conversions, an ultracompact design method based on the inner-embedded subwavelength metallic gratings is provided. The principle of the method relies on modulating the phase velocities of spoof surface plasmon polaritons (SSPPs) supported on the gratings to generate π phase difference. Owing to the gradient matching the phase velocities of the waveguide modes and SSPPs, the efficiencies of the mode conversions are considerable. The output mode purities of the both demonstrations are very good. Besides, waveguide bend is also achieved by this method. The work in this paper hopes to establish a universal technique for compact waveguide mode control, especially in the terahertz frequency range.

A thin wall leaky waveguide antenna (TLA) obtained by replacing one of the narrow walls of a rectangular waveguide by a film of metal with thickness less than the skin depth is discussed. The radiation pattern for TLA has been evaluated by using Kirchhoff-Huygens approach. In using this method, the dispersion relation has been solved for the complex propagation constants in a parallel plate waveguide with an imperfect plate ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sigma &lt; \infty</tex> ), the thickness of which is less than the skin depth. Separate approximate expressions for the propagation constants for TEM, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM_{n0}</tex> , and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TE_{n0}</tex> modes in this parallel plate structure have been obtained by considering the mode of propagation as a perturbation of the waveguide mode. Experimental results at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</tex> band frequencies with thin bismuth films are reported and are found to be in fair agreement with the theoretical results. Phase velocity ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/\upsilon</tex> ) variation of TLA is essentially that of a simple rectangular waveguide. Very low values of attenuation constant can be obtained so that a long antenna can be used even with the beam directed away from endfire.

AbsIracf - Two-dimensional backward-wave meta-material based on evanescent parallel-plate waveguide filled with uniaxial magnetic tilling with negative transversal permeability is investigated experimentally. The prototype comprised a two-dimensional array of the double ring resonators sandwiched between two metallic plates and it was operating in 8 GHz frequency hand. The backward-wave propagation was verified by the measurement of negative lateral displacement of the beam impinging the meta-material slab at several different angles of incidence I. INTRODUCTION A material which supports propagation of the backward waves ('left-handed material' or 'backward-wave material') was predicted theoretically in sixties by Veselago (I) and realized experimentally for the first time by Smith at al. (2) in 2000. The rather simple type of one-dimensional backward-wave meta-material based on an evanescent waveguide loaded with the split ring resonators was presented in (3). Such a waveguide was interpreted as one-dimensional material with both negative permeability and negative permittivity (3). In (4,5), it was shown that the propagation below the cut-off frequency actually occurs due to uniaxial anisotropy of negative permeability of the meta-material based on the split ring resonators. The extension of this basic principle to two dimensions, similarly to the idea introduced in (6), is investigated experimentally in this paper. 11. THEORETICAL RESULTS Let us analyze the structure that comprises a slab of anisotropic uniaxial magnetic materials sandwiched within the two infinite parallel PEC plates (Fig.1). If the optical axis of the uniaxial material is perpendicular to the plates (the transversal direction), it is obvious that this structure is essentially isotropic in two dimensions. Thus, the electromagnetic energy will propagate in any direction parallel to the plates (the longitudinal direction) in the exactly the same way (the propagation for two perpendicular longitudinal directions is sketched in Fig. 5). For any particular longitudinal direction, one may think of this structure as a parallel plate waveguide. In the lossles case, the longitudinal propagation factor of such a waveguide is given by the simple set of equations (4,51: Here, k, stands for longitudinal propagation factor, ko is a free space propagation factor, g is relative permittivity and p,, and pi, stand for relative permeability in transversal and longitudinal directions of the waveguide, respectively. The symbol f stands for a frequency of the signal, whereas Lo and L are the cut-off frequency of an empty parallel-plate waveguide and the parallel-plate waveguide filled with a material, respectively. If the transversal permeability is negative &<0 ) and the longitudinal permeability is positive (ppO), one must choose the negative sign in front of the square root in (I) for frequencies below the cut-off. This yields the peculiar phenomenon of the backward-wave propagation. Hence, the structure actually behaves as two-dimensional backward-wave meta-material provided that the vector of the magnetic field is perpendicular to the walls (TE 01 mode). If one stacks more parallel plate waveguides in the transversal direction, it is possible to manufacture the two-dimensional 'backward-wave' material of arbitrary thickness, as proposed in (6). The backward-wave propagation below the cut-off frequency can he simply explained by two-dimensional transmission line equivalent circuit of the structure (Fig.2). The distributed series impedance (212) is associated with the transversal permeability. The distributed shunt admittance associated with the relative permittivity (Y,) and the admittance associated with the longitudinal permeability (Y,) form the tank circuit. In the case of the uniaxial material with positive permeability, the series admittance will he also positive. Below the cut-off, the tank circuit behaves as an inductance and, for any direction parallel to the plates, the whole structure behaves as a transmission line with the distributed series inductance and the distributed parallel inductance (L-L transmission line), This kind of transmission line does not support the propagating waves. If the transversal

[EN] The general goal of this thesis is the development of a basic tool for the analysis of passive microwave and millimetre-wave devices used in the new generation of communication systems. In particular, the work is focused on the development of a tool for an efficient and accurate modal characterization of passive devices composed of waveguides with arbitrary cross section.&#13;\nIn order to achieve this goal, a method recently arisen and called BI-RME (Boundary Integral - Resonant Mode Expansion) is extended. This method has been revealed as one of the most appropriate, for its high computational efficiency, to characterize waveguides with arbitrary cross section. Regarding the original BI-RME method, this one will improve the precision and the efficiency of the modal solution in presence of circular and elliptic perturbations in the arbitrary waveguide contour.&#13;\nThen, the first work consists on extending the original BI-RME method theory to circular and elliptic geometries including the possibility to connect these two new types of segments (circular and elliptic) with the straight segments used until now in the practical implementation of the method.&#13;\nIn order to validate the new developed theory, a set of parameters as the cut-off frequencies, the cut-off wavelengths or the electromagnetic fields of canonical waveguides and arbitrarily shaped waveguides of great practical interest, are calculated. These results are compared with analytical well-known solutions or results published in the literature.&#13;\nOnce the electromagnetic modal chart of different arbitrarily shaped waveguides is obtained, the following step is the computation of the coupling integrals between these modes and the ones of the rectangular waveguide that completely surrounds the arbitrary contour. Such integrals are essential for the modal analysis of microwave passive structures involving waveguides with arbitrary cross section.&#13;\nFor this purpose, the new extended BI-RME method is included into a software package tool based on the integral equation method, which allows to characterize efficiently waveguide discontinuities by means of inmitance matrices (admittance or impedance). This efficient full-wave analysis method requires the knowledge of the modal chart related to all the waveguides included in the devices under consideration, as well as the coupling integrals between the modes of the two waveguides composing the discontinuity. In the rectangular waveguides case this modal information can be easily obtained in an analytical way. However, when a waveguide or both have arbitrary cross section, the accurate and efficient modal analysis can be obtained by means of the BI-RME method.&#13;\nThe use of the integral equation method based on admittance or impedance matrices to characterize devices composed of cascading waveguides is specially advantageous because it leads to a linear system of equations. The coefficients matrix of this linear system presents a banded structure that can be appropriately exploited to increase the computational efficiency associated to the resolution of the aforementioned system, and to reduce considerably the computational cost associated to the analysis of complex devices.&#13;\nNext step is the demonstration that in waveguides with arbitrary cross section and with one or two symmetry axes you can generate, also by means of the BI-RME method, the whole family of waveguide modes, so that they verify a certain fictitious boundary condition of electric or magnetic walls in the symmetry plane, and the advantages regarding the computation of the modes are discussed. Furthermore, those different symmetry families of waveguide modes propagate in an independent way along the structures constituted by a connection of uniform waveguides. This situation can also be exploited to increase in a remarkable way the efficiency of the Computer-Aided-Design (CAD) electromagnetic software packages.&#13;\nOnce verified with success the efficiency and precision of the new CAD tool developed by means of the analysis of transitions published in the related technical bibliography, the analysis of really complex passive waveguide devices is performed. Some passive waveguide devices analyzed are for instance a filter with inductive coupling windows and tuning screws, a filter with rounded corners, dual mode filters in circular waveguide with elliptic irises or in elliptic waveguides, an evanescent filter, a low bandpass filter in coaxial waveguide, and finally a new 90º twist component for space applications. The simulated results are totally validated by means of comparison with other data published in the related technical bibliography, or by means of measurements of manufactured prototypes.&#13;\nFinally, the conclusions of this work are extracted and future research lines are outlined. Publications in journal and congress presentations in the framework of this thesis are also indicated.

This paper presents a new method for the analysis of a wide variety of microwave components like posts, circular junctions, or corners in rectangular waveguides. The method gives the impedance or admittance matrix of the microwave component by applying discrimination between the localized and accessible modes in the indirect mode matching (IMM) formulation. In this way, the method is more simple and versatile than previous modeling techniques where IMM is involved, since no use of a Green's function is required for the coupling between different ports. Furthermore, additional obstacles, like posts at the center of a circular junction, can be considered as different terminations of the corresponding transmission lines. The formulation is given for a two-port as well as a multiport system, and in the present paper numerical demonstration is provided for an abrupt and a step H-plane transition from a rectangular to a radial waveguide, a centrally located H-plane circular post in a rectangular waveguide, and a two-port circular junction between rectangular waveguides. The results are compared with the High Frequency Structure Simulator. In all of the aforementioned cases, the method proved to be fast, very accurate, and easy to implement.

The solution for the free vibrations of initially disturbed thin elastic rings is obtained by converting the differential equations of motion to two sets of finite difference equations: a conventional set, and a modified set based on a technique first presented by Chuang and Veletsos in their investigation of the static equilibrium of cylindrical shells. In the conventional set, both of the displacement variables are defined at the same location on the finite difference network, but in the modified set the two displacement variables are defined at staggered locations. The analytical frequency equation associated with both sets of difference equations is derived and shown to be correct by an actual computer solution. A sample ring is considered and the superiority of the modified difference equation is shown by comparing the natural frequencies of both sets of difference equations with the exact natural frequencies.

This article presents a novel semi-open resonator design based on a pair of evanescent waveguides (EWs) and a dielectric material under test (MUT) for high-sensitivity material structural health monitoring. Compared to conventional microwave resonant cavities, the electric field energy is primarily concentrated within the dielectric material at the resonant frequency, which significantly enhances the interaction between the electromagnetic field and the dielectric. Minor variations in dielectric properties caused by local defects will induce measurable shifts in the resonant frequency. Another distinctive feature of the designed resonant cavity is its semi-open architecture, where the dielectric material is not placed inside a rectangular waveguide but is positioned between two mirror-symmetric open-ended waveguides. This design enables the resonator to achieve scanning detection for larger-area MUTs. To mitigate electromagnetic leakage and enhance the quality factor of the resonator, a parallel plate waveguide (PPW) is incorporated at the open ends of the evanescent waveguides. To demonstrate the detecting ability, an extensive investigation of the sensitivity to local defects inside the material is presented. Two different dielectric materials, including F4B and ceramic samples, are tested using a prototype device. The experimental results demonstrated frequency shifts of 11 MHz at 4.32 GHz and 10 MHz at 3.65 GHz for subwavelength defects with diameters and thicknesses less than 2 mm. Compared to conventional microwave-based sensors, our proposed method offers advantages in detecting minute defects within relatively thick materials at lower operating frequencies.

The problem of radiation of TE-wave from the parallel plate waveguide with a corrugated flange is reduced to the hypersingular integral equation. It is shown that the solution of scalar problem gives a good approximation to the radiation pattern in H-plane for 3D case of rectangular waveguide. The influence of the corrugations on the shape of the boresight radiation pattern is investigated. The regions of the grating parameters characterized by the radiation patterns with different number of maxima are defined.

Electromagnetic radiation and coupling problems by the finite periodic strip grating geometry on a grounded dielectric slab are investigated from the viewpoints of both the leaky wave antenna and the grating coupler. It is shown that the present geometry gives almost the same radiation pattern as that of the finite periodic slot geometry in the parallel-plate waveguide (taken as the simplified problem of the finite leaky wave antenna of the rectangular waveguide with one sidewall replaced by an inductive grid grating) by comparing the radiation patterns of two geometries. It is also shown that the present geometry has the advantages of better impedance matching and higher radiation efficiency even for the smaller size as well as easy fabrication for the leaky wave radiator application for millimeter waves over the periodic notch geometry on a ground plane covered by a dielectric slab. Concerning the grating coupler application, the performance of the present geometry is observed to be comparable to those of the previous finite periodic notch geometry on a ground plane as well as grating couplers used in integrated optics. In conjunction with the method for improving the coupling efficiency in the grating coupler application, a the discussion on the method for improving the aperture efficiency of the travelling wave antennas of the leaky wave type is touched upon.

The study of natural convection from a heat source located at the bottom of partially top vented enclosure is presented. Air cooling for electronic component situated at the bottom of a top vented enclosure is studied theoretically. The flow is assumed to be laminar, steady and of constant physical properties. The process is described by continuity, momentum and energy partial differential equations, which are be expressed in Cartesian coordinates system. Due to the nature of the studied problem and with proper transformation of the problem dependent and independent variables, these governing equations are transformed to a set of dimensionless partial differential equations. This set of differential equations is transformed to set of difference equations by the implantation of finite difference technique. Accordingly, one can obtain the solutions for this problem, which are obtained by application of the well known Gauss Siedel iteration method. A computer program is developed to solve the present proposed mathematical model. According to this solution, the values of Nusselt number, for different values of Rayleigh number are obtained. Comparisons between the present obtained results and those results obtained in previous theoretical results are performed for parametric variations of the vent opening size and shape. Also a correlation for Nusselt number as a function of Rayleigh number and equivalent diameter of the vent is proposed.

In this paper, two waveguiding structures for arbitrary wavefront manipulation in the terahertz spectral region were proposed, designed and characterized. The first structure consists of parallel stack copper plates forming an array of parallel-plate waveguides (PPWGs). The second structure is three-dimensional metal rectangular waveguides array. The phase delay of the input wave after passing through the waveguide array is mainly determined by the effective index of the waveguides. Therefore, the waveguide array can be engineered using different core width distribution to generate any desired light beam. Examples, working at the frequency of 0.3 THz show that good focusing phenomenon with different focus lengths and spot sizes were observed, as well as arbitrarily tilted propagation of incident plane waves. The structure introduces a new method to perform wavefront manipulation, and can be utilized in many important applications in terahertz imaging and communication systems.

Based on phased-gradient metasurfaces, an efficient method is developed to convert waveguide modes in a parallel-plate waveguide. An effective wavevector is imparted to the incident mode at each interaction with matesurfaces, resulting in the mode conversion. The numerical simulation indicates that the efficiency and purity of TE1-to- TE2 conversion can significantly reach up to 99.98% and 99.99% at 9GHz. The conversion between TM3 and TM4 is also achieved, which gives a fair verification of the polarization insensitivity. This work can provide technical guidance for compact modes control.

Recently, great interest has been devoted to the realization of artificial magnetic conductors (AMC) through the use of periodic surfaces. This kind of structure can be used in several practical applications. An interesting application of AMC surfaces is concerned with the possibility of realizing quasi-TEM waveguides, characterized by a uniform field distribution and by a phase velocity equal to the speed of light. A quasi-TEM waveguide implementation based on printed FSS on two walls of a rectangular waveguide has been proposed (Fei-Ran Yang et al., IEEE Trans. Microwave Theory and Techniques, vol.47, no.11, 1999). At the resonance frequency of the FSS, the side walls act as magnetic walls, thus supporting the propagation of quasi-TEM mode. Using a boundary condition on the wall imposes annulment of both the E and H components along the direction of propagation; thus the hard surface is well suited for supporting a quasi-TEM propagation inside waveguides. A hard surface can be realized by printing dipoles on a thin grounded dielectric slab. We use this solution for loading walls in both rectangular and circular waveguides to obtain quasi-TEM waveguides. The design process is described. The results are compared with those from an independent code.