Microwave processing of materials has emerged as a new method for processing of a variety of materials in the recent years. Microwaves have been used effectively with significant advantages, particularly in food processing and chemical synthesis. They are also found to be efficient for processing polymers, ceramics, polymeric composites, and ceramic composites. The physics of interaction of microwaves with characteristically different materials is not yet explored well; consequently, there are challenges in microwave processing of metal-based materials. Industrial processing of bulk metal is yet to be popular in spite of the fact that the feasibility of metal powder sintering was demonstrated a few decades ago. This article provides a summary of fundamental aspects of microwave processing of metal-based materials and their interaction with metallic materials. The processing challenges have been surveyed; developments in terms of techniques and tooling have been analyzed. Possible effects of microw...

A Review of Research Trends in Microwave Processing of Metal-Based Materials and Opportunities in Microwave Metal Casting

Phaseless near-field far-field transformations based on nonconvex optimization suffer from local stationary points. In order to avoid the corresponding suboptimal solutions, the incorporation of combinations of probe signals into the cost functional is proposed and investigated. The nonconvex optimization based on the Wirtinger Calculus relies on forward and adjoint radiation operator evaluations via the fast irregular antenna field transformation algorithm (FIAFTA) and is performed by a memory-limited Quasi-Newton method. This allows the solution of large scale problems with utilization of the full flexibility of the FIAFTA in terms of measurement probe correction and arbitrary sample locations. Results for synthetic and measured near-field data reveal significant improvements when the knowledge of probe signal combinations is utilized, especially in applications with a large number of unknowns.

Phaseless Near-Field Far-Field Transformation Utilizing Combinations of Probe Signals

A novel mm-wave microstrip-fed patch antenna with broad bandwidth and wide angular coverage suitable for integration in planar arrays is designed, analyzed, and verified by measurements. The antenna provides a bandwidth of 13.1% between 34.1 and 38.9 GHz, which is achieved by a slotted multiple resonances microstrip patch and a matching circuit in microstrip technology. The antenna is built on RO3003 substrate with top and ground layers, which is low cost compared with other techniques. For simple integration with microstrip and frontend circuits, the feeding happens at the top layer with a microstrip coupling gap feed. The wide half-power beamwidth (HPBW) is achieved by suitably designed parasitic patches for the first resonant mode. The second resonant mode has a wide HPBW by default. The HPBW is between 100° and 125° within the matched bandwidth, which is a very good value for a microstrip patch antenna radiating over a ground plane. The measured input impedance and radiation characteristic show very good agreement with simulation results.

A mm-Wave Patch Antenna with Broad Bandwidth and a Wide Angular Range

Electromagnetic field transformations are important for electromagnetic simulations and for measurements. Especially for field measurements, the influence of the measurement probe must be considered, and this can be achieved by working with weighted field transformations. This paper is a review paper on weighted field transformations, where new information on algorithmic properties and new results are also included. Starting from the spatial domain weighted radiation integral involving free space Green's functions, properties such as uniqueness and the meaning of the weighting function are discussed. Several spectral domain formulations of the weighted field transformation integrals are reviewed. The focus of the paper is on hierarchical multilevel representations of irregular field transformations with propagating plane waves on the Ewald sphere. The resulting Fast Irregular Antenna Field Transformation Algorithm (FIAFTA) is a versatile and efficient transformation technique for arbitrary antenna and scattering fields. The fields can be sampled at arbitrary irregular locations and with arbitrary measurement probes without compromising the accuracy and the efficiency of the algorithm. FIAFTA supports different equivalent sources representations of the radiation or scattering object: 1) equivalent surface current densities discretized on triangular meshes, 2) plane wave representations, 3) spherical harmonics representations. The current densities provide for excellent spatial localization and deliver most diagnostics information about the test object. A priori information about the test object can easily be incorporated, too. Using plane wave and spherical harmonics representations, the spatial localization is not as good as with spatial current densities, but still much better than in the case of conventional modal expansions. Both far-field based expansions lead to faster transformations than the equivalent currents and in particular the orthogonal spherical harmonics expansion is a very attractive and robust choice. All three expansions are well-suited for efficient echo suppression by spatial filtering. Various new field transformation and new computational performance results are shown in order to illustrate some capabilities of the algorithm.

Electromagnetic Field Transformations for Measurements and Simulations

Measurements of turbulent electron temperature fluctuation amplitudes, δTe⊥/Te, frequency spectra, and radial correlation lengths, Lr(Te⊥), have been performed at ASDEX Upgrade using a newly upgraded Correlation ECE diagnostic in the range of scales k⊥ 50% for all simulations within the sensitivity scans performed, good quantitative agreement is found for Lr(Te⊥) and αnT. A validation metric is used to quantify the level of agreement of individual simulations with experimental measurements, and the best agreement is found close to the experimental gradient values.

/pdf/validation-of-gyrokinetic-simulations-with-measurements-of-1poe2vmvoe.pdf

Validation of gyrokinetic simulations with measurements of electron temperature fluctuations and density-temperature phase angles on ASDEX Upgrade

/pdf/electromagnetic-field-transformations-for-measurements-and-4nh3oo11yt.pdf

Electromagnetic Field Transformations for Measurements and Simulations (Invited Paper)

A Newton-type method for the reconstruction of inhomogeneous 3-D complex permittivity variation of arbitrary shaped materials loaded in a rectangular waveguide is presented. The problem is first formulated as a system of integral equations consist of the well-known data and object equations, which contain the dyadic Green's function of an empty rectangular waveguide. Two unknowns of this system are solved in an iterative fashion by linearizing one of them, i.e., the data equation in the sense of the Newton method, which corresponds to a first-order Taylor expansion of the related integral operator. Since the problem is severely ill posed by nature, a regularization in the sense of Tikhonov is applied to the data equation. A detailed numerical implementation of the method, together with some numerical examples are also given to show the capabilities and validation limits of the method.

3-D Imaging of Inhomogeneous Materials Loaded in a Rectangular Waveguide

An iterative technique for the reconstruction of longitudinally inhomogeneous lossy dielectric material loaded in a uniform metallic waveguide is addressed in this paper. The problem is first formulated as an inverse scattering one based on an integral equation in terms of the unknown complex permittivity. By using knowledge of scattering parameters, the Gauss-Newton technique is employed to solve the inverse problem with both the Tikhonov regularization and multiplicative regularization. In order to show the effectiveness of the method, it is tested against numerical and experimental data at X- and K-bands and satisfactory reconstructions are obtained. Experimental results show that the inverse algorithm with multiplicative regularization can be used for the characterization of multilayered structures in a waveguide without needing a priori information.

Regularized 1-D Dielectric Profile Inversion in a Uniform Metallic Waveguide by Measurement and Simulation

 The characterization of antenna radiation patterns by transformed near-field measurements requires accurate amplitude and phase data This represents a problem since expensive measurement equipment is required, especially at millimeter and submillimeter wavelengths (Isernia et al, 1996) Amplitude-only antenna field measurements are theoretically sufficient for the unique determination of antenna far-fields Therefore, phaseless techniques are of special interest However, the required field transformations are extremely challenging, since they are nonlinear and strongly ill-posed In this work, the amplitude-only or phaseless near-field far-field transformation problem is formulated as a nonlinear optimization problem The linear radiation operator within the nonlinear formulation is evaluated using the fast irregular antenna field transformation algorithm (FIAFTA) A hybrid solution procedure is described which combines a genetic algorithm with an iterative conjugate gradient (CG) search method Numerical results prove the efficiency and flexibility of the formulation and it is shown that the algorithm remains stable when the noise level in the measurements is moderate Nevertheless, regularization techniques might be beneficial to further improve the robustness of the algorithm

/pdf/fast-near-field-far-field-transformation-for-phaseless-and-3ca9u5yjs0.pdf

Emre Kilic

Papers

Electromagnetic Field Transformations for Measurements and Simulations (Invited Paper)

3-D Imaging of Inhomogeneous Materials Loaded in a Rectangular Waveguide

Regularized 1-D Dielectric Profile Inversion in a Uniform Metallic Waveguide by Measurement and Simulation

Fast near-field far-field transformation for phaseless and irregular antenna measurement data

Solution of 3D inverse scattering problems by combined inverse equivalent current and finite element methods