Torleif Martin

An improved near- to far-zone transformation for the finite-difference time-domain method

Near- to far-zone transformation for the finite-difference time-domain (FDTD) method can be performed by integration of the equivalent electric and magnetic currents originating from scattered electric and magnetic fields on a surface enclosing the object. Normally, when calculating the surface integrals, either the electric or magnetic fields are averaged since the electric and magnetic fields are spatially shifted in the FDTD grid. It is shown that this interpolation is unnecessary and also less accurate than if an integration is performed on two different surfaces. It is also shown that the accuracy of the far-zone transformation can be further improved if the phase is compensated with respect to a second-order dispersion corrected wavenumber. For validation, scattering results for an empty volume, a circular disk, and a sphere are compared with analytical solutions.

Compressive Sensing Techniques for mm-Wave Nondestructive Testing of Composite Panels

This paper presents imaging results from measurements of an industrially manufactured composite test panel, utilizing two introduced algorithms for data postprocessing. The system employs a planar near-field scanning setup for characterizing defects in composite panels in the 50–67-GHz band, and can be considered as a complementary diagnostic tool for nondestructive testing purposes. The introduced algorithms are based on the reconstruction of the illuminating source at the transmitter, enabling a separation of the sampled signal with respect to the location of its potential sources, the scatterers within the device under test or the transmitter. For the second algorithm, an

Validation method for numerical simulations of large objects using the T-matrix for a collection of scatterers

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A novel method to increase the accuracy of material characterization using free space transmission measurements

-minimization problem formulation is introduced that enables compressive sensing techniques to be adapted for image retrieval. The algorithms are benchmarked against a more conventional imaging technique, based on the Fourier transform, and it is seen that the complete imaging system provides increased dynamic range, improved resolution, and reduced measurement time by removal of a reference measurement. Moreover, the system provides stable image quality over a range of frequencies.

T-matrix simulations of a collection of scatterers for validation of numerical electromagnetic solvers

This paper presents a validation method for large-scale electromagnetic scattering simulation problems, especially such performed using the Finite-Difference Time-Domain (FDTD) method. Large computational volumes imply that the results suffer from numerical dispersion. In order to validate the simulation results for large geometries a T-matrix solution for a collection of spheres can be used. In this paper the solution has been restriced to two spheres separated along the z-axis. Monostatic and bistatic scattering results using FDTD and the T-matrix method are compared. It is concluded that the grid resolution in FDTD must be adjusted in order to maintain the accuracy if the size of the computational volume increases substantially.

60 GHz imaging of panels for defect detection using planar scanning

This paper describes a method to accurately determine the electromagnetic material properties of thin dielectric sheets. For non-magnetic materials, free space transmission measurements relative to an empty setup are sufficient to determine the complex permittivity of the sheet material. Measurements have been performed on a thin resistive sheet and a transformation procedure has been applied where the edge distortion at the lower frequency band is reduced. This is accomplished by shifting the band limited frequency spectrum down to DC before applying an inverse Fourier for gating purpose. The procedure yields accurate material characterization over the entire measured frequency band.

Scattering from a multilayered sphere - Applications to electromagnetic absorbers on double curved surfaces

This paper presents the use of the T-matrix method for a collection of scatterers for validation purposes of large-scale numerical electromagnetic scattering problems. In order to validate the simulation results for large geometries the T-matrix solution for a collection of spheres and disks can be used. Scattering results using the T-matrix method, FDTD and MoM are compared. The results show that the T-matrix method for a small collection of scattering objects gives accurate reference solutions for large-scale scattering problems.

Scattering for doubly curved functional surfaces and corresponding planar designs

This paper presents imaging results of measurements conducted at 60 GHz, using the planar rectangular near-field technique. Utilizing the stated techniques at higher frequencies enables detection of smaller defects, and allows for a small measurement set-up in a laboratory environment. An algorithm based on the Fast Fourier Transform (FFT) has been developed in order to process the data. The paper provides measurement results for an illustrative panel, which show clear detection of the distributed defects. The purpose of this work is to enable processing of recent data from measurements of industrial composite panels.