Planar sensors for dielectric and magnetic materials measurement: A quantitative sensitivity comparison

Planar transmission lines are frequently used to characterize the RF properties of materials. However, the question arises which geometry should be chosen for optimal measurement sensitivity to the material under test. Thus far, this question appears to go unanswered. In this paper, the suitability of the three most popular planar geometries is compared for material characterization. To this end, the impact of a material under test on the apparent properties (i.e. the equivalent homogeneous cross-sections) is examined. This is done for the complex permittivity and the complex permeability, using conformal mapping methods, full-wave simulations and measurements. It is shown that the coplanar waveguide (without conductor backing) is the most suitable structure of the three, since it is the most sensitive to changes in the properties of the material under test

Effect of finite precision on em simulations for high-contrast biological media at low frequencies

At low frequencies, biological media are characterized by extremely high permittivities. As a result, the most commonly used simulation methods, i.e. finite-difference time domain (FDTD), finite element method (FEM), and domain integral equations (DIE), suffer from severe limitations in accuracy. These limitations are caused by the round-off errors in finite-precision floating point operations. Finite precision causes error accumulation in FDTD due to the large number of time steps required to simulate one period and to maintain stability. In FEM, finite precision causes the numerical derivative to collapse due to the dependence on the mesh size. While the DIE is hardly influenced by the mesh size, the extreme permittivities cause a large difference in the order of magnitude of the various terms in the DIE

Non-invasive brain stimulation : from field modeling to neuronal activation

Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) are the most commonly studied non-invasive brain stimulation treatment options. Over the past years, modeling and simulation of stimulation-induced electric fields have received increased attention. Modeling can take place at three different levels of abstraction. Although some validation of these models has taken place at these levels separately, coupling between the levels through a multi-scale approach and experimental validation of the overall approach has only recently started. This specific coupling might be an important step to unravel the mechanism of action and to ultimately improve the clinical efficacy of non-invasive brain stimulation