Statistical measurements of fast changing electromagnetic fields
The present works aims at describing important statistical indexes such as the field uniformity, the field inhomogeneity and the statistics near the cavity walls for a special case of fast changing random electromagnetic
fields. We generate this kind of electromagnetic environment by means of a vibrating intrinsic reverberation chamber. The use of several, fast, in-house, 3-axis field sensors allows for extensive, "real-time" measurements inside such a chamber.
Reverberation Chamber Characterization Using Enhanced Backscatter Coefficient Measurements
The primary focus of this thesis is to evaluate the use of the enhanced backscatter coefficient as a quantity for characterizing the spatial uniformity of the reverberation chamber (RC). An RC is a statistical measurement environment constructed from a large metallic hallow cavity containing an irregularly shaped paddle stirrer for mixing electromagnetic fields to obtain many statistically independent samples. The average power, measured from antennas placed inside of the RC, is ideally uniform over any chosen antenna position and orientation within the RCs working volume. Spatial uniformity is a fundamental assumption to all RC theoretical analysis, and thus the spatial dependence of the RC is directly related to the uncertainty of the measurements. Comparisons are made with standard multiple-configuration measurements (different antenna positions) to show that comparable values of standard deviation caused by spatial non-uniformity are obtained using the transfer function (as typically used) and the enhanced backscatter coefficient. Additionally, it is shown that the enhanced backscatter coefficient for an ideal RC is theoretically a constant value of 2, but has variation over frequency when measured. This variation over frequency is used in a single-configuration measurement to obtain values of standard deviation that are nearly the same as those found using multiple-configuration measurements. This is possible because the statistical variation of the reverberation chamber is similar over frequency and over different measurement positions. Furthermore, data is presented from various tests showing that the value of the enhanced backscatter coefficient is sensitive to calibration issues, and the improper use of frequency stirring. This helps to justify that the enhanced backscatter coefficient can be used as a benchmark quantity for determining if computations from the RC measured data are useful and can be trusted. One-and two-dimensional Greens function models are also used to explore the enhanced backscatter coefficient value for different types of stirring mechanisms. Lastly, application of the enhanced backscatter coefficient for determining the total efficiency of an antenna measured in the reverberation chamber is presented. Along with the estimates of total efficiency, the confidence interval of the results is computed from the frequency variation of the enhanced backscatter coefficient.
NMR characteristics of intracellular K in the rat salivary gland: a 39K NMR study using double-quantum filtering.
Intracellular K of the perfused rat mandibular salivary gland was measured by 39K NMR spectroscopy at 8.45 T. Multiple-quantum NMR arising from multiple-exponential decay was used to eliminate the resonance due to extracellular K in the perfused gland at 25 degrees C. The resonance due to intracellular K consisted of two Lorentzian signals stemming from the [spin 1/2 to -1/2] coherence (sharp resonance) and the [spin -1/2 to -3/2], [spin 3/2 to 1/2] coherences (broad resonance). The transverse relaxation time (T2) corresponding to the [spin 1/2 to -1/2] coherence was ca. 2.5 ms, and that corresponding to the [spin -1/2 to -3/2], [spin 3/2 to 1/2] coherences was ca. 0.4 ms. The relaxation time of the double-quantum coherence of rank 3 (originating from product operators like Ix2Iz) was determined to be ca. 0.2 ms. These results suggest the possibility of the presence of a single homogeneous population of intracellular K with a correlation time of ca. 2.5 x 10(-8) s and a quadrupolar coupling constant of ca. 1.4 MHz.