126 research outputs found
Probability Distributions of Random Electromagnetic Fields in the Presence of a Semi-Infinite Isotropic Medium
Using a TE/TM decomposition for an angular plane-wave spectrum of free random
electromagnetic waves and matched boundary conditions, we derive the
probability density function for the energy density of the vector electric
field in the presence of a semi-infinite isotropic medium. The theoretical
analysis is illustrated with calculations and results for good electric
conductors and for a lossless dielectric half-space. The influence of the
permittivity and conductivity on the intensity, random polarization,
statistical distribution and standard deviation of the field is investigated,
both for incident plus reflected fields and for refracted fields. External
refraction is found to result in compression of the fluctuations of the random
field.Comment: 23 pages, 11 figures, accepted for publication in Radio Scienc
Probability Distribution of the Quality Factor of a Mode-Stirred Reverberation Chamber
We derive a probability distribution, confidence intervals and statistics of
the quality (Q) factor of an arbitrarily shaped mode-stirred reverberation
chamber, based on ensemble distributions of the idealized random cavity field
with assumed perfect stir efficiency. It is shown that Q exhibits a
Fisher-Snedecor F-distribution whose degrees of freedom are governed by the
number of simultaneously excited cavity modes per stir state. The most probable
value of Q is between a fraction 2/9 and 1 of its mean value, and between a
fraction 4/9 and 1 of its asymptotic (composite Q) value. The arithmetic mean
value is found to always exceed the values of all other theoretical metrics for
centrality of Q. For a rectangular cavity, we retrieve the known asymptotic Q
in the limit of highly overmoded regime.Comment: accepted for publication in IEEE Trans. Electromagn. Compat., 201
Nonstationary random acoustic and electromagnetic fields as wave diffusion processes
We investigate the effects of relatively rapid variations of the boundaries
of an overmoded cavity on the stochastic properties of its interior acoustic or
electromagnetic field. For quasi-static variations, this field can be
represented as an ideal incoherent and statistically homogeneous isotropic
random scalar or vector field, respectively. A physical model is constructed
showing that the field dynamics can be characterized as a generalized diffusion
process. The Langevin--It\^{o} and Fokker--Planck equations are derived and
their associated statistics and distributions for the complex analytic field,
its magnitude and energy density are computed. The energy diffusion parameter
is found to be proportional to the square of the ratio of the standard
deviation of the source field to the characteristic time constant of the
dynamic process, but is independent of the initial energy density, to first
order. The energy drift vanishes in the asymptotic limit. The time-energy
probability distribution is in general not separable, as a result of
nonstationarity. A general solution of the Fokker--Planck equation is obtained
in integral form, together with explicit closed-form solutions for several
asymptotic cases. The findings extend known results on statistics and
distributions of quasi-stationary ideal random fields (pure diffusions), which
are retrieved as special cases.Comment: 54 pages, 8 figures, to appear in J. Phys. A: Math. Theo
Photochemistry and photophysics of thienocarbazoles
Two methylated thienocarbazoles and two of their synthetic nitro-precursors have been examined by absorption, luminescence, laser flash photolysis and photoacoustic techniques. Their spectroscopic and photophysical characterization involves fluorescence spectra, fluorescence quantum yields and lifetimes, and phosphorescence spectra and phosphorescence lifetimes for all the compounds. Triplet-singlet difference absorption spectra, triplet molar absorption coefficients, triplet lifetimes, intersystem crossing S-1 similar tosimilar to--> T-1 and singlet molecular oxygen yields were obtained for the thienocarbazoles. In the case of the thienocarbazoles it was found that the lowest-lying singlet and triplet excited states, S, and T-1, are of pi,pi* origin, whereas for their precursors S-1 is n,pi*, and T-1 is pi,pi*. In both thienocarbazoles it appears that the thianaphthene ring dictates the S, T, yield, albeit there is less predominance of that ring in the triplet state of the linear thienocarbazole, which leads to a decrease in the observed Phi(T) value.info:eu-repo/semantics/publishedVersio
Postcolonial manifestations of African spatiality in Europe : the invisible 'public' spaces of Ghent
The focus of this chapter is on everyday spaces of African migration in the mid-sized city of Ghent. One manifestation of African spatiality is discussed in-depth in relation to its (in)visibility and publicity: an African shop located in an ordinary terraced house. With no less than 12 activities taking place in the building, the shop is rather a âpublicâ place than solely a space of commercial transactions, although this is not signaled in very visible ways. By analyzing the modest stylistic appropriations of the façade and the significant re-arrangements of the buildingsâ interior spaces that challenge more conventional usages of spaces in Ghentâs ordinary houses, this chapter puts this African shop to the fore as emblematic of how the process of materialization of transnational lifestyles and connections is always a balancing act between the visibility necessary for functioning as a (semi-)pubic place and the invisibility required to circumvent hegemonic regulatory regimes
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