150 research outputs found
Radome design for hat-fed reflector antenna
We present the design of a low loss homogenous radome to house a hat-fed reflector antenna for the Ku satellite communication band. The radome is made of polycarbonate and can easily be vacuum formed in series. Simulated and measured data show that the radome has a low reflection coefficient and low ohmic loss, as well as very little effect on the antenna radiation patterns over the required frequency band
Design and optimization of compact wideband hat-fed reflector antenna for satellite communications
We present a new design of the hat-fed reflector antenna for satellite communications, where a low reflection coefficient, high gain, low sidelobes and low cross-polar level are required over a wide frequency band. The hat feed has been optimized by using the Genetic Algorithm through a commercial FDTD solver, QuickWave-V2D, together with an own developed optimization code. The Gaussian vertex plate has been applied at the center of the reflector in order to improve the reflection coefficient and reduce the far-out sidelobes. A parabolic reflector with a ring-shaped focus has been designed for obtaining nearly 100% phase efficiency. The antenna's reflection coefficient is below -17 dB and the radiation patterns satisfy the M-x standard co- and cross-polar sidelobe envelopes for satellite ground stations over a bandwidth of 30%. A low-cost monolayer radome has been designed for the antenna with satisfactory performance. The simulations have been verified by measurements; both of them are presented in the paper
Interplay of the volume and surface plasmons in the electron energy loss spectra of C
The results of a joint experimental and theoretical investigation of the C60
collective excitations in the process of inelastic scattering of electrons are
presented. The shape of the electron energy loss spectrum is observed to vary
when the scattering angle increases. This variation arising due to the electron
diffraction of the fullerene shell is described by a new theoretical model
which treats the fullerene as a spherical shell of a finite width and accounts
for the two modes of the surface plasmon and for the volume plasmon as well. It
is shown that at small angles, the inelastic scattering cross section is
determined mostly by the symmetric mode of the surface plasmon, while at larger
angles, the contributions of the antisymmetric surface plasmon and the volume
plasmon become prominent.Comment: 11 pages, 3 figure
Hybridization-related correction to the jellium model for fullerenes
We introduce a new type of correction for a more accurate description of
fullerenes within the spherically symmetric jellium model. This correction
represents a pseudopotential which originates from the comparison between an
accurate ab initio calculation and the jellium model calculation. It is shown
that such a correction to the jellium model allows one to account, at least
partly, for the sp2-hybridization of carbon atomic orbitals. Therefore, it may
be considered as a more physically meaningful correction as compared with a
structureless square-well pseudopotential which has been widely used earlier.Comment: 16 pages, 10 figure
Vacancy decay in endohedral atoms: the role of non-central position of the atom
We demonstrate that the Auger decay rate in an endohedral atom is very
sensitive to the atom's location in the fullerene cage. Two additional decay
channels appear in an endohedral system: (a) the channel due to the change in
the electric field at the atom caused by dynamic polarization of the fullerene
electron shell by the Coulomb field of the vacancy, (b) the channel within
which the released energy is transferred to the fullerene electron via the
Coulomb interaction. % The relative magnitudes of the correction terms are
dependent not only on the position of the doped atom but also on the transition
energy \om. Additional enhancement of the decay rate appears for transitions
whose energies are in the vicinity of the fullerene surface plasmons energies
of high multipolarity. % It is demonstrated that in many cases the additional
channels can dominate over the direct Auger decay resulting in pronounced
broadening of the atomic emission lines. % The case study, carried out for
Sc@C, shows that narrow autoionizing resonances in an
isolated Sc within the range \om = 30... 45 eV are dramatically
broadened if the ion is located strongly off-the-center. % Using the developed
model we carry out quantitative analysis of the photoionization spectrum for
the endohedral complex ScN@C and demonstrate that the additional
channels are partly responsible for the strong modification of the
photoionization spectrum profile detected experimentally by
M\"{u}ller et al. (J. Phys.: Conf. Ser. 88, 012038 (2008)).Comment: 32 pages, 11 figure
Gene Sequence Based Clustering Assists in Dereplication of Pseudoalteromonas luteoviolacea Strains with Identical Inhibitory Activity and Antibiotic Production
Some microbial species are chemically homogenous, and the same secondary metabolites are found in all strains. In contrast, we previously found that five strains of P. luteoviolacea were closely related by 16S rRNA gene sequence but produced two different antibiotic profiles. The purpose of the present study was to determine whether such bioactivity differences could be linked to genotypes allowing methods from phylogenetic analysis to aid in selection of strains for biodiscovery. Thirteen P. luteoviolacea strains divided into three chemotypes based on production of known antibiotics and four antibacterial profiles based on inhibition assays against Vibrio anguillarum and Staphylococcus aureus. To determine whether chemotype and inhibition profile are reflected by phylogenetic clustering we sequenced 16S rRNA, gyrB and recA genes. Clustering based on 16S rRNA gene sequences alone showed little correlation to chemotypes and inhibition profiles, while clustering based on concatenated 16S rRNA, gyrB, and recA gene sequences resulted in three clusters, two of which uniformly consisted of strains of identical chemotype and inhibition profile. A major time sink in natural products discovery is the effort spent rediscovering known compounds, and this study indicates that phylogeny clustering of bioactive species has the potential to be a useful dereplication tool in biodiscovery efforts
Reservoir-Excess Pressure Parameters Independently Predicts Cardiovascular Events in Individuals With Type 2 Diabetes.
The parameters derived from reservoir-excess pressure analysis have prognostic utility in several populations. However, evidence in type 2 diabetes (T2DM) remains scarce. We determined if these parameters were associated with T2DM and whether they would predict cardiovascular events in individuals with T2DM. We studied 306 people with T2DM with cardiovascular disease (CVD; DMCVD, 70.4±7.8 years), 348 people with T2DM but without CVD (diabetes mellitus, 67.7±8.4 years), and 178 people without T2DM or CVD (control group [CTRL], 67.2±8.9 years). Reservoir-excess pressure analysis-derived parameters, including reservoir pressure integral, peak reservoir pressure, excess pressure integral, systolic rate constant, and diastolic rate constant, were obtained by radial artery tonometry. Reservoir pressure integral was lower in DMCVD diabetes mellitus and than CTRL. Peak reservoir pressure was lower, and excess pressure integral was greater in DMCVD diabetes mellitus than and CTRL. Systolic rate constant was lower in a stepwise manner among groups (DMCVD< diabetes mellitus <CTRL). Diastolic rate constant was greater in DMCVD than CTRL. In the subgroup of individuals with T2DM (n=642), 14 deaths (6 cardiovascular and 9 noncardiovascular causes), and 108 cardiovascular events occurred during a 3-year follow-up period. Logistic regression analysis revealed that reservoir pressure integral (odds ratio, 0.59 [95% CI, 0.45-0.79]) and diastolic rate constant (odds ratio, 1.60 [95% CI, 1.25-2.06]) were independent predictors of cardiovascular events during follow-up after adjusting for conventional risk factors (both P<0.001). Further adjustments for potential confounders had no influence on associations. These findings demonstrate that altered reservoir-excess pressure analysis-derived parameters are associated with T2DM. Furthermore, baseline values of reservoir pressure integral and diastolic rate constant independently predict cardiovascular events in individuals with T2DM, indicating the potential clinical utility of these parameters for risk stratification in T2DM
Nucleation of a sodium droplet on C60
We investigate theoretically the progressive coating of C60 by several sodium
atoms. Density functional calculations using a nonlocal functional are
performed for NaC60 and Na2C60 in various configurations. These data are used
to construct an empirical atomistic model in order to treat larger sizes in a
statistical and dynamical context. Fluctuating charges are incorporated to
account for charge transfer between sodium and carbon atoms. By performing
systematic global optimization in the size range 1<=n<=30, we find that Na_nC60
is homogeneously coated at small sizes, and that a growing droplet is formed
above n=>8. The separate effects of single ionization and thermalization are
also considered, as well as the changes due to a strong external electric
field. The present results are discussed in the light of various experimental
data.Comment: 17 pages, 10 figure
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