COCHANNEL INTERFERENCE IN SATELLITE COMMUNICATION SYSTEMS DERIVED FROM RAIN ATTENUATION MEASUREMENTS AT 20 GHz

This paper reports experimental results on cochannel interference during rain, derived from measurements of simultaneous and copolar rain attenuations, in the 20 GHz band, in two convergent slant paths to fixed receivers at Spino d'Adda (Northern Italy), from the geostationary satellites Olympus (19°W) and Italsat (13°E). The results show that during rain the interference can be larger than that in clear sky, with probability 0.5 and that, in this case, the interfering level I = A_C − A_n (dB), i.e. the difference between the simultaneous rain attenuations (dB) in the interfered path, A_C (dB), and in the interfering path, A_n (dB), increases as the system power margin increases. For system design, the paper reports the cumulative distributions of I, conditioned to the maximum value of the attenuation not exceeded (i.e. system power margin) in the interfered link. It is shown how the results could be used in systems with regenerative or transparent transponders on board.     

VISCOELASTIC PROPERTIES OF ALGINATE GELS BY OSCILLATORY DYNAMIC TESTS

The response of several mannuronic and guluronic alginate gels to oscillatory tests was studied by varying the alginate concentration (c) from 1.0 to 1.75% w/w. Frequency dependence of the complex shear modulus G* discriminated between the alginate gels in terms of their different orders of the relaxation function α and "gel stiffness" A_α values using Friedrich and Heymann theory (1988). Guluronic alginate gels were approximately 4-5 times stronger than mannuronic ones, their networks exhibiting higher or smaller rates of stress relaxation, respectively. For both alginate types the gel stiffness parameter A_α was found to be a power function of alginate concentration and type (intrinsic viscosity [η], GG block diad frequency, F_GG, and average G-block length, N_G), its sensitivity with respect to c and N_G being greater than that to [η] and F_GG.     

Mechanical and histological properties of an electrospun scaffold with a modified surface by plasma polymerization implanted in an <i>in vivo</i> model

This article presents the construction of scaffolds composed of polylactic acid (PLA) with different concentrations of hydroxyapatite (HA) by electrospinning, which were superficially modified with polypyrrole (PPy/I) by plasma polymerization. A preliminary study was conducted of the biological and mechanical behavior of the scaffolds when they were implanted in the back of rabbits for 30 days; bone cells differentiated from mesenchymal stem cells (MSCs) were used. The bone cell and scaffold structures were characterized by histological, immunohistochemical, and mechanical stress tests. Hematoxylin–eosin staining showed good tissue conformation. The immunohistochemical tests highlighted the presence of the main bone tissue proteins, such as collagen, osteocalcin, and osteopontin. The PLA/HA scaffolds were observed to exhibit cell adhesion and proliferation properties; however, the response was much better in the scaffolds that had a higher concentration of HA and that were coated with PPy/I. The results of the mechanical tests of the scaffolds indicated that the plasma treatment improved the adhesion and cell proliferation properties and contributed to the mechanical support, allowing the formation of neotissues with good viability of cell growth.     

MULTILAYER ADSORPTION ON FRACTAL SURFACES: EFFECT OF THE ADSORBATE-ADSORBATE INTERACTIONS ON THE SORPTIONAL EQUILIBRIUM

ABSTRACT

An analytical isotherm equation that can be applied to water vapor-biopolymer systems, was developed. The model describes the multilayer adsorption on fractal surfaces with energetic interactions adsorbate-adsorbate (measure in terms of free energy) different from that of bulk water. Assuming explicit mathematical functionalities for the variation of the free energy with the number of adsorbed layers, it is possible to evaluate the effect of the adsorbate-adsorbate interactions on the adsorption capacity of solids of high degree of surface irregularity. For those surfaces with relatively low degree of irregularity (D values slightly larger than 2) it results that the free energy variation with the layer number in the multilayer region affects considerably the sorption capacity of the adsorbent, even for water activities lower than those corresponding to the monolayer moisture content. This effect becomes less marked as the fractal dimension increases (D approaching to 3), being relevant for water activities much larger than those corresponding to the monolayer value, only. The isotherm was tested, using published experimental equilibrium data of various biopolymers.     

Stability of Permanent Magnet Bearings under Parametric Excitations

Permanent magnet pairs offer negligible friction and no wear, but also static instability. Stability may be recovered if a suitable periodic axial motion is imposed on the levitated element of the pair.     

On efficiency of both single fuel cells and stacks II

The model developed in the first part of this work is used to predict the cell potentials and the irreversible Gibbs free energy of a stack of 15 cells. The model starts from a phenomenological equation of a polarization curve with the extent of reaction as the independent variable. Two extreme kinds of flow of reagents, defined as Chain and Separate Flows respectively, are considered. The cell potentials are obtained by a combination of the potential of the two extreme cases of flow. The stack cell potentials and the efficiencies, estimated by the model, reproduce the general characteristics obtained by the experiments.