477 research outputs found
A high gain antenna system for airborne satellite communication applications
A high gain antenna for commercial aviation satellites communication is discussed. Electromagnetic and practical design considerations as well as candidate systems implementation are presented. An evaluation of these implementation schemes is given, resulting in the selection of a simple top mounted aerodynamic phased array antenna with a remotely located beam steering unit. This concept has been developed into a popular product known as the Canadian Marconi Company CMA-2100. A description of the technical details is followed by a summary of results from the first production antennas
Entropy production for coarse-grained dynamics
Systems out of equilibrium exhibit a net production of entropy. We study the
dynamics of a stochastic system represented by a Master Equation that can be
modeled by a Fokker-Planck equation in a coarse-grained, mesoscopic
description. We show that the corresponding coarse-grained entropy production
contains information on microscopic currents that are not captured by the
Fokker-Planck equation and thus cannot be deduced from it. We study a
discrete-state and a continuous-state system, deriving in both the cases an
analytical expression for the coarse-graining corrections to the entropy
production. This result elucidates the limits in which there is no loss of
information in passing from a Master Equation to a Fokker-Planck equation
describing the same system. Our results are amenable of experimental
verification, which could help to infer some information about the underlying
microscopic processes
Entropy production in systems with unidirectional transitions
The entropy production is one of the most essential features for systems
operating out of equilibrium. The formulation for discrete-state systems goes
back to the celebrated Schnakenberg's work and hitherto can be carried out when
for each transition between two states also the reverse one is allowed.
Nevertheless, several physical systems may exhibit a mixture of both
unidirectional and bidirectional transitions, and how to properly define the
entropy production in this case is still an open question. Here, we present a
solution to such a challenging problem. The average entropy production can be
consistently defined, employing a mapping that preserves the average fluxes,
and its physical interpretation is provided. We describe a class of stochastic
systems composed of unidirectional links forming cycles and detailed-balanced
bidirectional links, showing that they behave in a pseudo-deterministic
fashion. This approach is applied to a system with time-dependent stochastic
resetting. Our framework is consistent with thermodynamics and leads to some
intriguing observations on the relation between the arrow of time and the
average entropy production for resetting events.Comment: (Accepted for publication in Physical Review Research
Flory theory for Polymers
We review various simple analytical theories for homopolymers within a
unified framework. The common guideline of our approach is the Flory theory,
and its various avatars, with the attempt of being reasonably self-contained.
We expect this review to be useful as an introduction to the topic at the
graduate students level.Comment: Topical review appeared J. Phys.: Condens. Matter, 46 pages, 8
Figures. Sec. VIF added. Typos fixed. Few references adde
National indeterminacies at the periphery of the Habsburg Monarchy: Nationalisms versus multi-ethnic identities in Fiume/Rijeka and Trieste, 1848-1867
In the 1848–1867 period, the Habsburg Monarchy was shaken by the first waves of nationalism. Yet in the case of the Habsburg port cities of Fiume/Rijeka and Trieste, contended by several different opponents, Italian and Croatian nationalisms had to face centuries‐long traditions of municipal autonomy. In both cities, municipalism and attachment to the House of Habsburg were particularly strong and were coupled with local urban identities that defied national forms of identifications, insofar as they were ethnically and linguistically hybrid. Nationalist activists sought to exploit ethnic and linguistic elements as markers of defined national identities, yet without widespread success. The final demise of the Habsburg Monarchy in 1918 has been generally taken as proof of the cogency of nationalist discourse, especially the Italian, in the region. However, the northern Adriatic rim points to the forcefulness of Habsburg multinationalism and the existence of ethnic hybridity, which provided effective bulwarks against nationalisms for decades
Heteropolymers in a Solvent at an Interface
Exact bounds are obtained for the quenched free energy of a polymer with
random hydrophobicities in the presence of an interface separating a polar from
a non polar solvent. The polymer may be ideal or have steric self-interactions.
The bounds allow to prove that a ``neutral'' random polymer is localized near
the interface at any temperature, whereas a ``non-neutral'' chain is shown to
undergo a delocalization transition at a finite temperature. These results are
valid for a quite general a priori probability distribution for both
independent and correlated hydrophobic charges. As a particular case we
consider random AB-copolymers and confirm recent numerical studies.Comment: 4 pages, no figure
Species lifetime distribution for simple models of ecologies
Interpretation of empirical results based on a taxa's lifetime distribution
shows apparently conflicting results. Species' lifetime is reported to be
exponentially distributed, whereas higher order taxa, such as families or
genera, follow a broader distribution, compatible with power law decay. We show
that both these evidences are consistent with a simple evolutionary model that
does not require specific assumptions on species interaction. The model
provides a zero-order description of the dynamics of ecological communities and
its species lifetime distribution can be computed exactly. Different behaviors
are found: an initial power law, emerging from a random walk type of
dynamics, which crosses over to a steeper branching process-like
regime and finally is cutoff by an exponential decay which becomes weaker and
weaker as the total population increases. Sampling effects can also be taken
into account and shown to be relevant: if species in the fossil record were
sampled according to the Fisher log-series distribution, lifetime should be
distributed according to a power law. Such variability of behaviors in
a simple model, combined with the scarcity of data available, cast serious
doubts on the possibility to validate theories of evolution on the basis of
species lifetime data.Comment: 19 pages, 2 figure
Hidden scaling patterns and universality in written communication
The temporal statistics exhibited by written correspondence appear to be
media dependent, with features which have so far proven difficult to
characterize. We explain the origin of these difficulties by disentangling the
role of spontaneous activity from decision-based prioritizing processes in
human dynamics, clocking all waiting times through each agent's `proper time'
measured by activity. This unveils the same fundamental patterns in written
communication across all media (letters, email, sms), with response times
displaying truncated power-law behavior and average exponents near -3/2. When
standard time is used, the response time probabilities are theoretically
predicted to exhibit a bi-modal character, which is empirically borne out by
our new years-long data on email. These novel perspectives on the temporal
dynamics of human correspondence should aid in the analysis of interaction
phenomena in general, including resource management, optimal pricing and
routing, information sharing, emergency handling.Comment: 27 pages, 10 figure
Correlation functions by Cluster Variation Method for Ising model with NN, NNN and Plaquette interactions
We consider the procedure for calculating the pair correlation function in
the context of the Cluster Variation Methods. As specific cases, we study the
pair correlation function in the paramagnetic phase of the Ising model with
nearest neighbors, next to the nearest neighbors and plaquette interactions in
two and three dimensions. In presence of competing interactions, the so called
disorder line separates in the paramagnetic phase a region where the
correlation function has the usual exponential behavior from a region where the
correlation has an oscillating exponentially damped behavior. In two
dimensions, using the plaquette as the maximal cluster of the CVM
approximation, we calculate the phase diagram and the disorder line for a case
where a comparison is possible with results known in literature for the
eight-vertex model. In three dimensions, in the CVM cube approximation, we
calculate the phase diagram and the disorder line in some cases of particular
interest. The relevance of our results for experimental systems like mixtures
of oil, water and surfactant is also discussed.Comment: 31 pages, LaTeX file, 7 figure
Oscillatory Behavior of Critical Amplitudes of the Gaussian Model on a Hierarchical Structure
We studied oscillatory behavior of critical amplitudes for the Gaussian model
on a hierarchical structure presented by a modified Sierpinski gasket lattice.
This model is known to display non-standard critical behavior on the lattice
under study. The leading singular behavior of the correlation length near
the critical coupling is modulated by a function which is periodic in
. We have also shown that the common finite-size scaling
hypothesis, according to which for a finite system at criticality should
be of the order of the size of system, is not applicable in this case. As a
consequence of this, the exact form of the leading singular behavior of
differs from the one described earlier (which was based on the finite-size
scaling assumption).Comment: 9 pages (REVTEX), 2 figures (EPS), Phys. Rev. E (accepted
- …