37,376 research outputs found
Reflector antennas with low sidelobes, low cross polarization, and high aperture efficiency
Techniques are presented for computing the horn near field patterns on the subreflectors and for correcting the phase center errors of the horn pattern by shaping the subreflector surface. The diffraction pattern computations for scanned beams are described. The effects of dish aperture diffraction on pattern bandwidth are investigated. A model antenna consisting of a reflector, shaped subreflector, and corrugated feed horn is described
Nonasymptotic Homogenization of Periodic Electromagnetic Structures: Uncertainty Principles
We show that artificial magnetism of periodic dielectric or metal/dielectric
structures has limitations and is subject to at least two "uncertainty
principles". First, the stronger the magnetic response (the deviation of the
effective permeability tensor from identity), the less accurate ("certain") the
predictions of any homogeneous model. Second, if the magnetic response is
strong, then homogenization cannot accurately reproduce the transmission and
reflection parameters and, simultaneously, power dissipation in the material.
These principles are general and not confined to any particular method of
homogenization. Our theoretical analysis is supplemented with a numerical
example: a hexahedral lattice of cylindrical air holes in a dielectric host.
Even though this case is highly isotropic, which might be thought as conducive
to homogenization, the uncertainty principles remain valid.Comment: 11 pages, 5 figure
Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies
Tori of Active Galactic Nuclei are made up of a mixture of hot and cold gas,
as well as dust. In order to protect the dust grains from destruction by the
hot gas as well as by the energetic radiation of the accretion disk, the dust
is often assumed to be distributed in clouds. In our new 3D model of AGN dust
tori, the torus is modelled as a wedge-shaped disk in which dusty clouds are
randomly distributed, by taking the dust density distribution of the
corresponding continuous model into account. We especially concentrate on the
differences between clumpy and continuous models in terms of the temperature
distributions, the surface brightness distributions and interferometric
visibilities, as well as spectral energy distributions. To this end, we employ
radiative transfer calculations with the help of the 3D Monte Carlo code MC3D.
In a second step, interferometric visibilities are calculated from the
simulated surface brightness distributions, which can be directly compared to
observations with the MIDI instrument. The radial temperature distributions of
clumpy models possess significantly enhanced scatter compared to the continuous
cases. Even at large distances, clouds can be heated directly by the central
accretion disk. The existence of the silicate 10 micron-feature in absorption
or in emission depends sensitively on the distribution, the size and optical
depth of clouds in the innermost part of the torus, due to shadowing effects of
clouds there. This explains failure and success of previous modelling efforts
of clumpy tori. After adapting the parameters of our clumpy standard model to
the circumstances of the Seyfert 2 Circinus galaxy, it can qualitatively
explain recent mid-infrared interferometric observations performed with MIDI,
as well as high resolution spectral data.Comment: 15 pages, 23 figures, accepted by A&
Digital computer analysis and design of a subreflector of complex shape
Digital computer technique for computing scattered pattern of complex hyperboloid subreflector in Cassegrain antenna feed system
Multi-Agent Orbit Design For Perception Enhancement Purpose
This paper develops a robust optimization based method to design orbits on
which the sensory perception of the desired physical quantities are maximized.
It also demonstrates how to incorporate various constraints imposed by many
spacecraft missions such as collision avoidance, co-orbital configuration,
altitude and frozen orbit constraints along with Sun-Synchronous orbit. The
paper specifically investigates designing orbits for constrained visual sensor
planning applications as the case study. For this purpose, the key elements to
form an image in such vision systems are considered and effective factors are
taken into account to define a metric for perception quality. The simulation
results confirm the effectiveness of the proposed method for several scenarios
on low and medium Earth orbits as well as a challenging Space-Based Space
Surveillance program application.Comment: 12 pages, 18 figure
Exterior optical cloaking and illusions by using active sources: a boundary element perspective
Recently, it was demonstrated that active sources can be used to cloak any
objects that lie outside the cloaking devices [Phys. Rev. Lett. \textbf{103},
073901 (2009)]. Here, we propose that active sources can create illusion
effects, so that an object outside the cloaking device can be made to look like
another object. invisibility is a special case in which the concealed object is
transformed to a volume of air. From a boundary element perspective, we show
that active sources can create a nearly "silent" domain which can conceal any
objects inside and at the same time make the whole system look like an illusion
of our choice outside a virtual boundary. The boundary element method gives the
fields and field gradients (which can be related to monopoles and dipoles) on
continuous curves which define the boundary of the active devices. Both the
cloaking and illusion effects are confirmed by numerical simulations
A new approach for shaping of dual-reflector antennas
The shaping of 2-D dual-reflector antenna systems to generate a prescribed distribution with uniform phase at the aperture of the second reflector is examined. This method is based on the geometrical nature of Cassegrain and Gregorian dual-reflector antennas. The method of syntheses satisfies the principles of geometrical optics which are the foundations of dual-reflector designs. Instead of setting up differential equations or heuristically designing the subreflector, a set of algebraic equations is formulated and solved numerically to obtain the desired surfaces. The caustics of the reflected rays from the subreflector can be obtained and examined. Several examples of 2-D dual-reflector shaping are shown to validate the study. Geometrical optics and physical optics are used to calculate the scattered fields from the reflectors
Optically switched magnetism in photovoltaic perovskite CHNH(Mn:Pb)I
The demand for ever-increasing density of information storage and speed of
manipulation boosts an intense search for new magnetic materials and novel ways
of controlling the magnetic bit. Here, we report the synthesis of a
ferromagnetic photovoltaic CHNH(Mn:Pb)I material in which the
photo-excited electrons rapidly melt the local magnetic order through the
Ruderman-Kittel-Kasuya-Yosida interactions without heating up the spin system.
Our finding offers an alternative, very simple and efficient way of optical
spin control, and opens an avenue for applications in low power, light
controlling magnetic devices
X-ray iron line variability for the model of an orbiting flare above a black hole accretion disc
The broad X-ray iron line, detected in many active galactic nuclei, is likely
to be produced by fluorescence from the X-ray illuminated central parts of an
accretion disc close to a supermassive black hole. The time-averaged shape of
the line can be explained most naturally by a combination of special and
general relativistic effects. Such line profiles contain information about the
black hole spin and the accretion disc as well as the geometry of the emitting
region and may help to test general relativity in the strong gravity regime. In
this paper we embark on the computation of the temporal response of the line to
the illuminating flux. Previous studies concentrated on the calculation of
reverberation signatures from static sources illuminating the disc. In this
paper we focus on the more physically justified case of flares located above
the accretion disc and corotating with it. We compute the time dependent iron
line taking into account all general relativistic effects and show that its
shape is of very complex nature, and also present light curves accompanying the
iron line variability. We suggest that future X-ray satellites like XMM or
Constellation-X may be capable of detecting features present in the computed
reverberation maps.Comment: Accepted for publication in MNRAS, 11 pages, 12 figure
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