11,518 research outputs found
Properties of a Variable-delay Polarization Modulator
We investigate the polarization modulation properties of a variable-delay
polarization modulator (VPM). The VPM modulates polarization via a variable
separation between a polarizing grid and a parallel mirror. We find that in the
limit where the wavelength is much larger than the diameter of the metal wires
that comprise the grid, the phase delay derived from the geometric separation
between the mirror and the grid is sufficient to characterize the device.
However, outside of this range, additional parameters describing the polarizing
grid geometry must be included to fully characterize the modulator response. In
this paper, we report test results of a VPM at wavelengths of 350 microns and 3
mm. Electromagnetic simulations of wire grid polarizers were performed and are
summarized using a simple circuit model that incorporates the loss and
polarization properties of the device.Comment: 25 pages, 10 figures, accepted by Applied Optic
Field Trials for the Empirical Characterization of the Low Voltage Grid Access Impedance From 35 kHz to 500 kHz
The access impedance of low-voltage (LV) power networks is a major factor related to the performance of the narrow-band power line communications (NB-PLCs) and, in a wider sense, to electromagnetic compatibility (EMC) performance. Up to date, there is still a lack of knowledge about the frequency-dependent access impedance for frequencies above 9 kHz and up to 500 kHz, which is the band where the NB-PLC operates. The access impedance affects the transmission of the NB-PLC signal, and it determines the propagation of the non-intentional emissions that may disturb other electrical devices, including malfunctioning or reduced lifetime of equipment. This paper presents the results of field measurements of the LV access impedance up to 500 kHz in different scenarios, with measurement locations close to end users and near transformers. The results provide useful information to analyze the characteristics of the LV access impedance, including variation with frequency, ranges of values for different frequency bands, and analysis of specific phenomena. Moreover, the results reveal a diverse frequency-dependent behavior of the access impedance in different scenarios, depending on the grid topology, the number of end users (that is, number and type of connected loads), and the type of transformation center. Overall, the results of this paper offer a better understanding of the transmission of NB-PLC signals and EMC-related phenomena.The authors would like to thank Iberdrola for the availability and the collaboration of authorized staff for carrying out the field trials
Direction for the Future - Successive Acceleration of Positive and Negative Ions Applied to Space Propulsion
Electrical space thrusters show important advantages for applications in
outer space compared to chemical thrusters, as they allow a longer mission
lifetime with lower weight and propellant consumption. Mature technologies on
the market today accelerate positive ions to generate thrust. The ion beam is
neutralized by electrons downstream, and this need for an additional
neutralization system has some drawbacks related to stability, lifetime and
total weight and power consumption. Many new concepts, to get rid of the
neutralizer, have been proposed, and the PEGASES ion-ion thruster is one of
them. This new thruster concept aims at accelerating both positive and negative
ions to generate thrust, such that additional neutralization is redundant. This
chapter gives an overview of the concept of electric propulsion and the state
of the development of this new ion-ion thruster.Comment: 10 pages, contribution to the CAS-CERN Accelerator School: Ion
Sources, Senec, Slovakia, 29 May - 8 June 2012, edited by R. Bailey. appears
in CERN Yellow Report CERN-2013-007, pp.575-58
Voltage Stabilization in Microgrids via Quadratic Droop Control
We consider the problem of voltage stability and reactive power balancing in
islanded small-scale electrical networks outfitted with DC/AC inverters
("microgrids"). A droop-like voltage feedback controller is proposed which is
quadratic in the local voltage magnitude, allowing for the application of
circuit-theoretic analysis techniques to the closed-loop system. The operating
points of the closed-loop microgrid are in exact correspondence with the
solutions of a reduced power flow equation, and we provide explicit solutions
and small-signal stability analyses under several static and dynamic load
models. Controller optimality is characterized as follows: we show a one-to-one
correspondence between the high-voltage equilibrium of the microgrid under
quadratic droop control, and the solution of an optimization problem which
minimizes a trade-off between reactive power dissipation and voltage
deviations. Power sharing performance of the controller is characterized as a
function of the controller gains, network topology, and parameters. Perhaps
surprisingly, proportional sharing of the total load between inverters is
achieved in the low-gain limit, independent of the circuit topology or
reactances. All results hold for arbitrary grid topologies, with arbitrary
numbers of inverters and loads. Numerical results confirm the robustness of the
controller to unmodeled dynamics.Comment: 14 pages, 8 figure
Development of large radii half-wave plates for CMB satellite missions
The successful European Space Agency (ESA) Planck mission has mapped the
Cosmic Microwave Background (CMB) temperature anisotropy with unprecedented
accuracy. However, Planck was not designed to detect the polarised components
of the CMB with comparable precision. The BICEP2 collaboration has recently
reported the first detection of the B-mode polarisation. ESA is funding the
development of critical enabling technologies associated with B-mode
polarisation detection, one of these being large diameter half-wave plates. We
compare different polarisation modulators and discuss their respective
trade-offs in terms of manufacturing, RF performance and thermo-mechanical
properties. We then select the most appropriate solution for future satellite
missions, optimized for the detection of B-modes.Comment: 16 page
Modeling and Analysis of Composite Antenna Superstrates Consisting on Grids of Loaded Wires
We study the characteristics and radiation mechanism of antenna superstrates
based on closely located periodical grids of loaded wires. An explicit
analytical method based on the local field approach is used to study the
reflection and transmission properties of such superstrates. It is shown that
as a result of proper impedance loading there exists a rather wide frequency
band over which currents induced to the grids cancel each other, leading to a
wide transmission maximum. In this regime radiation is produced by the magnetic
dipole moments created by circulating out-of-phase currents flowing in the
grids. An impedance matrix representation is derived for the superstrates, and
the analytical results are validated using full-wave simulations. As a
practical application example we study numerically the radiation
characteristics of dipole antennas illuminating finite-size superstrates.Comment: 9 pages, 11 figures. In the second version we have clarified the
analysis related to the prototype, and re-desinged the prototype antenn
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