Cross-polarization measurements at Martlesham Heath (UK) using OTS

Since 1985, a dual-frequency radar-radiometer, built by the Communications Research Laboratory of Japan, has been flown in several NASA-related experiments to test and to develop methods for the estimation of rain rate from space. These measurements have proved useful for developing and justifying methods of rain estimation for the radar to be launched as part of the Tropical Rainfall Measuring Mission. Over this period of time, the instrument was modified to provide, in addition to the usual copolarized data, cross-polarized measurements at X-band (10 GHz) and Ka-band (34.45 GHz). Additional upgrades have been completed to enable measurements of linear depolarization ratio (L/sub DR/) and differential reflectivity ratio (Z/sub DR/) at both frequencies for ground-based observations. Plans are presently underway to combine these measurements with the S-band Spandar radar and the microwave and millimeter-wave link experiments located at Wallops Flight Facility. In this paper, the authors first describe the instrument modifications, then present results taken during a case of light stratiform rain case. Because the calibration of the system is not yet complete, the authors focus on some of the quantitative features of the data obtained at and near the melting layer.

We report the results of an investigation of the binding of a series of bisphosphonate drugs to human bone using 2H, 13C, 15N, and 31P nuclear magnetic resonance spectroscopy. The 31P NMR results show that the bisphosphonate groups bind irrotationally to bone, displacing orthophosphate from the bone mineral matrix. Binding of pamidronate is well described by a Langmuir-like isotherm, from which we deduce an approximately 30-38 A2 surface area per pamidronate molecule and a deltaG = -4.3 kcal mol(-1). TEDOR of [13C3, 15N] pamidronate on bone shows that the bisphosphonate binds in a gauche [N-C(1)] conformation. The results of 31P as well as 15N shift and cross-polarization measurements indicate that risedronate binds weakly, since it has a primarily neutral pyridine side chain, whereas zoledronate (with an imidazole ring) binds more strongly, since the ring is partially protonated. The results of 2H NMR measurements of side-chain 2H-labeled pamidronate, alendronate, zoledronate, and risedronate on bone show that all side chains undergo fast but restricted motions. In pamidronate, the motion is well simulated by a gauche+/gauche- hopping motion of the terminal -CH2-NH3(+) group, due to jumps from one anionic surface group to another. The results of double-cross polarization experiments indicate that the NH3(+)-terminus of pamidronate is close to the bone mineral surface, and a detailed model is proposed in which the gauche side-chain hops between two bone PO4(3-) sites.

While propagating through a jungle environment radiowaves get substantially depolarized due to their scattering by the vegetation. Due to its conductivity, the foliage supports induced currents that tend to be randomly oriented and, therefore, they produce a depolarization of the overall field. Results of cross-polarization measurements in tropical moist deciduous and tropical wet evergreen forests in the VHF/UHF band are reported in this communication. It was found that vertically polarized waves suffer 5-15-dB higher depolarization than horizontally polarized waves. The crosspolarization discrimination is also found to be dependent on frequency as well as the separation distance between the transmitter and the receiver.

We present novel spectropolarimetric observations of the hydrogen Hα line taken with the Zürich Imaging Polarimeter (ZIMPOL) at the Gregory Coudé Telescope of the Istituto Ricerche Solari Locarno (IRSOL). The linear polarization is clearly dominated by the scattering of anisotropic radiation and the Hanle effect, while the circular polarization is dominated by the Zeeman effect. The observed linear polarization signals show a rich spatial variability, the interpretation of which would open a new window for probing the solar chromosphere. We study their spatial variation within coronal holes, finding a different behaviour for the U/I signals near the north and south solar poles. We identify some spatial patterns, which may facilitate the interpretation of the observations. In close-to-the-limb regions with sizable circular polarization signals, we find similar asymmetric Q/I profiles. We also show examples of net circular polarization profiles (NCP), along with the corresponding linear polarization signals. The application of the weak field approximation to the observed circular polarization signals gives 10 G (40–60 G) close to the limb quiet (plage) regions for the average longitudinal field strength over the spatio-temporal resolution element.

view Abstract Citations (21) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Circular Polarization and the Dielectric Nature of Dust Grains Chlewicki, Grzegorz ; Greenberg, J. Mayo Abstract The relationship between interstellar linear and circular polarization is reexamined. The derivation of optical constants for relevant materials is described, emphasizing general constraints on the electromagnetic response of an optical medium (Kramers-Kronig relations) which also provide the basis for the subsequent analysis of interstellar polarization. The formulas that relate the observed polarization to the properties of individual grains are summarized. It is shown that the apparent upper limit on the visual absorptivity of grains derived from earlier Mie theory calculations results from a violation of the Kramers-Kronig relations in the assumed optical constants of the particles. The analysis of circular and linear polarization in the interstellar medium is presented, and the implications of this analysis for grain models are addressed. Publication: The Astrophysical Journal Pub Date: December 1990 DOI: 10.1086/169474 Bibcode: 1990ApJ...365..230C Keywords: Circular Polarization; Dielectrics; Interstellar Extinction; Interstellar Matter; Optical Properties; Interstellar Gas; Linear Polarization; Mie Scattering; Astrophysics; INTERSTELLAR: GRAINS; POLARIZATION full text sources ADS |

The effects of ZnO morphology on photocatalytic efficiency of ZnO/RGO nanocomposites

This paper presents the configuration and experimental results of a novel wideband dual-polarized base station antenna with superior performance. The proposed antenna consists of four electric folded dipoles arranged in an octagon shape that are excited simultaneously for each polarization. Experimental results show that this element has a wide bandwidth of 46.4% from 1.69 GHz to 2.71 GHz with ≥ 15 dB return loss. Across this wide band, the variations of the half-power-beamwidths (HPBWs) of the two polarizations are all within 66.5° ± 5.5°, the port-to-port isolation is > 28 dB, and the cross-polarization discrimination (XPD) is > 25 dB.

Context: Radio astronomical receivers are now expanding their frequency range to cover large (octave) fractional bandwidths for sensitivity and spectral flexibility, which makes the design of good analogue circular polarizers challenging. Better polarization purity requires a flatter phase response over increasingly wide bandwidth, which is most easily achieved with digital techniques. They offer the ability to form circular polarization with perfect polarization purity over arbitrarily wide fractional bandwidths, due to the ease of introducing a perfect quadrature phase shift. Further, the rapid improvements in field programmable gate arrays provide the high processing power, low cost, portability and reconfigurability needed to make practical the implementation of the formation of circular polarization digitally. Aims: Here we explore the performance of a circular polarizer implemented with digital techniques. Methods: We designed a digital circular polarizer in which the intermediate frequency signals from a receiver with native linear polarizations were sampled and converted to circular polarization. The frequency-dependent instrumental phase difference and gain scaling factors were determined using an injected noise signal and applied to the two linear polarizations to equalize the transfer characteristics of the two polarization channels. This equalization was performed in 512 frequency channels over a 512 MHz bandwidth. Circular polarization was formed by quadrature phase shifting and summing the equalized linear polarization signals. Results: We obtained polarization purity of -25 dB corresponding to a D-term of 0.06 over the whole bandwidth. Conclusions: This technique enables construction of broad-band radio astronomy receivers with native linear polarization to form circular polarization for VLBI.

In this paper, a single-feed microstrip antenna (MA) equipped with a transmission-mode focusing metasurface (MS) is proposed to achieve dual-polarization capabilities and superior high-gain radiation performance. The original-feed MA comprises two distinct layers of coaxial-fed tangential patches, enabling it to emit a circular polarization (CP) wave with a gain of 3.5 dBic at 5.6 GHz and linear polarization (LP) radiation with a gain of 4 dBi at 13.7 GHz. To improve the performance of the single-feed MA, a dual-polarization transmission focusing MS is proposed and numerically substantiated. By positioning the originally designed MA at the focal point of the MS, we create a transmission-mode MS antenna system capable of achieving CP and LP radiations with the significantly higher gains of 12.9 dBic and 14.8 dBi at 5.6 GHz and 13.7 GHz, respectively. Measurements conducted on the fabricated dual-polarization focusing MS antenna closely align with the simulation results, validating the effectiveness of our approach. This work underscores the significant potential of dual-polarization high-speed data systems and offers a practical solution for enhancing antenna gains in contemporary wireless communication systems.

Results of laboratory-scale measurements of long-range across-slope pulse propagation in three-dimensional wedge-shaped oceanic waveguides with a sandy bottom are reviewed. The experimental data considered were collected during two laboratory-scale experimental campaigns that were led in 2006 and 2007 (after a required calibration phase) in the large indoor shallow-water tank of the LMA-CNRS laboratory in Marseille. Operational frequencies and water depths were chosen to produce a reduced number of propagating modes so as to facilitate the analysis of the received signals. The experimental data contain strong 3-D effects like mode shadow zones and multiple mode arrivals, all consistent with well-known three-dimensional effects described in the literature. The data are compared with numerical solutions obtained using a fully three-dimensional parabolic equation based model. Comparisons are performed in both time domain (received signals at distinct ranges and depths) and frequency domain (TL-versus-range curves at distinct depths for several discrete frequencies). In both cases, the experimental data are in good agreement with the numerical predictions.

In this letter, a 3-D pattern reconfigurable antenna array with multi-polarization is presented. It comprises four vertically placed reconfigurable end-fire dipole-pair elements and a compact four-path switchable feed network. By utilizing switchable operating dipoles of each element and appropriate pathways on the feed network, the antenna can achieve adaptive control of broadside patterns and backfire patterns with independent left-handed/right-handed circular polarization (LHCP/RHCP) and ±45° linear polarization (LP) in the elevation plane. Moreover, four end-fire patterns with vertical polarization (VP) can be adaptively controlled in the azimuth plane. The proposed antenna is fabricated and measured. With a compact 3-D structure of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math>$0.49\lambda_0 \times 0.49\lambda_0 \times 0.39\lambda_0$</tex-math></inline-formula> , the measured -10 dB overlapped impedance band for the twelve operating states is 2.3 GHz-2.75 GHz (17.8%). Besides, the corresponding crosspolarization discrimination (XPD) is more than 17.4 dB.

Integrated sensing and communication (ISAC) in the Industrial Internet of Things (IIoT) presents unique challenges in terms of localization techniques. While three-dimensional (3D) environments offer extra challenges to enhanced accuracy and realism, research in this area remains limited. To bridge this gap, we propose a novel localization technique assisted by a single base station (BS) in 3D IIoT scenarios. Our approach employs the MUltiple SIgnal Classification (MUSIC) algorithm to jointly estimate the angle of arrival (AoA) in azimuth and elevation, as well as the time of arrival (ToA). Compared to conventional multi-BS-assisted or MUSIC-based algorithms, our technique offers flexibility, easy implementation, and low computational cost. To improve performance, we integrate the Taylor-series into the iterative process after a MUSIC-based joint azimuth, elevation angle and delay estimation (JAEDE), resulting in a significant 99% reduction in computational complexity compared to a two-step MUSIC-based approach utilizing coarse-fine grid searching. Through numerical simulations, we compare our algorithm with three other MUSIC-based joint or separate estimation approaches, demonstrating its superior performance in azimuth angle of arrival (AoAz), elevation angle of arrival (AoAe), TOA, and overall location estimation across varying signal-to-noise ratio (SNR) conditions.

We report the results of parsec-scale, multi-frequency VLBA observations of the core region of 3C 279 in Stokes I, linear polarization, and circular polarization. These full polarization spectra are modeled by radiative transfer simulations to constrain the magnetic field and particle properties of the parsec-scale jet in 3C 279. The polarization properties of the core region, including the amount of linear polarization, the amount and sign of Faraday rotation, and the amount and sign of circular polarization can be explained by a consistent physical picture. The base of the jet is modeled as an inhomogeneous Blandford-Konigl style conical jet dominated by a vector-ordered poloidal magnetic field along the jet axis, and we estimate its net magnetic flux. This poloidal field is responsible for the linear and circular polarization from this inhomogeneous component. Farther down the jet the magnetic field in two homogeneous features is dominated by local shocks and a smaller fraction of vector-ordered poloidal field remains along the jet axis. In this picture, we find the jet to be kinetically dominated by protons with the radiating particles being dominated by electrons at an approximate fraction of >~ 75%. Based on the amounts of Faraday conversion deduced for the homogeneous components, we find a plausible range for the lower cutoff in the relativistic particle energy spectrum to be 5 <~ gamma_l <~ 35. The physical picture described here is not unique if the observed Faraday rotation and depolarization occur in screens external to the jet; however, we find the joint explanation of linear and circular polarization observations from a single set of magnetic fields and particle properties internal to the jet to be compelling evidence for this picture. (Abridged)

We find for infrared wavelengths that there are broad ranges of particle sizes and refractive indices that represent fog and rain, where circular polarization can persist to longer ranges than linear polarization. Using polarization tracking Monte Carlo simulations for varying particle size, wavelength, and refractive index, we show that, for specific scene parameters, circular polarization outperforms linear polarization in maintaining the illuminating polarization state for large optical depths. This enhancement with circular polarization can be exploited to improve range and target detection in obscurant environments that are important in many critical sensing applications. Initially, researchers employed polarization-discriminating schemes, often using linearly polarized active illumination, to further distinguish target signals from the background noise. More recently, researchers have investigated circular polarization as a means to separate signal from noise even more. Specifically, we quantify both linearly and circularly polarized active illumination and show here that circular polarization persists better than linear for radiation fog in the short-wave infrared, for advection fog in the short-wave and long-wave infrared, and large particle sizes of Sahara dust around the 4μm wavelength. Conversely, we quantify where linear polarization persists better than circular polarization for some limited particle sizes of radiation fog in the long-wave infrared, small particle sizes of Sahara dust for wavelengths of 9-10.5μm, and large particle sizes of Sahara dust through the 8-11μm wavelength range in the long-wave infrared.

Information about the poloidal field inside the plasma and about the current density profile is essential for the understanding of the behavior of tokamak plasmas. The poloidal field outside the plasma edge is easily measured by a set of magnetic coils, whereas measurements of the magnetic field distribution inside the plasma requires dedicated and often complex diagnostic systems. One of the methods is polarimetry using beams of electromagnetic waves traversing the plasma. It makes use of the fact that the magnetic field modifies the refractive index of the plasma and as a consequence affects the state of polarization of the passing waves. Specifically, Faraday rotation refers to the case of a linearly polarized wave propagating parallel to a magnetic field. In this case, the rotation of the polarization vector is proportional to the product of electron density and parallel magnetic field component. This thesis is devoted to the design of a specific polarimeter system and its implementation on the TCV tokamak. The conceptual design took into account the particular requirements of the TCV experiments and its experimental program with particular interest in measurements of current density profiles in low-density, EC-heated plasmas. Aiming at optimum performance of the polarimeter in this parameter range, the FIR laser wavelength of 432.5 µm was chosen. Since TCV is already equipped with an interferometer (at 214 µm) for measurements of the line-integrated density along 14 chords, the polarimeter was built as a separate instrument. The number of 10 spatial channels to cover the plasma diameter was found as a compromise between desired spatial resolution, access constraints and cost. The measurement of the Faraday rotation angle is based on a method suggested by Dodel and Kunz [1], which uses optical beams with rotating linear polarization. In this case, the Faraday rotation angle is retrieved from a phase measurement comparing the modulated signals from the probe detectors with that from a reference detector and to first order is insensitive to amplitude variations of the signals. Waveguide detectors based on Schottky barrier diodes are used as detectors for their sensitivity and large electrical bandwidth. Signal processing and analysis is performed in two branches : a) using specifically built analog electronic phase detectors followed by slow (250 kHz) ADCs b) using fast (5 MHz) ADCs and numeric signal processing on a mainframe computer. The thesis covers the description of the design and the various components of the polarimeter, presents initial tests and an evaluation of its performance based on simulations using real plasma configurations of TCV. The analysis of specific tests with the system installed on TCV revealed the presence of perturbations leading to parasitic contributions to the measured phase angles. The results of first measurements of the Faraday rotation angle for different plasma conditions on TCV are presented. Comparing the measured Faraday rotation angles with the results of calculations showed qualitative agreement, in particular the effects of an increase in electron density and of the reversal of the current direction were clearly seen. The system is also capable to detect a radial displacement of the magnetic axis of the plasma, as was demonstrated by comparing measurements from two specific plasma configurations. However, the absolute values still show significant deviations from the expected ones based on calculations. The discrepancies increase with increasing Faraday angle and depend on the direction of the plasma current. At high electron densities beam refraction becomes a problem and may lead to significant errors in the measurements and eventually to complete loss of the signal in several channels. In its present status, the polarimeter cannot yet provide results that are suitable to reconstruct the poloidal field or the current density profile of the plasma. Tentative explanations for this problem are given, but further specific tests are necessary to confirm them. Even in its present state, the polarimeter may be used to detect transient relative changes in the profiles of electron density and current. This was demonstrated by a series of experiments in plasmas with sawtooth activity. Comparing the signals from the polarimeter with those from other multi-chord diagnostics (interferometer and soft X-ray detectors) clearly revealed correlations but also specific differences. These experimental studies showed that the Far-Infrared Polarimetry still requires further improvements, but has potential to become a valuable diagnostic capable of directly measuring the current density profile in the TCV tokamak.