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Wiley Online Library : Radio Science
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Issue Information

Thu, 03/15/2018 - 16:05

No abstract is available for this article.

Power Flow, Energy Density, and Group/Energy Transport Velocities in Spatially Dispersive Media

Thu, 03/15/2018 - 15:50
Abstract

Electromagnetic power flow, energy density, and the energy transport velocity that equals the group velocity of a lossless wave packet are straightforwardly derived for the recently formulated electric and magnetic anisotropic representation of spatially dispersive materials or metamaterials. Boundary conditions in spatially dispersive media are used to confirm analytically and numerically the expressions obtained for the power flow and energy density. A long-standing alternative expression for the energy transport velocity in spatially dispersive media is shown to be invalid. Plausible formulation-independent definitions of spatial and temporal dispersion for normal modes are proposed in terms of group and phase velocities.

The Embrace Magnetometer Network for South America: Network Description and Its Qualification

Wed, 03/14/2018 - 15:36
Abstract

The present work is the first of a two-part paper on the Embrace Magnetometer Network. In this part, we present the new Embrace Magnetometer Network (Embrace MagNet) in South America, which is originally planned to cover most of the eastern portion of the Southern America longitudinal sector by installing and operating fluxgate magnetometer stations. We discuss the purpose and scientific goals of the network, associated with aeronomy and space weather. We provide details on the instrumentation, location of the sensors, sensitivity matching process, gain matching process, and magnetometer installation. In addition, we present and discuss details about the data storage, near-real time display, and availability.

Incoherent Scatter Spectra Based On Monte Carlo Simulations of Ion Velocity Distributions Under Strong Ion Frictional Heating

Tue, 03/13/2018 - 14:41
Abstract

Under strong electric field conditions often found at high latitudes, the ion velocity distribution of the weakly ionized F region plasma can differ enough from a Maxwellian shape to substantially change incoherent scatter (IS) spectra and thus the analysis of those spectra. With the goal to provide a quantitative and reliable description of the IS spectra, this study directly uses for the first time an advanced Monte Carlo calculation of the ion velocity distribution to derive IS spectra for a range of electric fields and aspect angles. For most cases the spectra associated with NO+ maintains a shape that closely resembles that of a spectrum derived from a Maxwellian distribution with the same line-of-sight ion temperature as the equivalent Monte Carlo simulated distribution. This study also fully characterizes the spectral shape as well as the ion temperature and its anisotropy for two different models of the resonant charge exchange between O+ and O. It confirms that the distortions from the Maxwellian shape can be substantial for this particular interaction. The distortions are also such that along the magnetic field direction, the extracted apparent electron temperature is always greater than the real temperature. This work also includes a determination of the stability of the plasma against magnetic field-aligned electrostatic instabilities. It is found that the NO+ distribution is always stable, whereas the O+ distribution may or may not be stable, depending on the model chosen for the resonant charge exchange cross section in collisions with the background atomic oxygen gas.

Wireless Sensor Network for Radiometric Detection and Assessment of Partial Discharge in HV Equipment

Sat, 03/10/2018 - 14:39
Abstract

Monitoring of PD activity within high-voltage electrical environments is increasingly used for the assessment of insulation condition. Traditional measurement techniques employ technologies that require either off-line installation or have high power consumption and are hence costly. A wireless sensor network is proposed that utilizes only received signal strength to locate areas of PD activity within a high voltage electricity substation. The network comprises low-power and low-cost radiometric sensor nodes which receive the radiation propagated from a source of PD. Results are reported from several empirical tests performed within a large indoor environment and a substation environment using a network of nine sensor nodes. A portable PD source emulator was placed at multiple locations within the network. Signal strength measured by the nodes is reported via WirelessHART to a data collection hub where it is processed using a location algorithm. The results obtained place the measured location within 2 m of the actual source location.

Experimental Demonstration of In-place Calibration for Time-domain Microwave Imaging System

Sat, 03/10/2018 - 14:35
Abstract

In this study, the experimental demonstration of in-place calibration was conducted using the developed time-domain measurement system. Experiments were conducted three calibration methods—in-place calibration and two existing calibrations, i.e., array rotation and differential calibration. The in-place calibration uses dual receivers located at an equal distance from the transmitter. The received signals at the dual receivers contain similar unwanted signals i.e., the directly received signal and antenna coupling. In contrast to the simulations, the antennas are not perfectly matched and there might be unexpected environmental errors. Thus, we experimented with the developed experimental system to demonstrate the proposed method. The possible problems with low signal-to-noise ratio and clock jitter, which may exist in time-domain systems, were rectified by averaging repeatedly measured signals. The tumor was successfully detected by the three calibration methods according to the experimental results. The cross-correlation was calculated using the reconstructed image of the ideal differential calibration for a quantitative comparison between the existing rotation calibration and the proposed in-place calibration. The mean value of cross-correlation between the in-place calibration and ideal differential calibration was 0.80, and the mean value of cross-correlation of the rotation calibration was 0.55. Furthermore, the results of simulation were compared with the experimental results to verify the in-place calibration method. A quantitative analysis was also performed and the experimental results show a tendency similar to the simulation.

A Novel Electromagnetic Power-Based Characteristic Mode for Magneto-Dielectric Materials

Thu, 03/08/2018 - 05:20
Abstract

In this paper, we focus on the characteristics of electromagnetic power of characteristic modes (CMs) for magneto-dielectric materials (MDM). A generalized integral form of Poynting's theorem is proposed. Based on the generalized integral form of Poynting's theorem, an explicit formulation for electromagnetic power of traditional VIE (volume integral equation)-based CM and PMCHWT (Poggio, Miller, Chang, Harrington, Wu, and Tsai)-based CM of homogeneous MDM is presented for the first time. Furthermore, from the concept of electromagnetic power, a novel electromagnetic power-based CM for MDM is proposed. Numerical results are given to show that the proposed CM are effective in solving scattering problem.

The Embrace Magnetometer Network for South America: First Scientific Results

Wed, 03/07/2018 - 22:55
Abstract

The present work is the second of a two-part paper on the Embrace Magnetometer Network. In this part, we provide some of the first scientific findings that we have already achieved with this network. We identified the diurnal and the seasonal natural variations of the H component. We provided the precise determination of SSC and SI. We showed that the ∆H amplitudes derived from the Embrace MagNet during intense magnetic storm are in very good agreement with the Dst index. We showed that it is possible to investigate of the effects of the Sq systems response to the X-class solar flares occurring during daytime under magnetically quiet conditions.

Relationship between the electromagnetic wave energy coupled by overhead lines and the radiation source current explored in the laboratory

Wed, 03/07/2018 - 22:55
Abstract

In order to solve the damage and interference problems to the electronic devices, which are induced by over voltage excited by the coupling process between lightning electromagnetic wave and overhead lines, the lightning tunnel is set to be equivalent to a radiant wire antenna, Based on the integration model to lightning return stroke tunnel, transmission line and ground, we take advantage of the derived formula gotten from the transmission line model, by combing the theoretical and experimental methods, we conduct a comparative analysis on the coupling process between natural /simulated lightning and overhead line. Besides, we also calculate the amplitude and energy of over-voltage which is caused by the coupling process between lightning electromagnetic wave and overhead lines. Upon these experimental results, we can draw several conclusions as follows: when the amplitude of the lightning current in the tunnel is between 5 kA to 41 kA, it takes on an excellent linear relation between the amplitude of over voltage and the magnitude of the lightning current, the relation between coupling energy and magnitude of the lightning current takes on an exponential trend. When lightning wave transmits on the transmission lines, the high order mode will be excited. Through analysis on the high order mode's characteristics, we find the theoretical analysis is consistent with the experimental results, which has a certain reference value to the protection on overhead lines.

Effectiveness of Moving Signal-Average Method in K-Band FMCW Radar for Short-Range Vehicle Detection Using Antennas with Narrow Beamwidths

Wed, 03/07/2018 - 22:55
Abstract

This paper presents the implementation and examination of moving signal-average methods (MSAMs) to improve the defects of frequency modulated continuous wave (FMCW) radar in detecting short-range vehicles which may cause false target detection. The MSAM may smooth out the short-term signal fluctuations in the system responses of received data in time series. These signal fluctuations may appear when the antenna radiation has a narrow beamwidth to illuminate only a part of sophisticated vehicle structures. In this examination, the defect phenomena were first investigated by employing the electromagnetic (EM) backscattering simulation. Afterward, experimental measurements were performed by implementing MSAM in a radar system for side-looking vehicle detection. The experimental FMCW radar operates at 24.125 GHz with 250 MHz bandwidth, and implements a linear frequency modulation (LFM) for TX/RX with 2mW for TX power. The output responses to determine the detection are based on the peak value distribution of signals after being processed by the Fast Fourier Transform (FFT) and MASM. Both numerical simulations and experiments have demonstrated the validity of MSAM in the signal treatment to detect the vehicles within the coverage range.

Status of NeQuick G After the Solar Maximum of Cycle 24

Tue, 03/06/2018 - 18:01
Abstract

The solar cycle 24 will not be registered as the most intense of the last cycles. In fact, its intensity is roughly half of the previous cycle and the ionospheric effects experienced in this cycle have been far milder than originally expected, despite having several major ionospheric storms in this period, as the so-called St. Patrick's Day's ionospheric storm. On the other hand, in this same period of time, the Galileo system has started the deployment phase and it started the In-Orbit-Validation campaign on 2013 with the first four full operational satellites, following the launch of a number of additional satellites allowing the declaration of Initial Services in December 2016 and targeting the Full Operational Capability by 2020. Thus, during this period of time, Galileo has been broadcasting the 3 Az coefficients needed to use the NeQuick G for correcting the ionospheric delay for single-frequency users. In this work, the full analysis of the performance of the NeQuick G for the last solar cycle will be presented along with the detailed analysis of some of the most relevant ionospheric storms occurred during the very same period. In general, the NeQuick G presents around 50 cm better root mean square than the Global Positioning System broadcast model for all the period of study. As an internal measure of the goodness of the NeQuick G, the percentage of slant total electron content inside of target Galileo specification will also be analyzed.

Computationally Efficient Radio Frequency Source Localization for Radio Interferometric Arrays

Fri, 03/02/2018 - 17:05
Abstract

Radio frequency interference (RFI) is an ever-increasing problem for remote sensing and radio astronomy, with radio telescope arrays especially vulnerable to RFI. Localizing the RFI source is the first step to dealing with the culprit system. In this paper, a new localization algorithm for interferometric arrays with low array beam sidelobes is presented. The algorithm has been adapted to work both in the near field and far field (only the direction of arrival can be recovered when the source is in the far field). In the near field the computational complexity of the algorithm is linear with search grid size compared to cubic scaling of the state-of-the-art 3-D MUltiple SIgnal Classification (MUSIC) method. The new method is as accurate as 3-D MUSIC. The trade-off is that the proposed algorithm requires a once-off a priori calculation and storing of weighting matrices. The accuracy of the algorithm is validated using data generated by low-frequency array while a hexacopter was flying around it and broadcasting a continuous-wave signal. For the flight, the mean distance between the differential GPS positions and the corresponding estimated positions of the hexacopter is 2 m at a wavelength of 6.7 m.

Novel Spatial Domain Integral Equation Formulation for the Analysis of Rectangular Waveguide Steps Close to Arbitrarily Shaped Dielectric and/or Conducting Posts

Thu, 03/01/2018 - 06:05
Abstract

In this paper, a novel integral equation formulation expressed in the spatial domain is proposed for the analysis of rectangular waveguide step discontinuities. The important novelty of the proposed formulation is that allows to easily take into account the electrical influence of a given number of arbitrarily shaped conducting and dielectric posts placed close to the waveguide discontinuity. For the sake of simplicity, and without loss of generality, the presented integral equation has been particularized and solved for inductive rectangular waveguide geometry. In this case, the integral equation mixed-potentials kernel is written in terms of parallel plate Green's functions with an additional ground plane located on the waveguide step. Therefore, the unknowns of the problem are reduced to an equivalent magnetic surface current on the step aperture and equivalent magnetic and electric surface currents on the dielectric and conducting posts close to the discontinuity. The numerical solution of the final integral equation is efficiently computed after the application of acceleration techniques for the slowly convergent series representing the Green's functions of the problem. The numerical method has been validated through several simulation examples of practical microwave devices, including compact size bandpass cavity filters and coupled dielectric resonators filters. The results have been compared to those provided by commercial full-wave electromagnetic simulation software packages, showing in all cases a very good agreement, and with substantially enhaced numerical efficiencies.

VHF radar images of artificial field-aligned ionospheric irregularities in the subauroral E region

Thu, 03/01/2018 - 05:40
Abstract

Artificial E-region field-aligned plasma density irregularities (AFAIs) have been generated using the HAARP ionospheric modification facility in Gakona and observed with a 30-MHz coherent scatter radar imager in Homer, Alaska. The AFAIs were generated using a distinctive, twisted-beam antenna pattern that illuminated a particularly broad volume overhead. The broad beam facilitates studies of natural sporadic E layer patches when they are present. The center of the pattern was pointed at different angles between zenith and magnetic zenith to examine the effects on the AFAI morphology. Radar images of AFAIs generally resemble the radiation pattern of the HF source, but the irregularities are strongest within a narrow range of zenith angles bounded approximately by the Spitze angle. A number of factors which might influence AFAI generation and detection are examined. The most important is most likely the requirement for the pump mode to have a standing-wave component for thermal parametric instability to operate.

New Results on Ionospheric Irregularity Drift Velocity Estimation Using Multi-GNSS Spaced-Receiver Array During High-Latitude Phase Scintillation

Sat, 02/24/2018 - 06:06
Abstract

The spaced-receiver technique using Global Navigation Satellite Systems (GNSS) receivers offers an inexpensive approach for estimating ionospheric irregularity velocity during ionospheric scintillations. Our previous work has demonstrated that correlative studies of the GNSS carrier phase variations can be used to derive irregularity drift velocity at high latitudes. This study expanded upon our previous projects by incorporating Global Navigation Satellite System (GLONASS) signals, investigation on ionospheric irregularity height assumption, and all-sky imager measurements into the methodology. A case study is presented based on Global Positioning System, Galileo, and GLONASS measurements during a geomagnetic storm event on 20 December 2015, obtained from a closely spaced receiver array at Poker Flat Research Range near Fairbanks, Alaska. The GNSS-estimated irregularity drift velocities are in general agreement with the measurements from the Poker Flat Incoherent Scatter Radar and the Poker Flat all-sky imager. The study also shows that the irregularity altitude assumption will not lead to significant variations in the irregularity drift velocity estimates, especially for satellites with relatively high elevations. The techniques presented in this paper demonstrate that GNSS receiver arrays can be used as powerful means to monitor the ionospheric plasma dynamics during space weather events.

Pilot ionosonde network for identification of travelling ionospheric disturbances

Sat, 02/24/2018 - 05:50
Abstract

Travelling Ionospheric Disturbances (TIDs) are the ionospheric signatures of atmospheric gravity waves (AGWs). Their identification and tracking is important because the TIDs affect all services that rely on predictable ionospheric radio wave propagation. Although various techniques have been proposed to measure TID characteristics, their real-time implementation still has several difficulties. In this contribution, we present a new technique, based on the analysis of oblique Digisonde-to-Digisonde (D2D) “skymap” observations, to directly identify TIDs and specify the TID wave parameters based on the measurement of angle-of-arrival, Doppler frequency, and time-of-flight of ionospherically reflected high-frequency (HF) radio pulses. The technique has been implemented for the first time for the Net-TIDE project with data streaming from the network of European Digisonde DPS4D observatories. The performance is demonstrated during a period of moderate auroral activity, assessing its consistency with independent measurements such as data from auroral magnetometers and electron density perturbations from Digisondes and GNSS stations. Given that the different types of measurements used for this assessment were not made at exactly the same time and location, and that there was insufficient coverage in the area between the AGW sources and the measurement locations, we can only consider our interpretation as plausible and indicative for the reliability of the extracted TID characteristics. In the framework of the new TechTIDE project (European Commission H2020), a retrospective analysis of the Net-TIDE results in comparison with those extracted from GNSS TEC-based methodologies is currently being attempted, and the results will be the objective of a follow up paper.

Automatic Detection and Classification of Buried Objects Using Ground-Penetrating Radar for Counter-Improvised Explosive Devices

Thu, 02/22/2018 - 04:16
Abstract

In this paper, a technique that can automatically detect and classify objects buried under the ground is proposed. The technique employs a ground-penetrating radar that transmits electromagnetic waves in order to strike the objects and then receives the backscattering electromagnetic wave to perform signal processing. This signal processing is divided into four main steps as follows. First, preprocessing is used to reduce the clutter due to the effect of the media layer interface. Second, the late time of the scattering signal is estimated using a simple cross correlation. Third, a few successive poles are extracted from the scattering response at the estimated late time by using the short-time matrix pencil method. Finally, the extracted poles are fed for object classification with different constitutions and/or shapes using a support vector machine. Simulations according to the practical situation in three southern provinces of Thailand to counter the improvised explosive devices were set up. The performance of the proposed technique was evaluated. The simulation results showed that the proposed technique can efficiently detect and classify buried objects for counter-improvised explosive device operations in the military.

Broadband Planar Aperture-Coupled Antenna Array for WLAN and ITS Beam-Steering Applications

Thu, 02/22/2018 - 04:11
Abstract

A broadband multilayer beam-steering antenna array with a modified mushroom-like radiating element is introduced. The two-layer aperture-coupled array consists of four equal sets of mushroom-like elements, each of which is made of 16 unit cells in a 4 × 4 configuration. The feeding network is a modified quasi-Butler matrix, which benefits from improved double-box branch line couplers. The network has an isolation bandwidth between 4 and 7.9 GHz (65%) and a transmission bandwidth from 4.8 to 7.1 GHz (39%), which operates acceptably in most of the C-band, especially at wireless local area network and Intelligent Transport Systems (ITS). A broadband Butler matrix uses broadband 3 dB couplers and a middle network. The geometry of the element and feeding network is utilized in array form to enhance the impedance bandwidth and radiation efficiency of the antenna. The results show that the antenna impedance bandwidth covers 3.82 to 8.2 GHz (port 1) and 3.8 to 8.28 GHz (port 2) for Snn ≤ −10 dB, which covers the C-band fully (4–8 GHz). Moreover, it has a 3 dB gain-bandwidth of 2.25 GHz that extends from 4.65 to 6.9 GHz with a peak value of 13.65 dBi. The antenna array has a 3 dB axial ratio bandwidth of 1.65 GHz that extends from 4.9 to 6.55 GHz and involves wireless local area network (5.15–5.825 GHz) and ITS (5.795–6.400 GHz). The radiation efficiency of the array for both port excitations is in excess of 75% when circular polarization is achieved. Two firing beams at elevation angles of 20° and −35° are obtained for ports 1 and 2 upon excitation, respectively.

Internal Natural Resonances Prediction through the Theory of Characteristic Modes

Tue, 02/20/2018 - 14:25
Abstract

Some aspects in the interpretation of the resonances provided by the theory of characteristic modes for dielectric bodies are analyzed. The analysis has been performed analytically based on the Poggio-Miller-Chang-Harrington-Wu-Tsai (PMCHWT) formulation on a canonical problem, the infinite dielectric circular cylinder. Firstly, natural resonances and characteristic mode resonances are presented and compared. It has been observed that characteristic mode resonances are in general near to internal natural resonances but not to external ones. It is also demonstrated that characteristic resonances become closer to internal natural ones as the relative permittivity of the dielectric cylinder is increased.

Issue Information

Tue, 02/20/2018 - 05:14

No abstract is available for this article.

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