Dipole In Space Research Articles

Maximizing the effective diversity gain of two parallel dipoles by optimizing the source impedances

The embedded element patterns of two parallel half-wavelength dipoles are calculated for various source impedances and dipole spacings by using classical formulas for self and mutual impedances. These are used to calculate the effective diversity gain, which is shown to vary with source impedances and dipole spacing. The results are verified using a method of moments (MoM) code for wire antennas and via measurements in a reverberation chamber. The numerical model is used to find the source impedances that maximize the effective diversity gain, which has a maximum for the computed cases when the source impedances are conjugate-matched to the input impedances of each of the embedded dipole elements. Then the two antenna ports are uncorrelated. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 532–535, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21400

DIVERSITY TECHNIQUES WITH PARALLEL DIPOLE ANTENNAS: RADIATION PATTERN ANALYSIS

Two parallel dipoles are assessed for antenna diversity. The three-dimensional radiation pattern is considered for signals correlation coefficient. The pattern analysis reveals that, depending on dipole spacing, three types of diversity techniques are generated: space, amplitude-pattern and phase-pattern diversity. The weighting of each technique in signals correlation coefficient mitigation is investigated. The results show that for closely spaced dipoles, the generated phase- pattern diversity is the most dominant factor which greatly reduces the signals correlation coefficient. The diversity configuration is measured in a rich scattering environment. Results include signals correlation coefficient, diversity gain for selection combining and maximum ratio combining, effective diversity gain and antenna radiation efficiency can be demonstrated. We show that in rich multipath channel the minimum spatial distance, for effective diversity gain performance, is reduced from 0.5λ for uncoupled dipoles to 0.15λ for coupled dipoles.

Sub-Audio Magnetic Survey Experiments for High-Resolution, Subsurface Mapping of Regolith and Mineralisation at the Songvang Gold Mine near Agnew, Western Australia

Experimentation with Sub-Audio Magnetic (SAM) survey parameters over the Songvang mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and differential weathering in the regolith at high resolution to a depth of 100 m, as long as the transmitter electrodes are placed sub-parallel to the strike direction of features to be detected. The equivalent magnetometric resistivity (EQMMR) response produced by horizontal current channelling between transmitter electrodes has a very similar pattern to gradient-array apparent resistivity results using a 50 m or less dipole spacing. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and the 2 m along-line sample density provides much greater resolution. The same area was surveyed using the same parameters, except that the transmitter electrode directions were at a right angle, and this produced radically different EQMMR results. EQMMR anomaly trends running N–S were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The Songvang main lode is an elongated, shallow plunging, rod-like zone of gold-pyrite ore that is detected by a gradient-array induced polarization survey. SAM equivalent magnetometric induced polarization (EQMMIP) was ineffective at imaging the main lode using surface transmitter electrodes and 1 Hz transmitter frequency. However, the 4 Hz EQMMIP response of the main lode was later imaged in great detail when transmitter electrodes were placed down boreholes into the orebody. The downhole SAM EQMMIP results were very effective at highlighting zones of intense sulphide-related gold mineralisation ahead of drilling. SAM surveying provided EQMMR data that identified shallow, conductive shears that contained economic gold mineralisation, and the EQMMIP data helped to plan resource definition drilling by prioritising chargeable targets related to increased pyrite alteration and gold grade along the Songvang main lode.

Sub-audio magnetic survey experiments for high-resolution, subsurface mapping of regolith and mineralisation over a blind gold discovery near Agnew in Western Australia

Experimentation with sub-audio magnetic (SAM) survey parameters over a recently discovered Archaean mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at high resolution to a depth of 100 m, as long as the transmitter electrodes are placed sub-parallel to the strike direction of features to be detected. The horizontal total field magnetometric resistivity (TFMMR) response produced by current channelling during SAM surveying is shown to have a very similar pattern to gradient array apparent resistivity results using 50 m dipole spacing or less. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and 2 m along-line sample density provides much greater resolution. TFMMR anomaly trends were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The main ore zone forms an elongated, shallow plunging pod of weakly chargeable ore that shows up in gradient array and dipole-dipole IP surveys. SAM total field magnetometric induced polarisation (TFMMIP) was ineffective at imaging this ore zone using surface transmitter electrodes and 1 Hz transmitter frequency. However, the TFMMIP response of the main ore zone was later imaged in great detail by placing transmitter electrodes down boreholes into a shallow part of the ore body, and then down-plunge 700 m to the south. SAM surveying using optimal survey parameters identified shallow conductive shears and deeper chargeability anomalies within the main ore zone that correlated to economic gold mineralisation, and the SAM results helped to target resource definition drilling.

A new method for real radiation at next-to-next-to-leading order

We propose a new method of computing real emission contributions to hard QCD processes. Our approach uses sector decomposition of the exclusive final-state phase space to enable extraction of all singularities of the real emission matrix elements before integration over any kinematic variable. The exact kinematics of the real emission process are preserved in all regions of phase space. Traditional approaches to extracting singularities from real emission matrix elements, such as phase space slicing and dipole subtraction, require both the determination of counterterms for double real emission amplitudes in singular kinematic limits and the integration of these contributions analytically to cancel the resulting singularities against virtual corrections. Our method addresses both of these issues. The implementation of constraints on the final-state phase space, including various jet algorithms, is simple using our approach. We illustrate our method using ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}$ jets at $O({\ensuremath{\alpha}}_{S}^{2})$ as an example.

2-D GEOELECTRICAL SURVEY FOR THE PRELIMINARY ROUTE OF THE NATURAL GAS PIPELINE INSTALLATION AT ALISTRATI CAVES, N. GREECE

An extensive 2-D geoelectrical survey was carried out at the Alistrati caves, prefecture of Serres, Ν. Greece, as part of a preliminary route survey for the planned natural gas pipeline that was about to cross these regions of Northern Greece. Geophysics can play a significant role in this procedure. Synthetic modeling and inversion examples presented in this work support the above claim. The dipole-dipole array was used with inter-electrode spacing of 5m and maximum dipole separation of n equal to 8 dipole spacing. Two groups of personnel, including geophysicists and technicians, worked for one month to realize this survey. The number of measurements is really enormous for such type of research. A total of 7616 measurements (952 electrode points) were obtained. The area of investigation was chosen on the basis of the local geological conditions, which appeared to favor the existence of void structures. The survey line had a length of approximately 5 km. The 2-D geoelectrical survey proved to be an effective tool for delineating the subsurface lithology and structure of the surveyed area. Most importantly, the technique proved successful in locating highly karstified areas and can be used to aid void mapping as well as to provide useful information about their general setting.

Mapping the brain's orchestration during speech comprehension: task-specific facilitation of regional synchrony in neural networks

BackgroundHow does the brain convert sounds and phonemes into comprehensible speech? In the present magnetoencephalographic study we examined the hypothesis that the coherence of electromagnetic oscillatory activity within and across brain areas indicates neurophysiological processes linked to speech comprehension.ResultsAmplitude-modulated (sinusoidal 41.5 Hz) auditory verbal and nonverbal stimuli served to drive steady-state oscillations in neural networks involved in speech comprehension. Stimuli were presented to 12 subjects in the following conditions (a) an incomprehensible string of words, (b) the same string of words after being introduced as a comprehensible sentence by proper articulation, and (c) nonverbal stimulations that included a 600-Hz tone, a scale, and a melody. Coherence, defined as correlated activation of magnetic steady state fields across brain areas and measured as simultaneous activation of current dipoles in source space (Minimum-Norm-Estimates), increased within left- temporal-posterior areas when the sound string was perceived as a comprehensible sentence. Intra-hemispheric coherence was larger within the left than the right hemisphere for the sentence (condition (b) relative to all other conditions), and tended to be larger within the right than the left hemisphere for nonverbal stimuli (condition (c), tone and melody relative to the other conditions), leading to a more pronounced hemispheric asymmetry for nonverbal than verbal material.ConclusionsWe conclude that coherent neuronal network activity may index encoding of verbal information on the sentence level and can be used as a tool to investigate auditory speech comprehension.

Magnetoencephalography and its Achilles' heel

Magnetoencephalography (MEG) has practically unlimited temporal resolution. Fundamental physical reasons, however, restrict the capability of MEG to separate simultaneously active sources. After a brief tutorial introduction into MEG, various aspects of spatial resolution are reviewed with the help of examples. First the estimation of a single current dipole is examined. A consideration of the resolution field shows that the spatial selectivity of the estimated dipole moment is highly dependent on methodological issues. A subsequent consideration of various two-dipole configurations illustrates how the topography of the magnetic field depends on the distance between the two dipoles and their relative orientations. The resolution fields associated with the estimation of the dipole moments reveal a strong interference for closely spaced dipoles. A simple model suggests that the standard deviations of the estimated moments are inversely proportional to the distance of the dipoles. Spatial information provided by techniques like functional magnetic resonance imaging (fMRI) could help to overcome problems resulting from the limited spatial resolution of MEG (multimodal integration). But a straightforward synthesis, according to the principle that fMRI provides the spatial structure of the sources and MEG adds the temporal information, is probably doomed to failure in many situations. A serious dilemma, among other problems, is that the fMRI signal generally represents a temporal integral over several seconds: The knowledge that a certain brain region was active sometime or other is not necessarily helpful for disentangling the MEG activity within a specified short time window. An intriguing fact is that the spatio-temporal pattern of the MEG signals can be considered as a signature of the brain which is suitable for hypothesis testing with high temporal and spatial resolution.

Electromagnetic analysis of effective and apparent diversity gain of two parallel dipoles

Two parallel dipoles are often used as a reference case for measuring diversity gain. The present paper shows how to calculate the radiation efficiency, effective diversity gain, and apparent diversity gain of two parallel dipoles and how they depend on dipole spacing. We treat several cases for different terminations of the neighboring dipole; open-circuited, short-circuited, and 50 /spl Omega/ termination. The results for 50 /spl Omega/ termination are verified by measurements of effective diversity gain in a reverberation chamber.

Estimation of the Coupling Between Closely Spaced Transmitters Using the Method of Auxiliary Sources

In this paper, the coupling between closely spaced dipole antennas is examined on the basis of the method of auxiliary sources (MAS). This method is applied by introducing sets of fictitious sources that are situated within the dipoles for the direct description of the radiated electromagnetic (EM) field. The currents of the sources are obtained by imposing the boundary condition of the electric field on the physical surface of each dipole. After that, the current distribution on each dipole is determined from the magnetic field boundary condition and as a direct outcome the driving and the radiation properties of the dipoles are readily computable. The proposed technique is applied to circular arrays of dipoles for the purpose of comparison with available results. Moreover, several representative arrangements of closely spaced antennas are studied. In each case, the influence of the mutual coupling phenomena over the input impedance and the radiation properties of the antennas is investigated and the minimum spacing for their desensitization against mutual coupling is discussed.

Fully differential single-top-quark cross section in next-to-leading order QCD

We present a new next-to-leading order calculation for fully differential single-top-quark final states. The calculation is performed using phase space slicing and dipole subtraction methods. The results of the methods are found to be in agreement. The dipole subtraction method calculation retains the full spin dependence of the final state particles. We show a few numerical results to illustrate the utility and consistency of the resulting computer implementations.

Identification of dynamical Lie algebras for finite-level quantum control systems

The problem of identifying the dynamical Lie algebras of finite-level quantum systems subject to external control is considered, with special emphasis on systems that are not completely controllable. In particular, it is shown that the dynamical Lie algebra for an N-level system with symmetrically coupled transitions, such as a system with equally spaced energy levels and uniform transition dipole moments, is a subalgebra of so(N) if N = 2ℓ + 1, and a subalgebra of sp(ℓ) if N = 2ℓ. General criteria for obtaining either so(2ℓ + 1) or sp(ℓ) are established.

Supersensitivity in a chain of closely spaced electric dipoles with variable moments.

A chain of closely spaced oscillators is studied theoretically. The oscillators are interrelated electric dipoles whose moments may vary within a wide range. An expression for the oscillator interaction potential is suggested. On the basis of this potential, a one-dimensional nonlinear equation of motion is derived with allowance made for dissipation and external driving. A numerical investigation is carried out, and various nonlinear phenomena are revealed in the chain. Among them are the size effect and ultrasensitivity, i.e., a giant response of the chain to extremely weak periodic perturbations. The findings are compared with previously obtained experimental results on naturally occurring objects with similar structure. It is inferred that the model is realistic.

Comparison of phase space slicing and dipole subtraction methods for $\gamma^* \rightarrow Q\bar{Q}$

We compare the phase space slicing and dipole subtraction methods in the computation of the inclusive and differential next-to-leading order cross sections for heavy quark production in the simple process \(\gamma^* \rightarrow Q\bar{Q}\). For the phase space slicing method we study the effects of improvement terms that remove restrictions on the slicing parameter \(s_{\mathrm{min}}\). For the dipole method our comparison is a first check on some of its counterterms involving massive quarks, derived recently. In our comparison we address issues such as numerical accuracy and efficiency.

Experimental study for single-pass lead DDD pacing

We investigated the possibility of single-pass lead DDD pacing in animal experiments, using an original double balloon bipolar lead with variable dipole spacing. In the present study, P-wave sensing was best with a narrower dipole spacing, a smaller electrode surface area, and with the placement of atrial electrodes in the upper region of the atrium. The pacing threshold was not significantly different in various atrial sites, whereas the threshold energy increased dramatically under conditions of truly floating atrial pacing. Additionally, our simulation analysis revealed that the pacing thresholds were lower with a smaller electrode surface and the optimal dipole spacing was dependent on the distance between the atrial wall and the stimulating electrodes. Since stimulation at a distance from the atrial wall is barely feasible, further studies are required to achieve single-pass lead DDD pacing.

Modeling parameters for the spectral behavior of infrared frequency-selective surfaces

Comparisons of experiment and theory are presented for transmission spectra over the range 2-15 mum of a set of frequency-selective surfaces consisting of arrays of simple dipole patches of aluminum on or in silicon. The arrays are fabricated by direct-write electron-beam lithography. Important parameters controlling the spectral shape are identified, such as dipole length, spacing, resistance, and dielectric surroundings. The separate influence of these variables is exhibited. Encouraging agreement between simple model calculations and the measurements is found.

3D resistivity inversion using 2D measurements of the electric field

Field and ‘noisy’ synthetic measurements of electric‐field components have been inverted into 3D resistivities by smoothness‐constrained inversion. Values of electrical field can incorporate changes in polarity of the measured potential differences seen when 2D electrode arrays are used with heterogeneous ‘geology’, without utilizing negative apparent resistivities or singular geometrical factors. Using both the X‐ and Y‐components of the electric field as measurements resulted in faster convergence of the smoothness‐constrained inversion compared with using one component alone. Geological structure and resistivity were reconstructed as well as, or better than, comparable published examples based on traditional measurement types. A 2D electrode grid (20 × 10), incorporating 12 current‐source electrodes, was used for both the practical and numerical experiments; this resulted in 366 measurements being made for each current‐electrode configuration. Consequently, when using this array for practical field surveys, 366 measurements could be acquired simultaneously, making the upper limit on the speed of acquisition an order of magnitude faster than a comparable conventional pole–dipole survey. Other practical advantages accrue from the closely spaced potential dipoles being insensitive to common‐mode noise (e.g. telluric) and only 7% of the electrodes (i.e. those used as current sources) being susceptible to recently reported electrode charge‐up effects.

Determination of internal stresses in cyclically deformed copper single crystals using convergent-beam electron diffraction and dislocation dipole separation measurements

Single crystals of copper were cyclically deformed, in single slip, to presaturation at 298 K. The dislocation substructure was analyzed using conventional bright-field and dark-field transmission electron microscopy with particular attention directed towards the dislocation dipole spacing. It was found that, in both metals, the dipole spacing and statistical distribution of spacings were independent of the location in the heterogeneous substructure, which consisted of dense dipole bundles (or veins) and channels with relatively low dislocation density. Furthermore, the stress to separate the dipoles with largest spacing (upper-bound separation for stable dipoles) can be used to calculate the stress at the dipole location. This stress is within a factor of about two of the applied stress in both channels and veins. The stress necessary to pass dislocations through the dense veins was calculated to also be within a factor of about two of the applied stress. Convergent-beam electron diffraction (CBED) experiments were also performed at several locations very near the dipole bundles and within the channels. The lattice parameter measurements also suggested an absence of long-range internal stresses. The observations and calculations suggest a uniform state of stress throughout the heterogeneous dislocation substructure, without the presence of significant internal stresses.

New model for the computation of quasi-stationary fields due to arbitrary distributions of magnetic dipoles

The quasi-stationary field of a magnetic dipole in free space is modeled by using the field due to a succession of dipoles oriented along any line between the poles of the original. Dipole, the two fields being identical everywhere except for the points on that line. By choosing appropriately the path of the equivalent string of dipoles, the resultant field due to the original dipole can be obtained from the superposition of fields with a simpler structure. In this paper, the proposed modeling method is illustrated for the quasi-stationary ac field of an arbitrarily located and oriented dipole in the presence of a conducting spherical shell, with the analytic expression of the field quantities derived in terms of single series, which are much more convergent than the double series in the expressions available in the literature. The model presented can be applied efficiently for analyzing eddy-current problems and electromagnetic shielding by various spherical, cylindrical, and planar structures in the presence of arbitrary distributions of electric current.

Transient and steady‐state conduction in ethyl cellulose (EC)–poly(methyl methacrylate) (PMMA) blends

Transient discharging currents and steady-state conduction in solution-grown ethyl cellulose (EC)–poly(methyl methacrylate) (PMMA) blends measured as a function of temperature (30–80 °C) and field strength (10–100 kV cm−1) are reported. Transient currents are found to follow the Curie–VonSchweidler law, characterized by different slopes in short- and long-time regions, having different decay constant values lying between 0.75–0.99, and 1.68–1.95. The corresponding activation energies are found to increase with time of measurement of discharge current. Isochronal characteristics (ie current versus temperature plots at constant times) constructed from the data seem to reveal a broad peak observed at 60 °C. The dependence of dark current at different temperatures (30–80 °C) in a metal (1)–EC–PMMA blend–metal (1)/(2) system on the applied voltage in the range 10–100 kV cm−1 has also been studied; the current is found to be strongly temperature dependent. Dipole polarization and space charge resulting from trapping of injected charge carriers in energetically distributed traps and induced dipoles created because of the piling up of charge carriers at the phase boundary of the heterogeneous components of the blend are considered to account for the observed transient currents. The results of current–voltage measurement on blends are interpreted to show that the low-field steady-state conduction is ohmic in nature, and in high fields the charge carriers are generated by field-assisted lowering of coulombic barriers at the traps and are conducted through the bulk of the material by a hopping process between the localized states by a Jonscher–Ansari Poole–Frenkel mechanism. The modified P–F barrier is calculated to be 1.89 × 10−19 J (1.18 eV), 1.92 × 10−19 J (1.20 eV) and 1.95 × 10−19 J (1.22 eV) for P1, P2 and P3 blends, respectively. © 2000 Society of Chemical Industry