Field Intensity Measurements Research Articles

Reflector antenna diagnosis from phaseless data

The problem of determining the surface profile or a reflector antenna from only amplitude measurements arises whenever the phase cannot be accurately detected. We apply the quadratic approach to solve this holography problem by looking directly for the aperture field function from measurements of field intensities at the feed location under different defocusing conditions, which can be easily achieved in testing radioastronomical antennas. A suitable finite-dimensional approximation of the unknown function and two sets of square amplitude data are employed. The solution procedure consists of minimizing a nonquadratic functional and can be critically affected by the existence of local minima. This crucial problem is discussed and solved and the procedure is applied to both simulated and experimental data. The reliability of the whole algorithm is confirmed by the fact that results are obtained starting from completely random initial guesses. A viable and simple way to determine the positions of the individual panels making up large radioastronomical antennas is also employed.

Optical spectroscopy diagnostics of a helium surface wave sustained discharge. II: modelling and evaluation of experimental data

A theoretical model for the kinetics of the processes for production and destruction of atom excited states in microwave discharges sustained in helium gas at reduced pressures by surface wave propagation is developed. The excited state population densities and the line intensities at the axis of the discharge are obtained from the stationary Boltzmann equation and the set of the rate balance equations for the excited states solved in a semi-analytical - semi-numerical manner. The numerical evaluation of the results accounts for the experimental conditions investigated in part I of the study. The comparison of the theoretical results with the experimental data (part I) for the population densities of the , , and excited states allows one to deduce the values of the mean electron energy and the electron density at the axis of the discharge. By these means, the two parts of the study complete a procedure for determination of these two plasma parameters in discharges sustained by propagating surface waves. This procedure for plasma diagnostics is extended to obtaining the radial and axial profiles of the electron density. The measured radial and axial distribution of the 504.7 nm line emitted intensity and radial distribution (through line absorption) of the and population densities (part I) are used. The axial density distribution obtained from completing the optical spectroscopy measurements with the theoretical model is in good agreement with that evaluated from radial decay measurements of the surface wave electric field intensity.

Maps of the magnetic anomaly field at Earth's surface from scalar satellite data

Satellite measurements of the magnetic anomaly field intensity have been used to estimate components of the vector field at the Earth's surface, using a linear approximation to relate satellite scalar measurements to downward‐continued vector components. The relation between data and model was inverted using a depleted harmonic spline basis to find an almost minimum norm solution, enabling the detail present in satellite data to be preserved over continental‐sized areas. In their development of the depleted basis method, Parker and Shure (1982) advocated using a subset of the original data as points in the depleted basis. Here, it is shown that a depleted basis formed from vertical component data kernels is much more satisfactory than a subset of measured intensity data kernels; however, we do form the depleted basis at the physical locations of actual data. The downward‐continued map over Africa agrees well with its counterpart produced from Magsat orthogonal component data.

Traceability of electric field intensity measurements at frequencies from 0 TO 1000 MHz

The present state of the traceability of electric field intensity measurements at frequencies from 0 to 1000 MHz is considered. Calibration schemes of electric field measuring instruments at 0 to 20 kHz and 0.0003 to 1000 MHz are proposed.

Photon scanning tunneling microscopy of optical channel waveguides

Photon scanning tunneling microscopy (PSTM) has been used to characterize optical channel waveguides. Both He Ne laser and semiconductor diode laser wavelengths are utilized in the measurement of the evanescent field intensity associated with the propagating modes of optical waveguides. A tapered optical fiber tip, and also a semiconductor heterostructure tip attached to an optical fiber are employed for detection. Local values of effective refractive index are calculated from measurements of both TE and TM polarizations and compared to model calculations.

Experimental studies of electrohydrodynamics inside a two-stage electrostatic air cleaner

Experimental investigations were conducted for design of the discharge electrode in a two-stage wire-plate electrostatic air cleaner, which includes the sizing of corona wire diameter, plate length and wire-to-plate distance. Measurements of the electric field and particle charge intensity in section of discharge electrode were carried out by using Langmuir probe and aerosol electrometer, respectively. The effect of the design parameters on the particle discharge performance was evaluated and examined.

Use of integral Doppler anemometry in field-flow fractionation

Integral Doppler anemometry (IDA) is applied in analytical field-flow fractionation (FFF) of particles by registering the transverse concentration profiles of particle mixtures in FFF channels. Several applications of IDA in FFF are considered. First, its use in FFF systems with a linear focusing force is discussed, for two qualitatively different schemes: (a) a conventional FFF scheme with a single probe injection; and (b) a laterally non-equilibrium, time-independent scheme with a steady-state particle concentration at the channel inlet. In the conventional scheme IDA allows direct and precise measurements of the equilibrium focusing positions of fractions and, if necessary, the registration of the usual elution curve. In the stationary non-equilibrium regime it allows the analysis time and channel length to be decreased considerably compared with the conventional FFF regime. Second, the possibility of IDA measurements of the lateral field geometry and intensity inside FFF channels by registering the characteristic trajectories of test particles is shown theoretically and experimentally. The specially developed kinematic formalism valid in the case of large transverse Peclet numbers, i.e., for strong enough fields and/or large enough particles, is used. It allows the time-dependent concentration distribution of particles in a flat channel flow with a lateral force applied (FFF conditions) to be obtained analytically for arbitrary profiles of velocity and force immediately following the probe injection.

Geomagnetic field reconnaissance in Kuwait

Nearly 150 measurements of the total magnetic field intensity were made in Kuwait in early 1988. Observation points are located primarily along major highways over lines varying in length from few to several tens of kilometers. Measurements were made with a proton magnetometer. Station spacing varied from nearly 1 to 5 km. Diurnal correction to the data was made possible by frequent excursions to the base station. Preliminary analysis of the data indicates that the field intensity undulates in SW-NE direction nearly perpendicular to the regional tectonic trend. The amplitude of these variations range approximately from 30 to 90 γ over wavelengths varying nearly from 38 to 70 km. We have made a combined inversion of the data with the unpublished Bouguer gravity map of the country along a N-S profile in the central region and find that the data are reasonably well matched with predictions of a simple two-dimensional block model. The model consists of a layer overlying a half-space. The variables of the model are the thickness and density of the surface layer and the susceptibility of the basement rock. The observed variation of the magnetic field and Bouguer anomalies for the selected profile may be accounted for either by the simultaneous northward density reduction of about 0.06 g/cm 3 in the surface layer and approximately 15% increase of the basement susceptibility, or else by nearly 800 m variation of the sedimentary thickness.

III-V ternary semiconductor heterostructures: The choice of an appropriate compositional analysis technique

The composition of ternary III-V semiconductor heterostructures can now be measured in a number of ways. Conventional spectroscopic probe techniques are inappropriate for fine (nanometer scale) heterostructures: Here, those transmission electron microscopy techniques which are appropriate, are compared emphasizing that different alloy systems often require different approaches. The three representative methods discussed in detail are kinematic convergent beam diffraction, 200 dark field intensity measurement, and [001] bright field thickness fringe matching. Dark field intensity measurement is found to be the most sensitive analytical method for many alloys, but the accuracy that may be achieved by thickness fringe matching is generally little worse, and sometimes better. Those convergent beam diffraction techniques which rely on lattice parameter measurement would not appear to be particularly useful except for the purposes of distinguishing nonunique interpretations of other measurements. It is also noted that no technique currently available is capable of measuring the composition of AlxGa1−xSb alloys to an accuracy of better than ±0.05 in x, so that further technique development will be required if heterostructures in this alloy system are to be studied seriously.

Photon scanning tunneling microscopy

An optical tunneling microscope is presented that operates in exactly the same way as the electron scanning tunneling microscope (ESTM). It takes advantage of evanescent fields generated by the total internal reflection (TIR) of light at the interface between materials of different optical densities. The photon scanning tunneling microscope (PSTM) employs an optically conducting probe tip to map spatial variations in the evanescent and scattered field intensity distributions adjacent to a sample surface, which forms or is placed on the TIR surface. These variations are due to the local topography, morphology, and optical activity of the surface and form the basis of imaging. Evanescent field theory is discussed and the evanescent field intensity as a function of surface-probe separation is calculated using several probe tip models. After a description of PSTM construction and operation, evanescent field intensity measurements are shown to agree with the model calculations. PSTM images of various sample surfaces demonstrate subwavelength resolution exceeding that of conventional optical microscopy, especially in the vertical dimension. Limitations and interpretation of PSTM images are discussed as well as the PSTMs applicability to other forms of surface analysis.

II-VI Semiconductor Heterostructures: Pointers to Appropriate Analysis Techniques

ABSTRACTApproximate methods are used, for a variety of II–VI semiconductor alloys, to estimate the sensitivity to composition change of quantitative transmission electron microscopy techniques which have proved successful in characterising III–V heterostructures. It is shown that bright field thickness fringe matching at the [001] axis is likely to prove relatively more successful than 200 dark field intensity measurement for many alloy systems. It is also noted that alternative methods would be necessary if quantitative characterisation of (Mn,Zn) compounds were required.

Radiofrequency radiation in Sao Paulo City (Brazil)

Abstract This paper summarizes the results of eight series of radiofrequency field intensity measurements carried out by CETESB in the urban area of São Paulo City. The main objective in conducting these measurements was to obtain some information about the field intensities which can be produced by VHF broadcast antennas and the associated level of public exposure to such radiations. The power densities derived from electric field intensity measurements varied from 1 W.cm‐2 on ground level, to 336 μW.cm‐2 at a building roof under FM and TV transmission tower.

In-situ magnetostratigraphy: interpretation of magnetic logging in sediments

The interest of magnetic logging in different geological settings: oceanic crust, sedimentary basins and deep continental crust is discussed. In all these cases, a good estimate of both the magnetic susceptibility and the remanence of the surrounding rocks could yield invaluable information about the geological formations penetrated in boreholes. A calculation of all the components of the magnetic field produced in a borehole by horizontal magnetized layers is presented. The variations of these components are calculated along the axis and off axis through simple geological formations and particularly through a zone of transition of polarity of the earth's magnetic field. Both susceptibility and remanence are involved in the interpretation of the field variations recorded when a magnetometer is passed through a borehole drilled in sediments; the separation of these variations is envisaged using magnetic field and susceptibility logs recorded simultaneously. We present an example of magnetic logging in sediments by the measurement of total field intensity. The results are compared with a model calculated from laboratory measurements of the natural remanent magnetization and magnetic susceptibility of cores.

Measurement of the electric field intensity by a spherical measuring transducer near a conducting surface

Measurement of the electric field intensity by a spherical measuring transducer near a conducting surface

Electric Field Measurement in Liquid Dielectrics Using a Combination of AC Voltage Modulation and a Small Retardation Angle

A modified Kerr electro-optic effect technique has been applied to the direct measurement of electric field intensity in dielectric liquids such as nitrobenzene, tricresylphosphate, benzene, transformer oil and silicone oil. In particular, by utilizing the incident elliptically polarized light with retardation angle ?=1 degree and modulating applied alternating voltage, it was possible to measure the electric field using short electrode lengths of 6 cm for low Kerr constant liquids such as transformer oil and silicone oil.

Influence of power frequency electric and magnetic fields on human health.

Several reports have appeared in the last ten years in which the authors suggest an association between cancer/leukemia and exposure to electric and magnetic fields in the workplace or in the vicinity of distribution lines from overhead transmission systems. Several of these reports are reviewed and critiqued. The reports of clinical effects of electric and/or magnetic fields among human populations present serious difficulties which stem from the many publications in which pertinent material is not presented, and the data are variable or internally inconsistent. The reports presented so far are not very probing. The numbers of subjects are small, the epidemiologic methodologies are often weak, measurements of intensity of field are absent, and the statistical analyses utilized are not always appropriate. Although suggestive associations of electric/magnetic fields and cancer or leukemia have been made, no one has established a causal relationship between these fields and cancer or leukemia. All the reports on human exposure have numerous deficiencies which include: lack of or imprecise measurements of electric or magnetic field intensities, questionable subject identification, lack of statistical significance, confounding with uncontrolled variables such as socioeconomic differences, smoking, X rays, drugs, population mobility, and the unreliability of occupational classification. Nevertheless the possibility of a link between leukemia and cancer and exposure to electric and magnetic fields has been raised and only responsible research can refute or confirm these reports.

Magnetic field observations of the Sabzevar ophiolitic complex of northeast Iran

Surface measurements of the total magnetic field intensity along three sections transverse to the structural trends of the Joghatay Mountains ophiolitic complex extended north and west of Sabzevar, northeast Iran, are analyzed. The observed variations of the magnetic field intensity are compared with the synthetic results computed for a block structure consisting of several east-west oriented vertical parallelepiped down to 10 km depth which is also in agreement with the mapped shallow structural features of the complex. Shoaling of the basement and/or Curie surface may explain the observed northward decrease of the mean field measurements. However, in this study the variations are modeled in terms of magnetic enrichment of southern blocks.

Magnetic and electric measurements on the island of Lesbos and the detection of buried ancient relics

Total geomagnetic field intensity measurements have been performed with a portable magnetometer in a gridded archaeological site. Their analysis reveals linear features which correspond to ancient buried relics. Resistivity soundings in the same area allowed estimates of the thickness of the soil with respect to bedrock and revealed artificially made voids.

Multiple-source, multiple-frequency error of an electric field meter

Electric field meters (EFM's) are typically calibrated using single-frequency, single-source standard fields. The response to multiple sources or nonsinusoidal time dependence may be different, however. Possible errors in a multiple-source, multiple-frequency environment are analyzed for an EFM consisting of an electrically short dipole antenna with a diode load and a radio-frequency (RF) filter transmission line. Also considered are errors in the assumption of equal electric and magnetic energy densities in a multiple plane-wave environment. Typical errors of field-intensity measurements are about one to 3 dB, but in some circumstances they can exceed 10 dB.

The Shape of Analytical Curves in Zeeman Atomic Absorption Spectroscopy III: Extended Dynamic Range

The analysis of the shape of analytical curves in Zeeman atomic absorption spectroscopy has been extended towards very high concentration. Beyond the absorption maximum the slope of the Zeeman analytical curve depends on the remaining absorption at maximum field strength. The absorbance and concentration at roll-over can be estimated from the initial slope of the Zeeman analytical curve at low concentration and the decreasing part of the curve at very high concentration. It is shown that the dynamic range of atomic absorption analytical curves can be readily extended with an ac Zeeman system. Analytical curves of reduced sensitivity are obtained when the maximum field intensity measurement is combined with a measurement at intermediate field strength rather than at zero field strength. However, the maximum absorbance in the extended range Zeeman analytical curve is lower than in the normal Zeeman analytical curve.