Far-field Distance Research Articles

Recordable depth of view and allowable farthest far-field distance of in-line far-field holography for micro-object analysis

We have obtained, based on the general irradiance distribution of in-line far-field holography, analytical solutions of the recordable depth of view (RDV) and the allowable farthest far-field distance (AFFD) of the in-line far-field holography for different illumination modes. The analytical solutions of RDV and AFFD show that AFFD cannot be limited if micro-objects are positioned in one special space and illuminated with convergent beams, but the RDV is not improved simultaneously. When micro-objects are placed in another special space and illuminated with a divergent beam, the RDV and AFFD can be improved simultaneously, but the recordable object space is split into two subspaces. These results are important in the design of a holographic recording system.

Teleseismic waveform modeling including geometrical effects of superficial geological structures near to seismic sources

AbstractWe analyze observed seismograms of 21 events recorded at teleseismic distances in France from nuclear explosions detonated at Nevada Test Site (NTS). Variations of the displacement waveform, duration, and amplitude are studied in terms of influence of the explosion's source medium and location in the laterally heterogeneous Yucca Flat basin. The analysis is made using numerical modeling of data that simulate the modifications of the original source signal by the geometry of the surrounding geological structure. Spalling and nonlinear effects are not included in computations. The numerical simulations of the variations are processed using a mixed symbolic and numerical algorithm developed to simulate the ground motions that may be recorded in any kind of two-dimensional heterogeneous nonvertical structures. This algorithm is linked (1) to the discrete wavenumber-boundary integral equation method and to the reciprocity theorem to simulate the displacement field radiated to far-field distances by the source site geological structure, and (2) to ray propagation to propagate the displacement field across the mantle from the source region to the receiver. Variations of the computed displacement amplitudes are as large as a factor of 2 from one detonation point to another. The shapes of the observed seismograms are modeled with good reliability for most of the 21 events in the frequency band studied, i.e., 0.2 to 2.5 Hz. A set of 21 relative yield estimates are derived, which include the source site response and thus the amplitude variations induced by the heterogeneous structure of the source region.

Numerical solution of Euler equations for aerofoils in arbitrary unsteady motion

AbstractThis paper is part of a DLR research programme to develop a three-dimensional Euler code for the calculation of unsteady flow fields around helicopter rotors in forward flight. The present research provides a code for the solution of Euler equations around aerofoils in arbitrary unsteady motion. The aerofoil is considered rigid in motion, and an O-grid system fixed to the moving aerofoil is generated once for all flow cases. Jameson's finite volume method using Runge-Kutta time stepping schemes to solve Euler equations for steady flow is extended to unsteady flow. The essential steps of this paper are the determination of inviscid governing equations in integral form for the control volume varying with time in general, and its application to the case in which the control volume is rigid with motion. The implementation of an implicit residual averaging with variable coefficients allows the CFL number to be increased to about 60. The general description of the code, which includes the discussions of grid system, grid fineness, farfield distance, artificial dissipation, and CFL number, is given. Code validation is investigated by comparing results with those of other numerical methods, as well as with experimental results of an Onera two-bladed rotor in non-lifting flight. Some numerical examples other than periodic motion, such as angle-of-attack variation, Mach number variation, and development of pitching oscillation from steady state, are given in this paper.

Pattern measurement distance for broad beam antennas

Measurement distance requirements for electrically large, broad beam antennas are investigated in this paper. Design guidelines are developed for determining far-field measurement distances for various pattern types. Results show that while the popular 2D2/λ measurement distance usually ensures accurate measurements of broad beam antenna patterns, measurement distances less than 2D2/λ can provide acceptable results.

Resolution characteristics of holographic particle image velocimetry

The resolution characteristics of in-line holographic imaging systems for application to velocity field measure- ment in turbulent and other vortical flows has been investigated experimentally. It is shown that the diameter of seed particles strongly influences the maximum size of the test volume that can be used as well as the spatial resolution of the measurement. Velocity measurements in a vortex ring flow using a two-axis holographic recording system are reported. The spatial resolution of this system is approximately 1 mm. OLOGRAPHIC imaging of flows seeded with small particles can be a useful technique for velocity field mea- surement in turbulent and other vortical flows. Several investi- gators have reported velocity measurements using holographic techniques.1'2 Two basic approaches have been proposed. In most cases in-line doubly exposed holograms of the seeded flow are obtained. The velocity is determined from the dis- placement of the particles on the reconstructed images. This approach is what we call holographic particle image velocime- try (HPIV) because it is a natural extension of planar particle image velocimetry.3 A second approach4'5 is to obtain a tempo- ral sequence of holograms. The velocity is determined by tracking individual particles as a function of time. The main advantage of HPIV compared to particle tracking techniques is that the particle displacement is determined by local analysis of the reconstructed images which results in reduced image processing requirements. Turbulent flow measurement requires a large spatial dy- namic range. This requirement becomes more severe as the Reynolds number is increased. A large spatial dynamic range requires imaging of very small particles in a large test volume. Research on in-line holographic imaging systems for particle size measurement has shown a limit to the test volume depth of 50-80 far-field distances.6'7 (The far-field distance is defined here as d2/ where d is the diameter of the particle and X the wavelength of the laser used to record the hologram.) The depth of field of the reconstructed particle images is of the order of one far-field distance.8 The depth of field of the particle image is a good measure of the spatial resolution of the velocity measurement. It follows that the resolution character- istics of the holographic imaging system can severely limit the spatial dynamic range of HPIV. In this paper we examine experimentally the resolution char- acteristics of in-line holographic imaging systems. A two-axis holographic recording system designed for high resolution ve- locity field measurements is described. Finally, velocity mea- surements in a vortex ring flow using this HPIV system are reported. The dynamic range requirements of typical HPIV recording systems dictate that the hologram be positioned at a large distance from the particles. In this case the light intensity distribution on the hologram plane is given by Fraunhofer theory9 _ , Trt/2 . 1=1- —— sin 2\z . Air2 |~ /T£/A/Tcfr) —— sin —— } \2Ji —— / —— 2\z \Xz/ |_ \\z/l XzJ '<Kdr

Gaussian beam-mode analysis and phase-centers of corrugated feed horns

The notion of phase center for a feed-horn is critically examined, and, using Gaussian beam-mode analysis, several variously-defined phase centers, including a maximal-gain phase center, are investigated in detail for corrugated feed-horns in balanced hybrid mode. Expressions for the locations of these phase centers are derived which are applicable for a wide range of corrugated horn dimensions and for near-field as well as far-field distances.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of phase-focused near-field testing for multiphase-center adaptive radar systems

Airborne or spaceborne radar systems often require tests before deployment to verify how well the system detects targets and suppresses clutter and jammer signals. The radar antenna diameter can be large and thus the conventional far-field test distance is impractical to implement. The theory and simulations of phase-focused near-field testing for adaptive phased array antennas is discussed. With near-field source deployment, standard phased-array near-field phase focusing provides far-field adaptive nulling equivalent performance at a range distance of one aperture diameter from the adaptive antenna under test. Both main beam clutter sources and sidelobe jammer sources are addressed. The phased array antenna elements analyzed are one-half wavelength dipoles over the ground plane. Bandwidth, polarization, array mutual coupling, and finite array edge effects are taken into account. Numerical simulations of an adaptive antenna that has multiple displaced phase centers indicate that near-field and far-field testing can be equivalent. >

Measurement distance effects on Bayliss difference patterns

The effects of measurement distance in distorting low-sidelobe difference patterns are examined. Previous calculations have used obsolete suboptimum aperture distributions. The Bayliss linear distribution is a versatile, highly efficient and robust optimum distribution; its use allows a single curve of sidelobe measurement error versus measurement distance (normalized to far-field distance 2D/sup 2// lambda ) for a given sidelobe level. Data are given for patterns from a uniform distribution to a 50-dB Bayliss. Difference patterns require slightly larger measurement distances than sum patterns. For example, the first sidelobe of a 40-dB Bayliss pattern is in error by 1 dB at a distance of 7D/sup 2// lambda . The results should apply approximately for circular apertures as well. >

Aberration limited resolution in Fraunhofer holography with collinated beams

The primary aberrations in standard in-line Fraunhofer holography with collimated beams of different recording and reconstruction wavelengths are described. Maréchal's criterion is then applied to establish the aberration limited size resolution of micro-objects. The resolution is described in terms of the recording and reconstruction wavelengths, the number of side lobes recorded, and the number of far-field distances. The particular case of a ruby laser recording with reconstruction by a HeNe laser is discussed in detail.

Technical memorandum: Phase retrieval radio holography in the Fresnel region: tests on the 30m telescope at 86 GHz

The application of phase retrieval radio holography in the Fresnel region has been studied using a 86 GHz transmitter as signal source. Maps of the aperture-plane field distribution were made with a surface resolution of about 100 independent pixels across the telescope diameter. Over the 2.7 km propagation path used (0.005 of the farfield distance), the measurement accuracy was found to be determined by atmospheric scintillation. The root mean square measurement errors in the derived aperture-plane phase distribution were about 65 microns when expressed as axially resolved surface errors. The signal to noise ratio and dynamic range requirements are easy to fulfill in the Fresnel zone, while the necessary near field corrections can be applied with high accuracy. There are thus definite advantages to phase retrieval radio holography in this near field zone.

Two-dimensional transonic aerodynamic design method

This paper demonstrates the capabilities of a new transonic, two-dimensional design method, based on the simultaneous solution of multiple streamtubes, coupled through the position of, and pressure at, the streamline interfaces. This allows the specification of either the airfoil shape (direct, analysis mode) or the airfoil surface pressure distribution (inverse, design mode). The nonlinear system of equations is formulated in a conservative manner, which guarantees the correct treatment of shocks, and is solved by a rapid Newton solution method. Viscous effects can also be included through a coupled integral boundary-layer analysis. The first set of results shows the effect of different far-field treatments, demonstrating the improvement in accuracy obtained by including the second-order doublet terms in addition to the usual first-order vortex term. The results are also compared to those obtained by specifying straight far-field streamlines (corresponding to solid-wall wind-tunnel experiments) or constant far-field pressure (corresponding to freejet experiments) to show the sensitivity to the farfield distance. In the second set of results, the design method is used to design a transonic airfoil with C/ = 1.000 at A/oo = 0.70. It is shown that the off-design performance is improved by specifying a surface pressure distribution with a very weak shock.

Electromagnetic Near Fields as a Function of Electrical Size

The near fields of a radiating rectangular plate placed over and normal to an infinite horizontal ground plane have been analyzed as a function of the electrical size of the plate. Both vertical and horizontal currents are taken into account. Two different methods are used to compute the near fields. The variation of the electric field along the normal to the plate is studied in detail as a function of the electrical length L/λ of the plate. The electric-field magnitude decreases as distance r increases until an oscillatory region is encountered about one wavelength from the plate. Oscillations continue until distance is equal to L2 /4λ, or one-eighth of the usual far-field distance. The location of various minima and maxima can be predicted by a consideration of Fresnel theory. Comparison is made with the near fields of a circular aperture.

Transient Heat Conduction from Buried Underground Radioactive Nuclear Waste

An exact analytical solution for transient heat conduction in the ground from buried radioactive nuclear waste has been derived. The heat sources are assumed to be an array of arbitrarily located point sources in an infinite ground. The solution accounts for orthotropic thermal conductivity effects in three dimensions and for the timevarying nature of the heat generation rate of each source due to radioactive decay. Owing to the point source nature of the assumed heat sources, the solution derived is valid only at intermediateand far-field distances from real finite-size sources. Numerical results are presented for several typical waste repository situations in shale, salt, and basalt.

Simulation of a proposed near-field-to-far-field antenna measurement system

A new antenna-pattern measurement method for antennas with large far-field distances is proposed. The method is applicable to conventional antenna ranges with two axis mounts and with pattern recording in amplitude only, It requires only a few minor items of additional equipment. A computer simulation has been carried out and the results are presented.

How far is far?

Recent comparisons of computed and measured backscattering cross sections of cylindrical bodies revealed unexpectedly large discrepancies which were apparently due to the finite range used in the measurements. This led to a reexamination of the farfield criterion for a variety of two-dimensional scatterers and one three-dimensional object. It was found that large errors can be incurred in measurements at the standard far-field distance, especially of bodies characterized by strong edge scattering. Errors as great as 6 dB can occur and it may be necessary to exceed the usual farfield distance by a factor of 5 or more in order to reduce the error to 1 dB.

Development of a Spinning-Mode Synthesizer to Study Complex Propagating Duct Modes

The use of commercial fan engines or model fan test rigs in studies attempting to relate the pertinent parameters of spinning modes to the acoustical characteristics of various duct treatments has been frustrated by the presence of a high level of broad-band noise and all the harmonics of the discrete tone spectrum. These facts reduce the possibility for meaningful parameter variation. Therefore, a flexible source of spinning modes has been developed which is capable of generating independent spinning waves of controlled complexity and spin speed without the introduction of broad-band elements. These features were accomplished through the use of broad-band elements. These features were accomplished through the use of eight commercial loudspeakers located in an equally spaced circular array and properly phased so that the system could generate a spinning wave. The constructed apparatus was tested in an anechoic environment and found capable of generating a plane, one- and two-lobed spinning wave of high quality at 120 dB, and a farfield distance of 7 ft. The phase velocities of those waves investigated varied from 8000 to 18 000 rad/sec, which represented propagating modal patterns.

On Short Antennas Immersed in Two Dielectrics

With the assumption that the customary current distribution and far-field distance approximations apply, this paper develops the far-field expressions for short linear antennas (2h=l<λ/10), each half immersed perpendicularly h meters in a dielectric. As an illustration of topics such as linear antennas, reflection and refraction at a dielectric interface, phasor addition of waves from several sources, etc., for the undergraduate student, the paper demonstrates that the E and H far fields at a given point in the first dielectric are invariant to the second dielectric provided the first dielectric is not changed.