Showing papers on "Fresnel zone published in 1991"
TL;DR: Approximate expressions for the aperture-averaging factor of optical scintillation in the turbulent atmosphere are developed that are within a factor of 2 of the measurements.
Abstract: We have developed approximate expressions for the aperture-averaging factor of optical scintillation in the turbulent atmosphere. For large apertures and weak path-integrated turbulence with a small inner scale, the variance of signal fluctuations is proportional to the -7/3 power of the ratio of the aperture diameter to the Fresnel zone size. If the inner scale is large, the variance is proportional to the -7/3 power of the ratio of the aperture diameter to the inner scale. In strong path-integrated turbulence, two scales develop. That portion of the variance associated with the smaller scale is proportional to the -2 power of the ratio of the aperture diameter to the phase coherence length. That portion of the variance associated with the larger scale is proportional to the -7/3 power of the ratio of the aperture diameter to the scattering disk. These simple approximations are within a factor of 2 of the measurements.
213 citations
TL;DR: In this paper, the authors consider the case of monochromatic illumination of a region consisting of small perturbations to a homogeneous background, and compare the two reconstruction algorithms to reveal the effect of neglecting scattering.
Abstract: Tomographic imaging is now in widespread use in geophysical inversion. Most early work in this field used the ray approximation to wave propagation, but more recently scattering effects have been addressed via the formalism of diffraction tomography. However, if the correct image cannot be adequately represented as a perturbation of a simple background, this may demand considerably more computational resources than ray tomography. Therefore it is of some interest to determine the scalelength of variation below which scattering cannot be neglected, so that ray tomography is no longer reliable. In the hypothetical case of monochromatic illumination of a region consisting of small perturbations to a homogeneous background, the way in which the two reconstruction algorithms map the data into the Fourier components of the object may be directly compared to reveal the effect of neglecting scattering. These mappings begin to differ significantly at a wave‐number corresponding to the first Fresnel zone radius; it...
210 citations
TL;DR: Focusing of and imaging with atoms by means of a spherical Fresnel zone plate has been observed for the first time and its properties are presented and compared with numerical calculations.
Abstract: Focusing of and imaging with atoms by means of a spherical Fresnel zone plate has been observed for the first time. An intense beam of metastable helium atoms with atomic de Broglie wavelength ${\ensuremath{\lambda}}_{\mathrm{dB}}$=0.5--2.5 \AA{} is passed through either a single or a double slip with dimensions in the 10-\ensuremath{\mu}m range. This transverse intensity distribution is imaged by a Fresnel zone plate, 210 \ensuremath{\mu}m in diameter and with an innermost zone diameter of 18.76 \ensuremath{\mu}m. For ${\ensuremath{\lambda}}_{\mathrm{dB}}$=1.96 \AA{} the focal length is 0.45 m. The imaging properties of the zone plate are presented and compared with numerical calculations.
148 citations
TL;DR: In this article, a bilateral approach for high-frequency time-harmonic and transient radiation from extended aperture sources and propagation of the resulting fields through perturbing environments is facilitated by simultaneous use of configurational and spectral information for suitably defined synthesizing wave objects.
Abstract: Analytical modeling of high-frequency time-harmonic and transient radiation from extended aperture sources and of propagation of the resulting fields through perturbing environments is facilitated by simultaneous use of configurational (space-time) and spectral (wave number–frequency) information for suitably defined synthesizing wave objects. Such a bilateral approach can be embodied within a configuration-spectrum phase space. The present investigation deals with radiation from extended aperture sources, with emphasis on alternative uses of the phase space at high frequencies, on promising wave objects as basis elements for field synthesis, and on extraction of physical information from exact wave solutions by asymptotic methods. Of special interest are beam-type wave objects that exhibit localization in the phase space because localized wave fields have favorable propagation characteristics in complex external environments. In this paper, alternative phase-space parameterizations are applied to time-harmonic plane aperture distributions and to the corresponding fields radiated into a homogeneous half-space. The parameterizations include nonwindowed continuum versions, in which localization occurs asymptotically through constructive interference; windowed continuum versions, in which localization is embedded inherently; and windowed discretized versions, in which the basis elements are situated on a self-consistent configuration–wave number lattice. By analysis and illustrative examples, it is shown how these alternative formulations are interrelated, how the localization around well-defined regions in the phase space takes place in each formulation, and how these localization properties, through the beam propagators, influence the synthesis of the radiation field. Transient phenomena will be addressed in separate publications.
86 citations
Patent•
14 May 1991
TL;DR: In this article, the concept of a "tuned" Fresnel lens was used to provide multifocal vision correction for small incision surgeries. But the limitation of the traditional Fresnel lenses is that the light passing through the lens is phase synchronized.
Abstract: Vision correcting lenses utilizing the principles of both Fresnel lenses and Fresnel zone plates are disclosed. The lenses of the present invention provide multifocal vision correction, and are most preferably used to provide intraocular lenses for small incision surgeries. Using the concept of a "tuned" Fresnel lens, intraocular and other types of vision correcting lenses are provided by the present invention which overcome the limitations found in typical Fresnel lenses by phase synchronizing the light passing through the lens. As disclosed, the groove width and depth of the lens may be varied to provide the synchronization, and also to provide multifocal vision. Also, the design of the present invention permits a relatively thin lens to be designed, therefore making it particularly applicable to small incision surgeries since it may be rolled or folded into a configuration which is easily inserted into an incision in the eye.
76 citations
TL;DR: In this article, the performance characteristics of Fresnel zone plates are described with emphasis on applications at millimeter wavelengths, and recent results along with a summary from numerous earlier investigations are included along with the design information, efficiency, bandwidth, focal characteristics, axial intensity dependence, aberrations, and far-field pattern.
Abstract: The performance characteristics of Fresnel zone plates are described with emphasis on applications at millimeter wavelengths. Recent results are included along with a summary from numerous earlier investigations. Parameters described include design information, efficiency, bandwidth, focal characteristics, off-axis performance, axial intensity dependence, aberrations, and far-field pattern. Both transmission and reflection configurations are discussed, mostly for cases where the focal length and diameter are comparable. This type of zone plate has advantages (compared to a lens or paraboloid) of reduced loss, weight, volume, and cost, as well as simple planar construction, with similar diffraction-limited beamwidth and major sidelobe performance, but lower efficiency.
67 citations
TL;DR: In this paper, the authors investigated the zero-offset response to circular reflectors of increasing Fresnel zone size and showed that reflection response is a constant and is independent of reflector size, except when the reflector diameter is so small that the diffractions interfere with the primary reflection.
Abstract: The investigation of zero-offset response to circular reflectors of increasing Fresnel zone size shows that reflection response is a constant and is independent of reflector size, except when the reflector diameter is so small that the diffractions interfere with the primary reflection. The extent of this effect is dependent upon vertical resolution and the time separation of the primary reflector and the diffraction. Interference occurs for reflectors smaller in diameter than the first Fresnel zone. Migration removes this interference.For broadband data the Fresnel zone solution breaks into two parts: the primary reflector and the edge-effects diffractor. With broadband seismic data, reflections and diffractions separate in time, except at locations near faults or very small bodies. Reflections are the seismic response to interlayer discontinuity and are independent of reflector size. Diffractions are the seismic response to lateral discontinuities and edges and depend on proximity to--and geometry of--the edge. Except in the locale of an edge, broadband reflections and diffractions are separated physically on the section and mentally by the interpreter. Furthermore, standard CMP processing attenuates diffractions, especially when CMP lateral offset is some distance from the diffractor.
49 citations
TL;DR: In this article, the authors presented the first results of high-spatial resolution x-ray imaging studies with an upgraded version of the scanning photoemission multiple application undulator microscope, which is a multilayer coated Schwarzschild objective that focuses undulator radiation onto the sample.
Abstract: We present the first results of high‐spatial resolution x‐ray imaging studies with an upgraded version of the scanning photoemission multiple application x‐ray imaging undulator microscope. The microscope is a multilayer coated Schwarzschild objective that focuses undulator radiation onto the sample. The recent upgrade improved the spatial resolution by a factor six reaching a full width at half maximum value of 0.5 μm. Highly polished mirrors reduced the diffuse background by almost two orders of magnitude and drastically improved the contrast. The improved microscope was used to perform a series of tests on microgrids and reverse Fresnel zone plates. The microscope capability to detect chemical and topological contrast was verified by using patterned metal overlayers on Si and GaAs substrates. Further improvements to increase the flux and the spatial resolution are underway; this includes the installation of a new undulator beamline.
43 citations
TL;DR: This paper showed that the focusing properties of Fresnel zone plates predicted by Fraunhofer diffraction theory are incorrect except for resolution defined by the Rayleigh criterion, and showed that these results are not consistent with those derived from Fresnel diffraction theories.
Abstract: Results published previously [ J. Opt. Soc. Am.57, 610 ( 1967)] concerning the focusing properties of Fresnel zone plates were obtained by implicitly invoking the Fraunhofer approximation. A comparison of these results with those derived from Fresnel diffraction theory shows that the focusing properties of Fresnel zone plates predicted by Fraunhofer diffraction theory are incorrect except for resolution defined by the Rayleigh criterion.
26 citations
TL;DR: In this article, the authors derived formulas for the P-SV Fresnel zone radius for both surface and VSP geometries, and showed that the relative changes in size and shape of the Fresnel zones as a function of offset are not large and are comparable to P-P case.
Abstract: Mode‐converted (P-SV) reflections in field data contain information about an area on the reflecting boundary, rather than a single point. For unmigrated data, the total area contributing to the observed reflection amplitude is approximated by the Fresnel zone. We derive formulas for the P-SV Fresnel‐zone radius for both surface and VSP geometries. In the surface case, the Fresnel radius can be expressed in a form similar to that of the P-P case by substitution of the P-SV single‐layer migration velocity. Numerical modeling shows that the relative changes in size and shape of the P-SV Fresnel zone as a function of offset are not large and are comparable to the P-P case. For a given depth and frequency, the P-SV Fresnel radius is smaller than the P-P Fresnel radius by a factor of about 0.8; hence the lateral resolution of converted‐wave (P-SV) data may be somewhat better than the P-P data. Two sample calculations using field data are presented. For the VSP example, the P-SV Fresnel radius (184 m) is smaller...
TL;DR: Numerical calculations using the Prony method have shown that the feedback coefficient in unstable laser resonators can be increased by factors of as much as 5 over the geometrical value using a mirror with a phase step at its center.
Abstract: Numerical calculations using the Prony method have shown that the feedback coefficient in unstable laser resonators can be increased by factors of as much as 5 over the geometrical value using a mirror with a phase step at its center. A phase shift of close to π over an area of an equivalent Fresnel number of 0.5 leads to minimum output losses. Experiments with a TEA CO2 laser confirm the prediction. The results can be attributed to a cancellation of the output wave by destructive interference, which confines the laser beam around the resonator axis.
TL;DR: Arrays of coherent focused spots generated by writing multiple Fresnel zone plates on optically and electrically addressed modulators are demonstrated and phase modulators have a better conversion efficiency than amplitude modulators.
Abstract: We demonstrate and characterize arrays of coherent focused spots generated by writing multiple Fresnel zone plates on optically and electrically addressed modulators. The programmable parameters of the array are its size (number of generated spots) and the transmissivity and focal length of each lens. As expected, phase modulators have a better conversion efficiency than amplitude modulators do.
TL;DR: In this article, the backscattering behavior of straight cylinders is examined whose lengths range from much less than the diameter of the first Fresnel zone of the source/receiver pair to much greater than the first fresnel zone, with special emphasis on the complicated "transitional region", where the cylinders occupy a finite number of Fresnel zones (≊1−5).
Abstract: The backscattering behavior of straight cylinders is examined whose lengths range from much less than the diameter of the first Fresnel zone of the source/receiver pair to much greater than the first Fresnel zone, with special emphasis on the complicated ‘‘transitional region,’’ where the cylinders occupy a finite number of Fresnel zones (≊1–5). In general, the scattering characteristics of cylinders in this region can only be described numerically. The scattering is described by first adapting the deformed cylinder formulation [T. K. Stanton, J. Acoust. Soc. Am. 86, 691–705 (1989)] to the point‐source/point‐receiver combination. Numerically evaluating this expression showed the scattering characteristics to be dominated by Fresnel zone effects—oscillations in the backscatter versus length curve caused by constructive and destructive wave interferences due to phase shifts from contributions along the cylinder axis. An experiment was performed that involved measurement of backscatter versus cylinder length in the transitional region, and there is reasonable agreement between the results and the trend as predicted by the approximate theory.
TL;DR: In this article, a diffraction-free, high-efficiency light beam has been generated using a specific Fresnel zone plate and a suitable convex lens, and the principle and experimental results are given.
Abstract: A diffraction-free, high-efficiency light beam has been generated using a specific Fresnel zone plate and a suitable convex lens. The principle and experimental results are given.
TL;DR: In this article, the diffraction pattern is calculated for the real region containing the focus, and also for the virtual region, because it is in the latter that the most sensitive measurements can be made.
Abstract: When monochromatic light is incident on a small converging lens which has no mount, the usual diffraction pattern is supplemented by light that has passed unobstructed around the lens. This problem arises in ice physics; very perfect lenses are formed by water occluded in the grain boundaries of polycrystalline ice, and measurement of the diffraction patterns they produce gives the ratio of the grain-boundary energy to that of an ice-water interface. The diffraction pattern is calculated here for the real region containing the focus, and also for the virtual region, because it is in the latter that the most sensitive measurements can be made. The intensity pattern depends entirely on a single parameter n, namely the number of half-period Fresnel zones subtended by the lens at its focus. The movement of the zeros of amplitude (wave dislocations) is followed as n changes. For discrete values of n an unstopped lens possesses a single anti-focus point, on axis in the virtual field, where the intensit...
TL;DR: In this paper, the concept of Fresnel zones for modes is introduced, which is analogous to the usual Fresnel zone introduced for rays, and the analysis of mode scattering at large-scale and random inhomogeneities of a medium in waveguides is simplified.
Abstract: The notion of Fresnel zones for modes is introduced, which is analogous to the usual Fresnel zones introduced for rays. It is shown that using Fresnel zones for modes one can simplify the analysis of mode scattering at large-scale and random inhomogeneities of a medium in waveguides. Simple formulae to calculate fluctuations of mode amplitudes are obtained. They are similar to well-known formulae of geometrical optics and to those of the Rytov method used to calculate fluctuations of ray complex amplitudes. Relations deduced can be used for calculating field fluctuations both at regular waveguide points and at caustics.
TL;DR: In this article, the diffractive elements were generated by electron-beam lithography on a Mo/C multilayer mirror obtained by means of the triode sputtering technique.
Abstract: The fabrication of linear, circular, and elliptical reflective zone plate lenses by a multistep process using microfabrication technologies is reported. The diffractive elements were generated by electron-beam lithography on a Mo/C multilayer mirror obtained by means of the triode sputtering technique. The patterns were transferred anisotropically into the multilayer mirror by reactive ion etching in a fluorinated plasma. An intermediate metallic mask made by the lift-off process was used for the transfer process. The groove depth could be monitored by following the reflectivity ofthe structure with a helium-neon laser during etching. Linear, circular (with 0.4-μm outer zones), and elliptical Fresnel zone plates (with 0.8-μ.m outer zones) were produced. Test patterns with a 50-nm top and 90-nm bottom resolution were produced. The groove profile and dimensional control were also investigated.
TL;DR: In this paper, the etch rate and etch profile were investigated as a function of the gas mixture, pressure, and plasma rf power, and the groove depth was monitored by following the reflectivity of the structure with a helium-neon laser during etching and comparing it with an optical model.
Abstract: Mo/Si multilayer mirrors deposited on silicon wafers by triode sputtering were patterned by electron‐beam lithography and etched by reactive ion etching in a fluorinated plasma with an intermediate metallic mask made by the lift‐off process. The etch rate and etch profile were investigated as a function of the gas mixture, pressure, and plasma rf power. The groove depth was monitored by following the reflectivity of the structure with a helium‐neon laser during etching and comparing it with an optical model. Large structures consisting of gratings and Fresnel zone plates with potential applications to high‐resolution alignment for projection x‐ray lithography were fabricated in Mo/Si. The fabrication feasibility of finer structures such as gratings with 0.3–0.5 μm pitch and linewidths smaller than 100 nm as well as linear and circular Fresnel zone plates with a 0.4‐μm‐wide outer zones were investigated in the similar Mo/C multilayer system.
01 Sep 1991
TL;DR: In this paper, the authors summarized recent MITRE efforts to validate the NEC-3, NEC-GS, and NEC-I versions of the Numerical Electromagnetics Code (NEC) developed by Lawrence Livermore National Laboratory for predicting the performance of antenna wire elements in close proximity to flat earth.
Abstract: : This paper summarizes recent MITRE efforts to validate the NEC-3, NEC-GS, and NEC-3I versions of the Numerical Electromagnetics Code (NEC) developed by Lawrence Livermore National Laboratory for predicting the performance of antenna wire elements in close proximity to flat earth. In an early version (NEC-1), the effect of the air-ground interface was included by applying a plane-wave Fresnel reflection coefficient approximation to the field of a point source. The NEC-2 version, while still retaining the Fresnel reflection coefficient model as an option, provides a more accurate ground model by numerically evaluating Sommerfeld integrals. The version NEC-3 extends the NEC-2 version to cases for bare wire segments below the air-earth interface. Version NEC-GS utilizes rotational symmetry to provide a more efficient version of NEC-3 for the case of a monopole element with a uniform radial wire groundscreen (GS). Version NEC-3I extends NEC-3 to include the case of insulated (I) wires.
TL;DR: In this article, the classical problem of Fresnel diffraction by a straightedge is extended to allow for loss in the medium, which modifies the usual Cornu spiral solution because the argument of the Fresnel integral becomes complex.
Abstract: The classical problem of Fresnel diffraction by a straightedge is extended to allow for loss in the medium. The loss modifies the usual Cornu spiral solution because the argument of the Fresnel integral becomes complex. For high loss the contributions of the outer Fresnel zones are highly attenuated.
TL;DR: In this paper, a method of scaling the light intensity distribution in the Fresnel region is presented, which enables a modification of the zone plate transmittance, which leads to a scale transformation of the output focal curve.
Abstract: The paper presents a method of scaling the light intensity distribution in the Fresnel region. This can be useful in the design of computer generated diffractive optical elements. In particular, the method enables a modification of the zone plate transmittance, which leads to a scale transformation of the output focal curve. The approach is useful in those cases where the focal curve to be transformed is localized in the plane parallel to that of the zone plate and the complex amplitude of the focusing element is known in an analytic form. The theoretical description is based on the similarity property of the Fourier transform. The method is demostrated by obtaining computer generated zone plates with elliptic and hyperbolic focal curves.
TL;DR: A generalization of the Fresnel approximation in diffraction theory is proposed, where the phase term in the diffraction integral is approximated by a parabolic variation, not by a binomial expansion but by matching up at the critical points for asymptotic evaluation of the integral.
Abstract: A generalization of the Fresnel approximation in diffraction theory is proposed. The phase term in the diffraction integral is approximated by a parabolic variation, not by a binomial expansion but rather by matching up at the critical points for asymptotic evaluation of the integral. The method provides a correction to the optical coordinates of the Fresnel diffraction theory that extends its region of validity. It is applied to diffraction of an inclined plane wave by a half-plane.
TL;DR: Two new optical free-space collinear cross-over interconnect schemes are suggested, which uses mirrors and beam splitters, while the second uses a Fresnel zone plate and lens combination.
Abstract: Two new optical free-space collinear cross-over interconnect schemes are suggested. The first optical implementation uses mirrors and beam splitters, while the second uses a Fresnel zone plate and lens combination. Some proof-of-principle experimental results are also presented.
Patent•
18 Apr 1991
TL;DR: In this article, the authors proposed to improve the efficiency of the reflecting plate by forming a reflecting plane for odd number order Fresnel zones only on reflecting plate and a non-reflecting plane for even number order FRZs only on a reflecting plate.
Abstract: PURPOSE:To improve the efficiency of the reflecting plate by forming a reflecting plane for odd number order Fresnel zones only on the reflecting plate and a non-reflecting plane for even number order Fresnel zones only on the reflecting plate. CONSTITUTION:Only odd number order Fresnel zones F1, F3,... on a reflecting plate 1 are formed as reflecting faces and even number order Fresnel zones F2, F4,... on the reflecting plate 1 are formed as non-reflecting faces. Since a reflecting wave on the even number order Fresnel zones F2, F4,... has a phase difference of lambda/2 with respect to the reflecting wave on the odd number order Fresnel zones F1, F3,... on the reflecting plate 1, the reflecting waves are cancelled together at a reception point. However, the even number order Fresnel zones F2, F4,... are formed as non-reflecting faces, then the reflecting wave having a phase difference of lambda/2 is lost and cancellation action is lost, and reception power is increased. Thus, the efficiency of the reflecting plate is improved.
28 May 1991
TL;DR: In this paper, the authors developed a Fresnel zone plate lens comprised of a serial stack of patterned minus-filters which allows broadband radiation to be focussed (or imaged) without longitudinal or transverse chromatic aberrations.
Abstract: : Significant progress has been made toward solving the century-old problem of chromatic aberrations in diffractive optics. Our approach exploits modern materials and microfabrication technology and is very different from the 'purely diffractive strategy', which is commonly employed and which results in multiple diffractive elements separated by a finite distance. We have developed a Fresnel zone plate lens comprised of a serial stack of patterned minus-filters which allows broadband radiation to be focussed (or imaged) without longitudinal or transverse chromatic aberrations. Diffractive optics, such as the Fresnel zone plate shown in Figure 1, are commonly used for imaging and focussing radiation at wavelengths for which refraction is not a viable option.
TL;DR: In this article, the authors analyzed P-wave and shear-horizontal wave Fresnel zone amplitudes with the use of physical models, which consist of Plexiglas cylinders mounted on a plate and placed in a water tank for P wave data, and a block of polysilicon with cylindrical holes drilled into the bottom for the SH wave case.
Abstract: Fresnel zone and diffraction amplitude responses have been calculated using numerical modeling techniques. (Hilterman, 1970, 1975, 1980, 1982, and Knapp, 1991) Our research involves analyzing P-wave and shear-horizontal wave Fresnel zone amplitudes with the use of physical models. These models consist of Plexiglas cylinders mounted on a plate and placed in a water tank for P-wave data, and a block of Plexiglas with cylindrical holes drilled into the bottom for the SHwave case. These collected data allow us to examine the effects of diffractions on various sized Fresnel zone reflectors. Amplitude analysis of the data confirm theoretical predictions. In addition, physical modeling of both acoustic and elastic waves allows a one to one comparison between P and SH-wave reflection and diffraction amplitudes.
TL;DR: In this paper, a photo-lithographic imaging (PLI) method was developed to produce high efficiency volume phase-only CGHs, which can be fabricated on a variety of substrates and in different configurations, providing substantial technological and economical advantages over existing conventional manufacturing techniques for diffractive binary CGHOEs.
Abstract: A new photo-lithographic imaging (PLI) method to produce high efficiency volume phase-only CGH has been developed. PLI technique utilizes Du Pont proprietary materials and processes to replicate a binary intensity CGH, recorded on a chrome photomask-master, into a dry photopolymer. By changing the refractive index of the photopolymer in the imaged areas the intensity modulated pattern in the chrome photomask-master is converted into a volume phase modulated pattern with a single lithographic step. This technique was used to fabricate a variety of computer generated holographic optical element (CGHOE), such as on-axis and off- axis Fresnel zone plates and array illuminators. Diffraction efficiencies up to 98% were observed for a CGHOE consisting of a 10 X 10 array of off-axis Fresnel zone plates. Such CGHOEs can be fabricated on a variety of substrates and in different configurations, providing substantial technological and economical advantages over existing conventional manufacturing techniques for diffractive binary CGHOEs.
01 Oct 1991
TL;DR: In this article, a measure for the shadowing influence of buildings and trees in an area with mixed land cover, based on the first fresnel zone ellipsoid, is given as well as an approach to calculate wave propagation in a city with regular building structure and a rectangular street grid.
Abstract: This article treats several aspects of field strength prediction, especially concerning the land cover influence on mobile radio links. A measure for the shadowing influence of buildings and trees in an area with mixed land cover, based on the first fresnel zone ellipsoid, is given as well as an approach to calculate wave propagation in a city with regular building structure and a rectangular street grid. In addition, different definitions of the transmitter's effective antenna height are discussed.
04 Jun 1991
TL;DR: In this paper, the authors describe a theoretical analysis of the illumination of a hollow, conductive object containing a wire by a non-planar wave and show that such near-field (Fresnel zone) testing produces coupling responses markedly different from that encountered due to plane wave (where the wave impedance is 377 ohm) incidence.
Abstract: Describes a theoretical analysis of the illumination of a hollow, conductive object containing a wire by a non-planar wave. Much EMC testing is performed at antenna-test object separation distances that are small compared to the wavelength. The authors show that such 'near-field' (Fresnel zone) testing produces coupling responses markedly different from that encountered due to plane wave (where the wave impedance is 377 ohm) incidence. There is practical verification for this-in 1983, the effect was noted in an aircraft trial.