Continuous Aperture Research Articles

High-frequency laser sonar system

A set of measurements recently completed at the Naval Undersea Warfare Center (NUWC) demonstrated that a laser-based sonar system can be used to detect acoustic particle velocity on the surface of a thin acoustically-compliant plate embedded beneath a standard acoustic window. The theoretical acoustic and measured surface particle velocity varied by less than 1 dB (reference m/s) over a wide frequency band (10 kHz to 100 kHz). However, the Polytec Model PSV-100 Scanning Laser Vibrometer System (SLVS) used in the experiments had relatively poor acoustic sensitivity, presumably due to high electronic noise, speckle noise, stand-off distance, and drifting laser focus. The laser’s acoustic sensitivity appears to be inversely proportional to the backscatter signal level. The existing SLVS can sample a grid of 512 by 512 points, with each grid point having a spot size of approximately 10 mm (0.0004 in.). Such fine sampling may be used to create essentially a continuous aperture, eliminating acoustic grating lobes at all frequencies of practical sonar interest.

Gore/Seam Architectures for Gossamer Structures

Gossamer spacecraft are ultra-low-mass (areal densities <1 kg/m 2 ) structures with applications in communications, imaging, and propulsion. Often large, for example, diameters greater than 10 m, aperture architectures canbe either filled (monolithic) or sparse (segmented). Although filled apertures provide the greatest performance, manufacture of large monolithic, that is, seamless, apertures is extremely challenging, Gore/seam architectures provide a compromise for fabrication of gossamer spacecraft. Segments (gores) are manufactured and then assembled into a continuous aperture by seaming together the gores. A similar process was chosen for the Inflatable Antenna Experiment and has been used for decades in the fabrication of large high-altitude scientific balloons. The architecture options for design and fabrication of gossamer spacecraft are reviewed, with particular emphasis on precision applications in communications and imaging. The near-term inevitability of seaming as a fabrication method is emphasized, and the point of view is taken of using to advantage. The possibility of seams to mitigate some of the aforementioned issues is discussed and extended to go further to find other uses for the active seams. Lessons learned from NASA's high-altitude balloon program are reviewed in improving structural integrity through use of novel gore/seam design.

Gaussian beam and pulsed-beam dynamics: complex-source and complex-spectrum formulations within and beyond paraxial asymptotics.

Paraxial Gaussian beams (GB's) are collimated wave objects that have found wide application in optical system analysis and design. A GB propagates in physical space according to well-established quasi-geometric-optical rules that can accommodate weakly inhomogeneous media as well as reflection from and transmission through curved interfaces and thin-lens configurations. We examine the GB concept from a broad perspective in the frequency domain (FD) and the short-pulse time domain (TD) and within as well as arbitrarily beyond the paraxial constraint. For the formal analysis, which is followed by physics-matched high-frequency asymptotics, we use a (space-time)-(wavenumber-frequency) phase-space format to discuss the exact complex-source-point method and the associated asymptotic beam tracking by means of complex rays, the TD pulsed-beam (PB) ultrawideband wave-packet counterpart of the FD GB, GB's and PB's as basis functions for representing arbitrary fields, GB and PB diffraction, and FD-TD radiation from extended continuous aperture distributions in which the GB and the PB bases, installed through windowed transforms, yield numerically compact physics-matched a priori localization in the plane-wave-based nonwindowed spectral representations.

Doping and defect inhomogeneities of polypyrrole tosylate films as revealed by μ-NEXAFS

The photoemission electron microscope (PEEM) based techniques micro-near-edge X-ray absorption fine structure (μ-NEXAFS) and micro-photoelectron spectroscopy (μ-PES) have been recently installed at the BESSY-I HE-TGM beam lines as novel techniques for material analyses. In the PEEM microscope, photoemitted electrons are imaged by electrostatic lenses having a lateral resolution of 40 nm. Images with a field of view between 800 and 20 μm are obtained. NEXAFS enables the simultaneous recording of the near-edge fine structures of the absorption coefficient while varying the incident photon energy. For μ-PES, at a fixed photon energy, core level and valence band spectra can be recorded by an electrostatic energy analyser to fit in the optical axis of the PEEM microscope. Spectra are recorded by analysing all the electrons of the PEEM image or by selecting segments of the image with a continuous iris aperture. The minimum size of the analysed spot is in the order of a few micrometers. Here, we use these techniques to determine the electronic structure of polypyrrole films within a well defined surface area. We found surface structures of such polypyrrole films which are corrugated in the micron range as well as areas that appear almost smooth. In addition, the PEEM microscope gives patterns due to a non-homogeneous surface potential distribution. In such patterns bright irregular stripes appear between dark patches. The stripes have a typical width in the order of several micrometers.

Retrodirective beams and their application to low sidelobe pattern synthesis in small planar phased arrays

Beam synthesis remains one of the most difficult problems for an antenna array designer, this task being made more difficult when one limits the number of elements to a handful. Pattern synthesis in planar arrays is often achieved by the sampling of a continuous aperture distribution at the element locations. In the case of arrays of limited numbers of elements this produces errors that can unacceptably distort the radiation pattern. When one also then considers the effects of mutual coupling between the antenna elements, the application of ideal element weights derived in this way are no longer valid. In the paper a technique is presented, using the concept of retrodirective beams, that can achieve a low sidelobe beam from a continuous distribution on a circular aperture. The theory is then extended to include a planar array of antenna elements. This is achieved by the addition of retrodirective beams to the uniform beam pattern of the array. Measured data is presented for a small planar array antenna using the derived method. A method for correction of the element weights, to minimise the effects of mutual coupling, producing low sidelobe radiation patterns is then applied. No knowledge of the antenna array&apos;s coupling coefficients are required.

Asymmetric-shaped beam patterns from a continuous linear aperture distribution

A design method previously developed to give symmetrical shaped patterns from a continuous linear aperture, with individual control of the amplitude of ripples and the heights of unshaped side lobes up to any specified order, is shown to be applicable to asymmetric patterns as well. The method is based on the introduction of complex roots (to achieve null filling in the shaped region) in the technique previously developed by R. S. Elliott to obtain asymmetric Taylor patterns. Power synthesis, where the radiation pattern becomes complex, and real field synthesis, which yields a pure real pattern, are studied. The procedure is illustrated with two examples corresponding to a cosecant squared pattern, obtained by power and real field synthesis, respectively. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 15: 288–291, 1997

On sampling continuous aperture distributions for discrete planar arrays

The theory of sampling continuous distributions for the purpose of determining planar array excitation coefficients is examined. The cause of pattern degradation associated with conventional sampling is identified, and a new method called integrated sampling is introduced which provides superior results. Implementation details are given for the specific case of a rectangular grid planar array with a roughly circular boundary. Numerical examples are provided for both uniform and Taylor distributions in order to perform comparative studies and to demonstrate the advantages of the new technique.

Three dimensional conformal treatment planning with multileaf collimators

Three dimensional conformal radiation treatments are complex, often involving large numbers of blocked or multileaf collimated fields that shape regions of high dose to conform to the treatment volume. As manual definition and digitization of aperture shapes and their corresponding multileaf configurations can be impractically time consuming, it was necessary to integrate the planning of multileaf fields into an existing three dimensional treatment planning system and improve the efficiency of treatment delivery to make these treatments feasible on a routine basis. A subfunction of the Beam's Eye View (BEV) component can be used to automatically generate a continuous aperture shape with a margin around the tumor to account for beam penumbra, and excluding any normal structures to be spared (each with its own margin). To convert a continuous aperture shape into one defined by the multileaf collimator (MLC), a leaf coverage mode is chosen to determine how leaves are fitted to aperture shapes. The conversion process also considers parameters of the specific MLC system, e.g., leaf thickness and the number of leaves. If normal structures to be shielded split the target into multiple regions, more than one multileaf aperture can result. An interactive leaf adjustment routine is also provided to allow for modification of individual leaf positions. Dose calculation programs then take into account multileaf apertures for computation of dose distributions using a pencil beam convolution model. Finally, prescription files specifying leaf and jaw configurations are prepared in treatment machine specific formats and downloaded to the computers driving the multileaf collimators and other components of the treatment machines. An example is presented of a prostate treatment plan, with MLC configurations, dose distributions, and treatment delivery description, along with discussion of clinical implementation at Memorial Hospital.

Wideband, derivative-matched, continuous aperture acoustic transducer

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Synthesis of shaped line-source antenna beams usingpure real distributions

Shaped line-source antenna beams can be synthesised by introducing complex roots in a Taylor line source distribution to achieve null filling in the shaped region. Optimisation of these roots and others allows individual control of the amplitude of ripples and the heights of unshaped sidelobes up to any specified order. The corresponding continuous aperture distribution can be made pure real by appropriate root-pairing, which makes the results applicable, via sampling, to resonantly spaced linear arrays.

A proof of the Woodward‐Lawson sampling method for a finite linear array

An extension of the continuous aperture Woodward‐Lawson sampling theorem has been developed for a finite linear array of equidistant identical elements with arbitrary excitations. It is shown that by sampling the array factor at a finite number of specified points in the far field, the exact array factor over all space can be efficiently reconstructed in closed form. The specified sample points lie in real space and hence are measurable provided that the interelement spacing is greater than approximately one half of a wavelength. This paper provides insight as to why the length parameter used in the sampling formulas for discrete arrays is larger than the physical span of the lattice points in contrast with the continuous aperture case where the length parameter is precisely the physical aperture length.

Representation of continuous circular aperture as a set of discrete rings for a prescribed radiation pattern

A method for discretization of a circular aperture with a set of rings which are further discretized as a set of elements on a ring is outlined. These elements have the same current magnitudes. For practical reasons the authors have only concentrated on the near side lobes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dilute aperture diffraction imagery and object reconstruction

Replacement of a large continuous aperture by a set of small apertures offers a number of advantages if the subapertures can be placed within the array so as to achieve minimal spatial frequency redundancy. Using an approach based on combinatorics and integer programming, we have found solutions for a linear array for N (number of subapertures) up to eight. In my opinion, too much emphasis has been placed on the subaperture positioning problem and not enough on the resultant diffraction imagery. To this end, we have calculated the diffraction images of extended incoherent objects for these optimum arrays. We then proceed to discuss inversion techniques whereby we determine the object intensity from the diffraction images.

Sonar system using acoustically transparent continuous aperture transducers for multiple‐beam beamformation

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Footprint patterns obtained by planar arrays

An extension of Taylor&apos;s circular aperture pattern synthesis technique permits the generation of a flat-topped beam that is rotationally symmetric, surrounded by ring side lobes of controllable height, with the ripple amplitude also controllable. The corresponding continuous aperture distribution can be made pure real by appropriate root-pairing, which makes the results applicable, via sampling, to resonantly-spaced waveguide slot arrays. A one-dimensional linear stretch extends the results to rectangular grid planar arrays possessing an elliptical boundary. In that case the pattern is a fiat-topped beam with an elliptical contour, surrounded by ring side lobes also of elliptical contour, but with amplitude and side lobe level still controlled. The principal application is to space vehicle communication systems required to place a footprint pattern on a selected portion of the earth&apos;s surface.

Normal oscillation modes of a conjugate resonator with an array-type controlled transparency

An analysis is made of the configuration of the field and losses experienced by normal oscillation modes of a conjugate resonator with an array-type controlled transparency. It is shown that the presence of gaps between the active areas of a transparency results in deformation of the normal modes in such a resonator, compared with a resonator which has a continuous aperture. Computer calculations are reported of the distribution of the field of normal modes on a resonator mirror and of the dependence of the losses experienced by the modes on the Fresnel number of the resonator. It is shown that the dependence has certain singularities corresponding to qualitative changes in the structure of normal modes, resulting in a considerable increase of the angular divergence. These features of the behavior of normal oscillation modes are supported by experiments carried out as part of the present study.

Shaped patterns from a continuous planar aperture distribution

A pattern synthesis procedure is developed for a planar aperture with a circular boundary. A (Ø-symmetric continuous distribution is assumed and required to produce a flat-topped beam with controlled ripple, surrounded by ring side lobes of controlled height. With M filled-in nulls in the shaped region, there are 2M solutions for the aperture distribution. Sampling can yield the excitation of a circular grid planar array. Linear stretching extends the results to an elliptical boundary.

Experimental Study of the Controllable Microwave Troughguide Applicator

The design and evaluation of a new class of focussed electromagnetic hyperthermia applicators to increase penetration depth and improve control over the radiated field pattern is presented. Unlike currently available RF and microwave applicators, this new troughguide applicator provides a continuous aperture distribution with built-in deposited power monitoring.

A new leaky wave antenna for millimeter waves using an asymmetric strip in groove guide, Part I: Theory

Traveling wave antennas for the millimeter wave range face two main problems: higher metal loss at the higher frequencies, and fabrication difficulties due to the smaller wavelengths. These two problems are addressed here by a new type of leaky wave antenna, in which the fabrication problem is minimized by employing a longitudinally continuous aperture, and the higher loss reduced by basing the antenna on a low-loss waveguide, the groove guide. A longitudinally continuous asymmetric metal strip provides the mechanism that transforms the initially bound mode into a leaky mode. An accurate analysis, based on a transverse equivalent network, is presented here for the properties of this leaky structure that takes into account the mode conversion from the bound mode to the leaky mode. All the constituents of this transverse equivalent network are obtained in closed form, thus yielding a dispersion relation in closed form. Part I of these two companion papers describes the new antenna and its principle of operation, and then derives the parameters of the complete transverse equivalent network. Part II is concerned with the properties of the leaky mode and their relations to leaky wave antenna performance; design considerations together with a variety of numerical results are contained in Part II.

Finite periodic structure approach to large scanning array problems

There are two conventional techniques dealing with mutual coupling problems for antenna arrays. The element-by-element method is useful for small to moderate size arrays. The periodic method deals with one cell of infinite periodic structures, including all the mutual coupling effects. It cannot, however, include edge effects, current tapers, and nonuniform spacings. A new technique called the periodic method, is presented and applied to represent the active impedance of an array, it involves two operations. The first is to convert the discrete array problem into a series of continuous aperture problems by the use of Poisson's sum formula. The second is to use spatial Fourier transforms to represent the impedance in a form similar to the infinite periodic structure approach. The active impedance is then given by a convolution integral involving the infinite periodic structure solution and the Fourier transform of the equivalent aperture distribution of the current over the entire area of the array. The formulation is particularly useful for large finite arrays, and edge effects, current tapers, and nonuniform spacings can also be included in the general formulation. Although the general formulation is valid for both the free and forced modes of excitation, the forced excitation problem is discussed to illustrate the method.