Fresnel zone

About: Fresnel zone is a research topic. Over the lifetime, 2337 publications have been published within this topic receiving 37650 citations.

A new Fresnel zone antenna with beam focused in the Fresnel region

Abstract: A new Fresnel Zone (FZ) Antenna to focus the microwave power in the radiation near field region is presented. FZ Antennas, in comparison with array antennas or reflector antennas, conventional focused antennas, take the advantages of design simplicity as well as lower sidelobe levels (SLL) in both axial and transverse directions. Simulation results and comparisons made between the new structure and aperture antennas based on the quadratic phase distribution show a reduction of sidelobe levels in both axial and transverse directions.

A quasi-deterministic path loss propagation model for the open sea environment

Abstract: In this paper, a quasi-deterministic propagation model for the open sea environment is presented, derived from 2 GHz measurement data collected in Trondheimsfjorden, Norway. The path loss results have been compared with both deterministic models and empirical models. From the comparison, it is found that the ITU-R P.1546-2 model with 50% of the locations among the empirical models fits the measurement results best. However, the significant fadings at a short distance from the shore are not included. On the other hand, the deterministic plane earth loss model is discovered to well match the measurement results at a short distance, but offset between them will become larger gradually with a increase of distance due to diffraction loss of earth curve. As a consequence, a quasi-deterministic propagation model comprising of the plane earth loss model and the ITU-R P.1546-2 model with a bound determined by the First Fresnel zone, is proposed. The model has been validated by measurements in two different measurement campaigns.

Reconstructing surface wave profiles from reflected acoustic pulses

Abstract: Surface wave shapes are determined by analyzing underwater reflected acoustic signals. The acoustic signals (of nominal frequency 200 kHz) are forward scattered from the underside of surface waves that are generated in a wave tank and scaled to model smooth ocean swell. An inverse processing algorithm is designed and implemented to reconstruct the surface displacement profiles of the waves over one complete period. The inverse processing uses the surface scattered pulses collected at the receiver, an initial wave profile (two are considered), and a broadband forward scattering model based on Kirchhoff's diffraction formula to iteratively adjust the surface until it is considered optimized or reconstructed. Two physical length scales over which information can be known about the surface are confirmed. An outer length scale, the Fresnel zone surrounding each specular reflection point, is the only region where optimized surfaces resulting from each initial profile converge within a resolution set by the inner length scale, a quarter-wavelength of the acoustic pulse. The statistical confidence of each optimized surface is also highest within a Fresnel zone. Future design considerations are suggested such as an array of receivers that increases the region of surface reconstruction by a factor of 2 to 3.

Diffraction phenomena inside thick Fresnel zone plates

Abstract: The parabolic wave equation method is used to describe the complex wave field inside the body of a thick zone plate used to focus X ray radiation Two analytical approaches are applied: (1) Diffraction of a plane wave incident onto a separate interface between opaque and open zones is considered We construct an approximate analytical solution to the classical problem of diffraction by a dielectric wedge in terms of the Fresnel integral and a new special function (2) Coupled wave theory is used to describe collective effects of diffraction by many interfaces The zone plate is considered as thick diffraction grating with slowly varying period An analytical solution is found for three interacting modes A possibility to optimize the zone plate performance is shown Taken together, these results describe all the main features observed in the output field of realistic X ray zone plates

Comparison of Fresnel zone plates and uniformly redundant arrays

Abstract: Several imaging systems in laser fusion, a-beam fusion, and astronomy employ a Fresnel zone plate (FZP) as a coded aperture. The recent development of uniformly redundant arrays (URAs) promises several improvements in these systems. The first advantage of the URA is the fact that its modulation transfer function (MTF) is the same as the MTF of a single pinhole, whereas the MTF of an FZP is an erratic function including some small values. This means that if inverse filtering is used, the URA will be less susceptible to noise. If a correlation analysis is used, the FZP will produce artifacts whereas the URA has no artifacts (assuming planar sources). Both the FZP and URA originated from functions which had flat MTFs. However, practical considerations in the implementation of the FZP detracted from its good characteristics whereas the URA was only mildly affected. The second advantage of the URA is that it better utilizes the available detector area. With the FZP, the aperture should be smaller than the detector in order to maintain the full angular resolution corresponding to the thinnest zone. The cyclic nature of the URA allows one to mosaic it in such a way that the entire detector area collects photons from all of the sources within the field of view while maintaining the full angular resolution. If the FZP is as large (or larger) than the detector, all parts of the source will not be resolved with the same resolution. The FZP does have some advantages, in particular its radial symmetry eases the alignment problem; it has a convenient optical decoding method; and higher diffraction order reconstruction might provide better spatial resolution.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.