Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

Offset multireflector antennas with perfect pattern symmetry and polarization discrimination

Abstract: Conditions are derived that are useful for designing reflector antennas with excellent cross-polarization discrimination. These conditions ensure circular symmetry and absence of cross-polarization everywhere in the far field of an antenna, provided a suitable feed such as a corrugated horn is employed. The spherical wave radiated by the fundamental mode of such a feed has circular symmetry around the axis, and it is everywhere free of cross-polarization. An arbitrary sequence of N confocal reflectors (hyperboloids, ellipsoids, paraboloids) is combined with such a feed. It is shown that it is always possible to ensure circular symmetry (and absence of cross-polarization) in the antenna far field by properly choosing the feed axis orientation. If the final reflector is a paraboloid, a simple geometrical procedure can be used. It is also shown that the asymmetry caused by an arbitrary number of reflections can always be eliminated by properly introducing an additional reflection. An application to the problem of producing a horizontal beam using a vertical feed is discussed. Two arrangements are described that may be useful for radio relay systems.

Bragg Scattering of Electromagnetic Waves from the Air/Sea Interface

Abstract: The advent of radar during World War II had consequences for research in air/sea interactions which were far from obvious in the face of the immediate wartime need to detect enemy aircraft. When the newly-discovered tool was put to use to detect targets on or near the surface of the sea, these targets were often obscured by strong echos from the ocean itself. This “sea echo” constituted a considerable nuisance to those engaged in locating enemy vessels so investigations into the nature of this unwanted return were launched. Early theoretical work concentrated on explaining sea echo in terms of either return from the sea surface itself or from the spray and bubbles above the surface. Methods of modelling electromagnetic scattering which had proved useful to the characterization of many types of targets were adapted for use modelling sea return. Specular reflection seemed to explain the characteristics of sea echo for small incidence angles but not for larger ones. The standard method of carrying out such calculations utilized the “Kirchoff principle,” also known as physical optics or the tangent plane method, which assumes that the surface is smooth in the sense that the radius of curvature is large compared to the electromagnetic wavelength. In addition to yielding disturbingly low return at large incidence angles, the method was unable to account for the observation that vertically polarized return was generally stronger than horizontal at such angles. Since interference between radiation directly incident on bubbles above the surface and radiation incident on the bubbles following reflection from the surface could in principal yield such polarization differences, theories based on Rayleigh scattering by the bubbles were also proposed.

The loop antenna as a probe

Abstract: The properties of circular and square loop antennas as probes for measuring the magnetic field are investigated. The response of an electrically small loop to the magnetic and the electric components of a given electromagnetic field is determined theoretically for singly- and doubly-loaded loops in terms of suitably defined constants. A system error ratio is defined as a measure of the ability of a given probe to discriminate against the effects of the electric field. An experimental procedure for measuring loop sensitivities in the elliptically polarized near field of a quarter-wave monopole is described. Experimental and theoretical results are compared. It is concluded that very large errors are possible when a singly-loaded loop is used to measure magnetic fields unless its diameter is less than 0.01\lambda . The doubly-loaded probe may be used with comparable accuracy when its diameter is as large as 0.15\lambda .

Modeling of a Cantilever-Based Near-Field Scanning Microwave Microscope

Abstract: We present a detailed modeling and characterization of our scalable microwave nanoprobe, which is a micro-fabricated cantilever-based scanning microwave probe with separated excitation and sensing electrodes. Using finite-element analysis, the tip-sample interaction is modeled as small impedance changes between the tip electrode and the ground at our working frequencies near 1GHz. The equivalent lumped elements of the cantilever can be determined by transmission line simulation of the matching network, which routes the cantilever signals to 50 Ohm feed lines. In the microwave electronics, the background common-mode signal is cancelled before the amplifier stage so that high sensitivity (below 1 atto-Farad capacitance changes) is obtained. Experimental characterization of the microwave probes was performed on ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or topographical signals can be realized using different reflection modes of the probe.