Showing papers on "Radiation pattern published in 1988"

A leaky-wave analysis of the high-gain printed antenna configuration

Abstract: A leaky-wave analysis is used to explain the narrow-beam resonance-gain phenomenon in which narrow beams may be produced from a printed antenna element in a substrate-superstrate geometry. It is demonstrated that the phenomenon is attributable to the presence of both transverse electric and transverse magnetic-mode leaky waves, that are excited on the structure. Asymptotic formulas for the leaky wave are compared with the exact patterns to demonstrate the dominant role of the leaky waves in determining the pattern. Results are presented as a function of frequency, the scan angle, and the permittivity of the superstrate. >

Modeling electromagnetic wave propagation in the troposphere using the parabolic equation

Abstract: A computational method is described for predicting electromagnetic wave propagation in the troposphere using the parabolic approximation of the Helmholtz wave equation. The model represents propagation over a spherical, finitely conducting Earth and allows specification of frequency, polarization, antenna pattern, antenna altitude, and elevation angle. The method enables calculations to be performed using either ideal or measured refractivity profiles that vary in both altitude and range. A brief discussion of the theoretical formulation and computational implementation of the propagation model is presented, followed by examples that demonstrate various features. Example calculations include 3-GHz propagation over a calm sea in the presence of both range-dependent and range-independent surface-based ducts as well as in standard atmosphere conditions. Comparisons with two other propagation models are also discussed. >

Microstrip transmission line excitation of dielectric resonator antennas

Abstract: A systematic investigation of the excitation of a cylindrical dielectric resonator antenna was undertaken to illustrate the possibility of such a feed structure and to characterise the coupling behaviour and radiation patterns of the system.

Near-field probe used as a diagnostic tool to locate defective elements in an array antenna

Abstract: Results of an experimental study are presented in which the near-field probe was used as a diagnostic tool to locate the defective elements in a planar array. The near-field data were processed not only to obtain the far-field patterns of the array under the test, but also to reconstruct the aperture field for diagnostic purposes. The backward transform enables the near-field probe to identify accurately aperture faults at a distance, free of interactions and couplings with the array elements. In practice, to recover the aperture field properly from the near-field distribution, the evanescent components in the computed far-field spectrum must be excluded from the inverse process with fast-Fourier-transform (FFT) techniques. For low-gain array antennas, a correction on the far-field spectrum is required to remove the contribution of the probe and the element factor before the inverse transform, strongly enhancing the resolution. >

Magnetic tuning of a microstrip antenna on a ferrite substrate

Abstract: The operating frequency of a microstrip antenna on a ferrite substrate can be tuned by varying the DC magnetic bias field applied to the ferrite. The letter reports experimental work where a 40% tuning range has been obtained, with usable patterns and return loss over this band.

Dielectric cone loaded horn antennas

Abstract: The radiation characteristics of conical horns loaded with dielectric cones are investigated. The horn radiates low crosspolarisation over a wide band of frequencies with cones made of foam or solid dielectrics. They make high performance feeds for reflector antennas. A modal-matching technique is used to investigate the behaviour of the higher order modes excited along the horn that influence the radiated cross-polarisation. A parametric study is described to assess the influence of the aperture and cone diameters, the flare angle, and the permittivity of the dielectric.

A selective modal scheme for the analysis of EM coupling into or radiation from large open-ended waveguides

Abstract: A selective modal scheme is proposed to efficiently analyze the problem of high-frequency (HF) electromagnetic (EM) coupling/penetration into or radiation from open-ended waveguides. This scheme is based on the phenomenon that at sufficiently high frequencies, the modes which contribute most significantly to the fields coupled into the waveguide are those whose modal ray directions are most nearly parallel to the incident-wave direction. This concept is illustrated by calculating the EM radiation and backscattering from open-ended parallel-plate, rectangular, circular, and sectoral waveguide geometries. The calculations use the usual geometrical optics, aperture field, and Ufimtsev edge current techniques. Also included are some measured results which further verify the accuracy of the above computations. >

Broadband electromagnetic energy absorber

Abstract: A radar absorbing material comprising multiple layers integrated to form a thin flexible and light weight structure. The material includes a substrate disposed thereon antenna elements that are relatively loaded to enable one to construct a device in relatively small and thin size. The broad handling of the device is carried out by multi-layering concepts in which different size antenna patterns are multi-layered with each layer designed to absorb frequencies in s specified range. The antenna elements are selected for their intrinsic broadband properties and to preferably be polarization insensitive.

A dielectric bifocal lens for multibeam antenna applications

Abstract: An iteration technique is applied to the design of a bifocal dielectric lens antenna. A bifocal lens has two conjugate off-axis focal points and offers good off-axis scanning performance. A computer model was developed to aid in the design analysis and far-field performance evaluation of a bifocal lens. The computer simulation results demonstrate that the bifocal dielectric lens can provide a higher-gain radiation pattern for multibeam antenna (MBA) applications over a large field-of-view. The computer study also indicates that the quadratic phase error can be significantly reduced by constraining the feed along an optimized focal arc. >

Use of sampling expansions in near-field-far-field transformation: the cylindrical case

Abstract: A modified sampling technique is applied to the reconstruction of the antenna far-field pattern from near-field measurements on a cylinder. It is shown that, if a proper phase factor is singled out from the tangential components of the measured electric field, the samples spacing along the axis of the measurement cylinder increases linearly with the cylinder radius. Thus, when this radius is substantially larger than the antenna's dimension, the sample's spacing can be much greater than the half-wavelength spacing commonly accepted as the maximum possible one, without losing the computational efficiency of the standard approach. >

Losses, sidelobes, and cross polarization caused by feed-support struts in reflector antennas: design curves

Abstract: In a symmetrical reflector antenna the feed system and its support struts block the aperture and thereby deteriorate the radiation characteristics. Simple design curves are presented for the efficiency reduction, the sidelobe levels, and the cross polarization caused by strut blockage. The results are obtained from an analytical study that includes the induced field ratio (IFR) of the struts. The most significant IFR values for struts with circular cross section are calculated and plotted in a way which makes them easy to use as design curves. The use of the design curves is demonstrated by an example. >

Aperture efficiencies of large axisymmetric reflector antennas fed by conical horns

Abstract: It is shown how the radiation patterns of conical horns can be understood in terms of Laguerre-Gaussian beam modes. This approach provides a simple method for calculating the horn radiation pattern as it propagates through a multielement optical system consisting of lenses or offset mirrors with large focal ratio and/or a small angle of incidence, and can be used in quantitatively determining aperture efficiencies for systems where conical horns are coupled to antennas via tertiary optics. The resulting aperture efficiency results and a practical quasioptical system fed by a conical horn are discussed. >

The wavenumber spectrometer—an alternative to the directional coupler for multimode analysis in oversized waveguides

Abstract: The problem of measuring a broad spectrum of discrete modes in oversized waveguides is discussed. An analysis of the standard solution, the directional coupler, shows that the measured signals can be ambiguous in mode selectivity and directivity. Therefore we have developed an instrument, the wavenumber spectrometer, where this problem of ambiguity is avoided. The principle of this new instrument ensures extremely high directivity and allows the measurement of all TE and TM modes with the same instrument. Furthermore, the device inherently has a large frequency bandwidth. This wavenumber spectrometer consists of a leaky-wave antenna structure in the wall of the waveguide and of a movable receiver antenna with high directivity. From the radiation pattern of the leaky-wave antenna, as measured by the receiver antenna, the spectrum of the modes propagating in the waveguide is deduced.

Broadband directional antenna

Abstract: A broadband antenna includes a center fed active radiating/receiving element formed of two contiguous conductive spirals. The two planar antenna spirals are secured to a cavity loaded with a wave absorbing lossy material such that the antenna is operative only in its front lobe direction opposite the loaded cavity. In accordance with the present invention, the cavity wall is formed of varying pattern of conductive and insulative material. The resultant composite antenna is characterized by a relatively narrow beam pattern for both horizontally and vertically polarized electric waves across a broad spectrum of frequencies.

Analysis of mutual coupling between a finite phased array of dipoles and its feed network

Abstract: The mutual coupling effects between a finite phased array of dipoles and its feed network are analyzed. The feed network is typically a corporate feed consisting of split-tee power dividers cascading to form a certain power distribution over the aperture. A simple iterative approach is used to solve the interaction between elements and feed. The radiation of a finite dipole array are first found for a given voltage excitation. These radiation impedances are then used as loads for the feed network, and the n+1 port network problem is analyzed. Due to the interaction between the feed network and dipoles, the antenna parameters such as mismatch, antenna pattern, and gain are all affected. These effects can be determined from the analysis of the network representation. Numerical results for a typical phased array with a corporate feed show that the resultant VSWR of the feed pattern degradation is due to the mutual coupling effects. >

Gain enhancement of microstrip antennas with overlaying parasitic directors

Abstract: The gain characteristics of rectangular microstrip antennas with one and two overlaying parasitic directors have been investigated experimentally. Results indicate that, when properly spaced, the parasitic directors can enhance the gain (dB) and reduce the beamwidth of the antenna by a factor of two or better.

Antenna patterns of nonsinusoidal waves with the time variation of a Gaussian pulse. III

Abstract: The development of antenna theory for nonsinusoidal electromagnetic waves has been based on the idealized rectangular pulse. In practice, an antenna that is designed to operate in the mode of an electric hertzian dipole would radiate a pulse that best approximates a Gaussian one when the driving current consists of a linear transient. The principle of radiation of nonsinusoidal electromagnetic waves with the time variation of Gaussian pulses is discussed. The properties of the Gaussian pulse are presented, i.e., the autocorrelation function, energy spectral density, and spectrum. Antenna patterns, such as peak-amplitude pattern, peak-power pattern, energy pattern, and slope pattern are derived for a Gaussian pulse received (or radiated) by a linear array antenna. Computer plots of the derived antenna patterns are presented that show a considerable improvement in the angular resolution capability over that of the antenna patterns that have been derived for a rectangular pulse. >

Magnetised microstrip antenna with pattern control

Abstract: Measured results are given for a novel microstrip antenna with in-built pattern control capabilities. The antenna element consists of a metal patch etched onto alumina substrate with a thick ferrite overlay, which is then magnetised by a permanent magnet, H-plane patterns showing a 30° scan are given and a simple analysis of this new device explains the principal mechanisms involved.

ICRF heating with mode control provided by a rotating field antenna

Abstract: A rotating field antenna set — a pair of two closely spaced dual half-turn antennas — is used to tailor the azimuthal spectrum of the antenna field. It is demonstrated that the azimuthal mode of ICRF waves can be controlled by proper phasing of the antenna set. Ion heating is shown to be strongly dependent on the azimuthal mode number. When the antenna set is operated in the m = −1 (left rotating) mode, the m = −1 modified ion cyclotron wave is selectively excited, resulting in efficient ion heating.

V-conical antenna

Abstract: A new angular antenna called the V-conical antenna was investigated. The characteristics of the antenna are frequency-independent, and the electromagnetic fields are pure spherical waves (transverse electromagnetic mode) even in the near region. The theoretical analysis makes use of conformal mapping. Closed-form solutions for the fields, currents, and characteristic parameters are obtained. The formulas are simple and precise. An experimental study of the antenna was conducted under pulse excitation. The shape of the measured electric-field pulses is the same as that of the source pulse from a picosecond pulse generator. The measured field patterns are very close to those predicted by the theory. Such a structure with a tapered resistive termination is an ideal directional pulse antenna or electromagnetic pulse simulator. >

Performance degradation of dielectric radome covered antennas

Abstract: Performance is evaluated by a method based on the reciprocity theorem. The asymptotic expression for the radome Green's function that allows the characterization of the radome by itself, independently of any specific antenna, is identified. The concept of the modified aperture distribution, which radiating in free space produces the same pattern as the radome-covered antenna, is introduced. It can be used for the design of the radome stratification and for the optimization of the antenna location. On the basis of the formulation of a computer code has been developed that analyzes the degradation induced by radomes with surfaces and dielectric stratifications that can be defined numerically. Theoretical predictions and experimental results are compared. >

Mode stability of radiation-coupled interinjection-locked oscillators for integrated phased arrays

Abstract: An analysis is presented of a two-element, 10-GHz array consisting of two oscillators coupled solely by means of the free-space interaction between their respective antenna elements. The oscillators are modeled as energy-storing L-C tank circuits in parallel with voltage-dependent negative conductances. A simplified far-field slot antenna model is used to derive the mutual admittance of the two antennas. Even-odd mode analysis yields the normal modes of the system, and a theorem from averaged potential theory is used to determine which mode is stable. Two microstrip Gunn diode oscillators were built to verify the essential features of the model. Oscillator frequencies, relative phases, and radiation patterns were measured as functions of the interantenna distance, and the periodic alternation of modes with distance predicted by theory was confirmed quite well. >

Synthesis of shaped-beam reflector antenna patterns

Abstract: The paper describes the synthesis of shaped-beam dual-reflector antenna patterns using two methods. In the first, for a prescribed far field the antenna aperture field is obtained initially and then a Geometric Optics synthesis method used to define the reflector coordinates. In the second method an optimisation procedure is employed to shape the reflectors directly, so as to generate, through Physical Optics, the desired far field. The two methods are compared for the pattern corresponding to the EUTELSAT 2 spacecraft. Close agreement is found and differences ascribed to initial constraints.

Microstrip patch antenna with omni-directional radiation pattern

Abstract: A microstrap patch antenna with a substantially omni-directional radiation pattern includes an upper hemisphere microstrip patch radiator working against a ground plane and a lower hemisphere microstrip patch radiator working against a ground plane, the ground planes being oriented in close proximity and spaced apart from each other. One of the radiators is excited in left hand circular polarization and the other in right hand circular polarization such that their fields add constructively across their ground planes to achieve an omni-directional radiation pattern.

Astronomical observations with a network of detectors of gravitational waves. I - Mathematical framework and solution of the five detector problem

Abstract: Two different representations for the antenna pattern of Earth-based gravitational wave detectors (laser interferometers and bars) observing arbitrarily polarized waves are investigated in the long wavelength limit. The authors show that the response of a gravitational wave detector can be written either as a contraction between two symmetric trace-free (STF) tensors, one associated with the wave and the other with the detector, or as a linear combination of generalized spherical functions. They then apply this formalism to solve the simplest form of the 'inverse problem' for bursts: determining the direction of an incoming wave, the orientation of its polarization ellipse and the wave's two indepedent amplitudes.

Antenna with enhanced gain

Abstract: The present invention will yield enhanced gain over a conventional antenna system by taking advantage of combining directional antennae. In one embodiment, the invention utilizes four 10 dB dipole, vertically-polarized, omni-directional antennae having a reflector added to each one to limit the horizontal beamwidth to 90° and increase the gain to 16 dB. The vertical beamwidth remains at 7°. By utilizing four of these antennae and utilizing power combining hybrids to connect each of the two opposed antennae together, excess gain over a 10 dB omni-directional system will be obtained. The effective antenna gain is equal to the directional antenna gain minus the omni-directional antenna gain minus the hybrid loss. Thus, the vertical beamwidth and physical height of the antenna are preserved with the increase in gain at the cost of a little antenna complexity.

Non-invasive quarter wavelength microwave applicator for hyperthermia treatment

Abstract: A printed circuit board structure fabricated to define a plurality of quarter wavelength microstrip antennas, each radiating microwave energy in a dipole pattern. The ends of the microstrip antennas are formed overlapping the ends of adjacent microstrip antennas so that a resultant additive effect of the radiated microwave energy is exhibited near the ends of the antennas. The overall effect is to produce a more uniform radiation pattern which does not have the typical radiation minima located at the ends of the dipole antennas.

Radiation field consideration of biconical horn antenna with different flare angles

Abstract: The radiation fields of a biconical horn antenna with different flare angles are computed from both the fields of an equivalent Huygens source on the spherical-surface aperture and the infinite biconical horn antenna with spherical transverse electromagnetic wave. Numerical data and experimental results are first presented for the symmetrical biconical horn antenna ka=1.57 and 3.14 (where k is the propagation constant and a=cone length) with equal flare angle. Curves of the beamwidth versus cone length are then given for various flare angles. At flare angles above 75 degrees , the beamwidth increases with the cone length, while at flare angles below about 75 degrees , the beamwidth decreases with the cone length in this region of ka. For the asymmetrical biconical horn antenna, with two different flare angles (including a conical coaxial type and a discone type), the numerical and the experimental patterns are both presented. For the asymmetrical biconical horn antenna, the pattern main lobe direction, with respect to the antenna axis, may be adjusted by using suitable flare angles. >

Technique for antenna array pattern synthesis with controlled broad nulls

Abstract: The author presents a new technique for synthesising antenna array pattern with controlled broad nulls. The approach taken in the paper is to integrate the power response of the array over a spatial region of interest and force it to be less than or equal to a small quantity. The optimal weight vector is obtained by minimising the weight vector norm subject to a linear and a quadratic constraint. Numerical techniques based on matrix factorisation are proposed to reduce the computational complexity. Subsequently, a set of linear constraints, also known as eigenvector constraints, are used to approximate the effect of the quadratic constraint in order to maintain a broad null over the spatial region of interest. This set of eigenvectors can be viewed as an efficient representation of array signals of specified angular occupancy. >

Accurate determination of planar near-field correction parameters for linearly polarized probes

Abstract: A procedure used by the US National Bureau of Standards (NBS) for accurately determining the plane-wave receiving parameters of both single- and dual-port linearly polarized probes is described. Examples are presented, and the effect of these probe receiving characteristics in the calculation of the parameters for the antenna under test is demonstrated using the required planar near-field theory. The planar near-field theory necessary to accomplish probe correction and to formulate probe parameter errors is presented in a concise and meaningful way to help understand when probe correction is or is not needed. >