Far-field Radiation Characteristics Research Articles

Microstrip-fed printed Lotus antenna for wideband wireless communication systems [Wireless corner

A novel printed antenna, fed by a microstrip line, has been designed for wideband wireless communication systems. The new antenna shows a wide -10dB bandwidth of over 60%. In addition to being small in size, the antenna exhibits stable far-field radiation characteristics in the entire operating band with relatively high gain, low cross polarization, wide beamwidth, and good front-to-back ratio.

Optimal Transmission Frequency for Ultralow-Power Short-Range Radio Links

Analysis determining the optimal transmission frequency for maximum power transfer across a short-range wireless link is introduced, including a comparison of near-field transmission with far-field transmission. A new near-field power transfer formula has been derived, which allows direct comparison with the well-known far-field Friis transmission formula. Operating charts are presented, which provide the designer with the preferred transmission frequency as a function of distance and antenna dimensions, together with surface plots which show the power transfer for this frequency. The analysis, performed for loop antennas, has been used to evaluate the oscillator transmitter as a low-power topology. It is shown that the requirement of a high-Q factor to realize a low-power oscillator need not be contradictory to achieving optimal far-field radiation characteristics. Based on this fact an approach to sizing loop antennas for low-power oscillator transmitters is suggested.

A uniqueness theorem for the time-domain elastic-wave scattering in inhomogeneous, anisotropic solids with relaxation

A uniqueness theorem for the (analytic or computational) time-domain modeling of the elastic wave motion in a scattering configuration that consists of inhomogeneous, anisotropic solids with arbitrary relaxation properties, occupying a bounded subdomain in an unbounded homogeneous, isotropic, perfectly elastic embedding, is presented. No direct time-domain uniqueness proof seems to exist for this kind of configuration. As an intermediate step, the one-to-one correspondence between the causal time-domain wavefield components and the constitutive material response functions on the one hand, and their time Laplace-transform counterparts for (a sequence of) real, positive values of the transform parameter on the other hand, seems a necessary tool. It is shown that such an approach leads to simple, explicit, sufficiency conditions on the inertial loss and compliance relaxation tensors describing the solid’s constitutive behavior for uniqueness to hold. In it, the property of causality plays an essential role. In Christensen [Theory of Viscoelasticity—An Introduction (Academic, New York, 1971)] a similar approach is applied to the problem of uniqueness of the elastodynamic initial-/boundary-value problem associated with a viscoelastic object of bounded extent, the surface of which is subject to an admissible set of explicit boundary values. In the scattering configuration of unbounded extent, no explicit boundary values occur and the far-field compressional and shear wave radiation characteristics at “infinity” in the embedding play a key role in the proof.

Improved Three-Dimensional Image Reconstruction Technique for Multi-Component Ground Penetrating Radar Data

For electromagnetic imaging the vectorial character of the emitted field and the radiation characteristics of both source and receiver play an important role. Recently, a new imaging algorithm was presented dedicated to the electromagnetic case. The radiation characteristics of GPR source and receiver antennas and the vectorial nature of the electromagnetic waves are taken into account for a monostatic fixed offset GPR survey. This resulted in a representative image of a point scatterer. Comparison with scalar imaging algorithms shows that for a homogeneous space the SAR image has an opposite sign compared to the multi-component image, whereas the Gazdag image has a phase shift of about 90° with respect to the multi-component image. In this paper, modified scalar imaging algorithms are introduced that minimize these differences. However, between the images obtained in a homogeneous half-space phase differences of 10–20° are still present. These differences indicate the possible error in nature of the physical property contrast when it is determined with the modified scalar imaging algorithms. The multi-component imaging algorithm returns a representative image because it uses the appropriate Greens functions to eliminate the propagation effects. For practical imaging strategies, only far-field radiation characteristics can be used to compensate for the propagation effects due to the large computing time needed to evaluate the total-field expressions. Synthetic analysis of the imaging of a point scatterer calculated using total-field expressions shows that using the far-field expressions in the multi-component imaging algorithm the images better approximate the actual contrast than using the modified scalar imaging algorithms. Experimental results are presented from imaging several buried objects with different medium properties and different orientations. The phase differences in the experimental data are similar to those obtained synthetically. This likeness indicates that using the multi-component imaging algorithm, a more reliable image is obtained than with the modified scalar imaging algorithms.

Direct-signal modulation using a silicon microstrip patch antenna

In this paper, a microstrip patch antenna is fabricated directly onto a high-resistivity silicon substrate without an insulating barrier, thereby forming a distributed Schottky diode between the patch radiator metallization and ground plane. By applying d.c. bias control to the patch metallization, direct amplitude modulation of a CW microwave carrier can be achieved. Experimental results are presented that show the far-field radiation characteristics of the structure and its response to applied base-band modulation signals. Also direct base-band signal detection using the patch antenna is discussed.

Active antenna array lumped ring configuration

Coupled active antenna oscillator arrays are used for power-combining at microwave and millimeter-wave frequencies. It is known that the relative phase determined by the element separation distance ultimately determines the array operational mode and, hence, far-field radiation characteristics. Separately, it is known that coupled oscillator modal stability is achieved by coupling oscillators through lumped capacitive elements. In this paper, an arrangement whereby lumped capacitive elements (placed across the oscillator loads) and radiative coupling are employed concurrently is investigated. The arrangement takes the form of a ring of coupled oscillators used to excite a 2/spl times/2 antenna array. The effect that these couplings have on array behavior are evaluated using time-domain analysis and analytically derived equations. Experimental results for far-field radiation patterns are discussed in relation to coupled oscillator dynamical behavior. These suggest that the theoretical predictions are valid, offering a robust design tool for studies of larger power-combining arrays.

Electromagnetic scattering and radiation involving dielectric objects

The influence of dielectric objects on electromagnetic fields is an important consideration in many engineering problems. In some applications, the scattering from dielectric objects for a known incident field is of interest. In other situations where dielectric objects interact with radiators - such as biological tissue near a wireless transceiver or a dielectric loaded antenna - the resulting input impedance, far-field radiation and power absorption characteristics are important quantities to determine. In this work an integral equation approach is used to solve these problems for which the method of moments is applied to facilitate a numerical solution. A comprehensive computer program was developed that handles both scattering and radiation problems. With the computer program both dipole and superquadric loop antennas can be modeled which have any arbitrary relative location and orientation to dielectric bodies of interest. The implementation is verified by comparison with finite-difference time-domai...

Canonical examples of reflector antennas for high-power microwave applications

The suitability of various radiating systems for generating a directive high-power microwave (HPM) beam is considered. It is concluded that offset reflector antenna systems are well suited for this purpose. An HPM reflector antenna may consist of one or two reflectors that are illuminated by single or array feeds. To accurately predict the performance of an HPM reflector antenna, a versatile analysis scheme is used to compute the fields of an array feed with general configurations, and the diffraction techniques of physical optics (PO) and physical theory of diffraction (PTD) are applied to analyze reflectors of conic or shaped surfaces. Design examples of offset dual-reflector antennas for HPM applications are presented. Analysis results are shown with emphasis on both the near-field and far-field radiation characteristics. Applications of optimization (mathematical programming) techniques to the diffraction synthesis of HPM reflector antenna systems are discussed, and examples are given to demonstrate their effectiveness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High brightness operation of a XeCl laser with negative branch unstable resonators

Confocal and non-confocal negative branch unstable resonators with an intracavity spatial filter have been applied to a high-gain short-pulse UV preionized XeCl laser. The near- and far-field radiation characteristics have been investigated in both configurations. Laser beams of larger brightness have been obtained with non-confocal schemes. A maximum beam brightness of 1.3×1014 Wcm−2Sr−1 has been achieved.

Narrow-linewidth SFUR applied to a XeCl laser

A self-filtering unstable resonator with an intracavity grating (2400 lines/mm) at a grazing-incidence angle (80°) has been applied to a UV-preionized XeCl laser. A 4.5 mm diameter output beam of 100 μJ, 19 ns long and with a linewidth of 0.13 Å has been obtained. The far-field laser radiation characteristics have been investigated. With the commonly used plane-plane cavity, with a grazing-incidence (80°) grating and spatial apertures of 1 mm diameter, laser radiation of larger linewidth (0.23 Å) and smaller energy (70 μJ) has been obtained.

Effect of Operating Frequency on Driving-Point Impedance and Radiation Characteristics of Helical Array

A rigorous theoretical method has been developed for the calculation of driving-point impedances and far-field radiation patterns of helical array of staggered half-wave dipole radiators at different operating frequencies. The method is based on induced E M F theory. The radiators of the array are considered to be thin, linear and centre-driven. The broad-band performance for the theoretically helical array has been investigated both in respect of driving-point impedances and far-field radiation characteristics.

Mathematical model for a free-flooded piezoelectric cylinder transducer

A mathematical model for the transmitting response and electrical impedance of a free-flooded, thickness-polarized piezoelectric cylinder transducer has been developed. The elasticity of the cylinder is described using membrane equations which have been modified to account for piezoelectric effects. An additional consequence of the piezoelectricity of the shell is a significant alteration in the boundary conditions for the ends of the cylinder. A particular solution to the membrane equations which satisfies these modified boundary conditions is obtained. The complete solution for the vibration of the cylinder is then found in the form of a sum of the particular solution and the normal modes appropriate to a nonpiezoelectric cylinder with free ends. The acoustic field loads the solution, couples the normal modes, further couples the particular solution and the normal modes, and changes the boundary conditions. The acoustic influence coefficients needed to incorporate these effects are calculated using the ship computer program. The water-loaded normal surface velocities are determined and the surface pressures and farfield radiation characteristics are calculated using ship. The model fully accounts for such effects as the cavity resonance, the velocity of the ends of the cylinder, distribution of velocity along the length of the cylinder, and the difference between the velocity on the inner and outer surfaces of the cylinder. Comparisons between theoretical and experimental results are presented.

Mathematical model for a free‐flooded piezoelectric cylinder transducer

A mathematical model for the transmitting response of a free‐flooded, thickness‐polarized piezoelectric cylinder transducer has been developed. The elasticity of the cylinder is described using thin shell equations which have been modified to account for piezoelectric effects. An additional consequence of the piezoelectricity of the shell is a significant alteration in the boundary conditions for the ends of the cylinder. A particular solution to the thin shell equations which satisfies these modified boundary conditions is obtained. The complete solution for the vibration of the cylinder is then found in the form of a sum of the particular solution and the normal modes appropriate to a nonpiezoelectric cylinder with free ends. The acoustic field loads the solution, couples the normal modes, further couples the particular solution and the normal modes, and changes the boundary conditions. The acoustic influence coefficients needed to incorporate these effects are calculated using the SHIP computer program (a fast Helmholtz integral radiation program). The water‐loaded normal surface velocities are determined and the surface pressures and farfield radiation characteristics are calculated using SHIP. The model fully accounts for such effects as the cavity resonance, the velocity of the ends of the cylinder, distribution of velocity along the length of the cylinder, and the difference between the velocity on the inner and outer surfaces of the cylinder. Comparisons between theoretical and experimental results are presented.

Array processing to eliminate standing wave effects in shell radiation measurements

The spectral measurement of a cylindrical shell's radiation characteristics in a tidal basin were desired to compare with deep ocean farfield radiation characteristics. Standing wave effects from the surface and bottom of the basin spectrally distorted the radiation measurements. The techniques evaluated to provide relief from standing wave effects were: large, highly directive arrays; line arrays in the nearfield with detection before averaging; and line arrays in the semifarfield with and without beamforming. Detection methods were linear, square law, and logarithmic. An analytical evaluation of the detection methods indicated linear or logarithmic detection had a lower standard deviation than square‐law detection. Also, a large, highly directive array wrapped around the shell (i.e., the very nearfield) was evaluated. The best smoothing of the standing wave effects of the medium were achieved using a ten‐element line array (normal to cylinder axis) in the semifarfield using logarithmic detection before averaging. Further experimental work simplified this array to a three‐element sloped line with logarithmic detection. This system allowed measurement of the shell's radiation characteristics over 212 decades without perturbation by standing waves.

A nearfield, underwater measurement system

A nearfield acoustic measurement technique for accurately computing the farfield radiation characteristics of underwater sound transducers was developed, implemented, and evaluated. The analysis was based on evaluation of a form of the Helmholtz integral which utilizes a Green’s function that vanished over the surface of integration, thereby requiring only knowledge of the nearfield acoustic pressure for computing the farfield radiation. An experimental program was conducted to evaluate the technique. Details of the nearfield measurement instrumentation are presented. Comparisons between conventionally measured farfield properties of four different types of test transducers and the results obtained by using the nearfield technique are given. The results showed that excellent agreement was achieved for the radiation properties compared. For the directional responses, the nearfield technique matched the conventionally measured results over the minor lobe structure as well as over the major lobe structure of the pattern. The results also showed that absolute sound-pressure levels could be determined accurately and with relative ease.

A nearfield, underwater measurement system

A nearfield acoustic measurement technique for accurately computing the farfield radiation characteristics of underwater sound transducers was developed, implemented, and evaluated. The analysis was based on evaluation of a form of the Helmholtz integral which utilizes a Green's function that vanished over the surface of integration, thereby requiring only knowledge of the nearfield acoustic pressure for computing the farfield radiation. An experimental program was conducted to provide corroboration of the technique. Details of the automated, digital, nearfield measurement instrumentation are presented. Comparisons between conventionally measured farfield properties of four different types of test transducers and the results obtained by using the nearfield technique are given. The results showed that excellent agreement was achieved for the radiation properties compared. For the directional responses, the nearfield technique matched the conventionally measured results over the minor lobe structure as well as over the major lobe structure of the pattern. The results also showed that absolute sound pressure levels and directivity indices could be ascertained accurately and with relative ease. [Work supported by NAVSEA 034.]

Far-field radiation characteristics of elastic waves and the elastodynamic radiation condition

In the present paper, a radiation condition for time-harmonic elastic waves in a homogeneous, isotropic, perfectly elastic medium is presented. It is based upon certain physical considerations pertaining to the elastodynamic radiation generated by a source of finite extent. In it, the particle displacement on and the traction across a spherical surface around the source occur. Separate conditions are imposed on the radial and the angular parts of these quantities. It is shown that the resulting far-field radiation characteristics for the particle displacement are in accordance with the properties that could be expected from the physical considerations we have started with.

Radiation Characteristics of Spherically Symmetric, Perfect Focusing Acoustic Lenses

It was shown in a paper by Morgan [J. Appl. Phys. 29, 1358–1368 (1958)] that there exist an infinite number of spherically symmetric, perfect-focusing lenses of which the Luneburg lens is but one example. Using the Helmholtz formulation of diffraction theory, we examine the farfield radiation characteristics of these lenses for both the case of an omnidirectional source and a particular directional source.