Offset dual-shaped reflectors for dual chamber compact ranges

The application of the theory of the synthesis of offset dual-shaped reflectors to the design of compact ranges is examined. The object of the compact range is to provide a uniform plane wave with minimum amplitude and phase ripple over as large a volume as possible for a given size reflector. Ripple can be lowered by reducing the edge diffraction from the reflector producing the plane wave. This has been done either by serrating or rolling the edge. An alternative approach is to use dual offset-shaped reflector synthesis techniques to produce a reflector aperture distribution that is uniform over most of the aperture, but with a Gaussian taper near the edge. This approach can be used together with rolling and/or serration if desirable. The amount of phase and amplitude ripple obtained with two different dual-shaped reflector designs is studied as a function of position in the plane wave zone and reflector size in wavelengths. The amount of both transverse and longitudinal (z-component) cross polarization is studied     

Method of moments analysis of the backscattering properties of a corrugated trihedral corner reflector

A method of moments (MoM) formulation is developed to analyze the backscattering properties of an anisotropic trihedral corner reflector, which is obtained by corrugating one or several of its interior faces. The proposed formulation treats the corrugated surface as ideally tuned to the incident wave frequency. The numerical analysis of the studied structures has been done using closed-form formulas and accurate numerical integration. The focus of the study reported in this paper has been the polarization responses of ideally tuned corrugated reflectors, which have interesting properties, particularly regarding elliptically or circularly polarized waves. We numerically verify that an appropriately corrugated reflector returns elliptically and circularly polarized waves with the same handedness as the incident wave. For a linearly polarized incident wave, the corner reflector is able to rotate them by 90/spl deg/. Also the effect of the direction of the corrugation to the backscattering properties is studied.     

Curved edge modification of compact range reflector

For many years, the compact range has successfully been used to design and evaluate many electromagnetic radiation and scattering systems. However, it has had limited use for large structures because of the large discrepancy between the reflector and target zone sizes. The limitation results from the large edge diffracted signal which emanates from the termination of the reflector. Previous attempts to solve this problem have involved using serrated edges to diffuse the diffracted field. While this reduces the edge diffracted field in the target zone, it is not eliminated. In addition, one has introduced many new corners which also diffract into the target zone. A curved edge modification is presented which reduces the edge diffraction by an order of magnitude or more and in the process does not create new mechanisms which perturb the plane wave in the target zone. Using this curved modification, one is able to design a compact range reflector whose target zone cross section approaches the size of the reflector parabolic surface. This makes the compact range ideal for scale model measurements since larger targets (antennas) can be measured. It also makes it more practical to consider systems which can measure full scale vehicles.     

Stray signal requirements for compact range reflectors based on RCSmeasurement errors

The authors present a performance criterion for compact range reflectors such that their edge diffracted stray signal levels meet a reasonable radar cross section (RCS) measurement error requirement. It is shown by example that one of the significant error sources is the diffracted fields emanating from the edges or junctions of the reflector. This measurement error is demonstrated by placing a diagonal square flat plate in the target zone and rotating it to appropriate angles. These angles are determined by bisecting the plane wave and stray signal directions. This results in a peak bistatic measurement of the edge diffracted stray signal. It is proposed that the diagonal flat plate be used to evaluate new reflector designs as well as existing systems. A reasonable stray signal performance level has been developed so that new reflector systems can be characterized in terms of an RCS measurement error requirement     

Broadband and compact 2-D photonic crystal reflectors with controllable polarization dependence

Two-dimensional (2-D) compact photonic crystal reflectors on suspended InP membranes were studied under normal incidence. We report the first experimental demonstration of 2-D broadband reflectors (experimental stopband superior to 200 nm, theoretical stopband of 350 nm). They are based on the coupling of free space waves with two slow Bloch modes of the crystal. Moreover, they present a very strong sensitivity of the polarization dependence, when modifying their geometry. A compact (50/spl times/50 /spl mu/m/sup 2/) demonstrator was realized and characterized, behaving either as a broadband reflector or as a broadband transmitter, depending on the polarization of the incident wave. Experimental results are in good agreement with numerical simulations.     

Compact range reflector analysis using the plane wave spectrumapproach with an adjustable sampling rate

An improved method for determining the test zone field of compact range reflectors is presented. The plane wave spectrum (PWS) approach is used to obtain the test zone field from knowledge of the reflector aperture field distribution. The method is particularly well suited to the analysis of reflectors with a linearly serrated rim for reduced edge diffraction. Computation of the PWS of the reflector aperture field is facilitated by a closed-form expression for the Fourier transform of a polygonal window function. Inverse transformation in the test zone region is accomplished using a fast Fourier transform (FFT) algorithm with a properly adjusted sampling rate (which is a function of both the reflector size and the distance from the reflector). The method is validated by comparison with results obtained using surface current and aperture field integration techniques. The performance of several serrated reflectors is evaluated in order to observe the effects of edge diffraction on the test zone fields     

Pattern measurements of reflector antennas in the compact range andvalidation with computer code simulation

The measurements were performed at the University's compact range facility. They demonstrated: (1) the excellent dynamic range that can be achieved with antenna pattern measurements in a compact range facility; and (2) the excellent validation achieved for the calculated patterns of two 8-ft diameter reflector antennas. The compact range has a rolled edge modification to its reflector and uses a pulsed radar system to eliminate the clutter interference such that a dynamic range of more than 80 dB can be obtained. The measured far field patterns of two 8-ft reflector antennas, a prime focus fed antenna and a Cassegrain antenna, at 11 GHz were compared with those calculated by Ohio State University's Reflector Antenna Code. The computer code simulation's approach is also briefly described     

The ohio state university compact radar cross-section measurement range

The ElectroScience Laboratory at The Ohio State University has recently installed a new radar cross-section measurement system. The system uses a Scientific Atlanta compact range reflector (an offset parabolic reflector) to generate a far-field plane wave in the confines of a40 times 20 times 60ft anechoic chamber. The system uses a computer controlled microwave frequency synthesizer and a multichannel computer controlled receiver. The target support/positioning system and an optical target alignment system are also interfaced to the computer. The parameters of the system are 1) operation from 1 to 30 GHz (eventual operation to 96 GHz has been confirmed by field probing); 2) a plane wave volume (test area volume) of 1.3 m diameter; and 3) a sensitivity of -50 dBsm. Of particular importance is the ability of the system to measure phase as well as amplitude. This permits vector subtraction of the background and system calibration using a reference (sphere) target. The development of this system and the performance characteristics obtained so far will be discussed. Some results which demonstrate the system performance will be shown. Of particular interest is the broad-band measurement of both amplitude and phase. This permits conversion of the results to the time domain. Examples will be shown in which the various system components (antennas, reflector, walls, ceiling, etc.) are separated in the time domain by this technique. The development of the range is continuing and planned future improvements will also be discussed.     

New approach to compact measurements on reflector antennas

The letter describes a novel scheme for very compact measurements on reflector antennas. The measurement configuration involves a plane reflector located just beyond (and parallel to) the aperture plane, so that radiated power is reflected and refocused back into the feed. Measurement of return loss can then give information on antenna efficiency and gain. The basic concept is described and its performance confirmed by means of experiments on a 10 ft (3.048 m) diameter reflector. Possible methods of extending the performance of the scheme are briefly discussed.     

Compact range measurement systems for electrically small test zones

Traditional range requirements are evaluated for spherical and compact range measurement systems. It is shown that a compact range system is more appropriate for targets three wavelengths (λ) and larger because of the chamber size needed to meet these requirements. Commercially available compact range systems are, however, normally restricted to 10 λ or larger targets because of the excessive diffraction associated with currently available compact range reflectors. It is shown that this limitation could be overcome by using a blended rolled edge compact range reflector. For example, a 9 λ×9 λ blended rolled edge reflector can be used to measure 3 λ targets at the lowest frequency of operation. As the frequency of operation increases, the test zone of the reflector approaches one half the reflector's linear dimension, which is consistent with presently available compact range systems     

Compact antenna range at 35 GHz

The feasibility of operating a compact antenna test range at 35 GHz has been demonstrated. A compact range of simple construction has been shown to have a performance only slightly inferior to the performance at X-band. Diffracted rays from the edges of the source reflector can degrate the purity of the electric field. This has been investigated and sucessfully controlled by serrating the rim of the reflector.     

Time-domain fields exterior to a two-dimensional FDTD space

A transformation algorithm for the near-zone and far-zone fields exterior to a two-dimensional (2-D) finite-difference time-domain (FDTD) field lattice has been developed entirely in the time domain. The fields are found from a surface integration of the convolution of the time derivative of equivalent currents and charges along a contour that encloses the scatterer or radiator of interest. The kernel of the convolution integral has a square-root singularity for which an efficient numerical integration rule is presented. Using this technique, a very accurate solution is obtained; however, convolution integrals are computationally expensive with or without singularities. As an alternative, a rapidly convergent approximate series expansion for the convolution integral is presented, which can be used both in the near and far zone. Results using the new 2-D transform are compared with analytical expressions for the fields generated by a modulated Gaussian pulse for TE and TM line sources. In addition, the 2-D transform solution for the near-zone fields scattered from an open-ended cavity for a TE incident modulated Gaussian pulse plane wave is compared against a full-grid FDTD solution for accuracy and efficiency. The 2-D transform far-zone fields are compared against an alternative technique, which uses a double Fourier transform to perform the convolution in the frequency domain     

An efficient plane wave spectral analysis to predict the focalregion fields of parabolic reflector antennas for small and wide anglescanning

An efficient approach is described for calculating the field distribution in the focal region of an electrically large, symmetric or offset parabolic reflector antenna with an arbitrary rim contour, when the concave reflector surface is fully illuminated by an obliquely incident arbitrary electromagnetic plane wave. The dominant contribution to the focal-region fields is found by transforming the physical-optics integral over the reflector surface into a plane-wave spectral (PWS) integral. The PWS integral is evaluated rapidly via the fast Fourier transform (FET) algorithm to furnish, in only a single computation, the field for every place in the focal plane (or any plane parallel to it) within the focal region for a given direction of the incident wave. The correction to the physical-optics field is relatively small in the focal region and may therefore be neglected. Numerical results based on this PWS approach are presented, and their accuracy is established by comparison with results based on other approaches     

一种高性能单反射面柱形紧缩场微波暗室

介绍了一个单反射面柱形紧缩场暗室,分析了柱面紧缩场的工作原理,其原理是采用一维算法将柱面波转换为平面波.整体暗室以紧缩场为基础,通过综合和优化选型,从暗室、吸波材料、紧缩场设备等多个方面配合设计,达到了较好的综合效果,具有很高的测试性能,可用于高精度的天线测量和雷达散射截面测量.     

A two-element antenna for suppression in multipath environments

The design of a two-element antenna for portable transceivers is considered. The antenna consists of a dipole terminated with a parallel loop-capacitor combination. The antenna has a single feed at a point on the loop opposite the junction and does not require external combining circuitry. The capacitor creates a phase shift between the dipole and loop currents, thereby greatly reducing the probability of deep s in the received signal when the antenna is deployed in free-space or in the vicinity of a fixed reflector, where standing wave patterns occur. Theoretical and simulation studies based on multiple incident/reflected plane wave fields typical of multipath environments are used to quantify the reduction in probability. Simulation results are presented for three antenna types: a dipole antenna, a loop-dipole antenna without a capacitor, and the loop-dipole antenna with capacitor. The results are verified by field measurements on an automated outdoor test range, where the incidence angle and distance from the reflector are varied. With a single incident plane wave, the loop-dipole-capacitor (LDC) design reduces the probability of deep s in the received signal by up to two orders of magnitude at low signal-to-noise ratios (SNRs) when compared with the standard dipole antenna and the loop-dipole antenna without the capacitor. The performance advantage of the new design decreases as the number of incident waves increases; however, it performs at least as well as the dipole antenna in all cases studied.     

全反射地震P波的Goos-Hanchen效应定量分析

利用反射波相角推导出了反射P波在反射界面的横向偏移和横向偏移渡越时间,讨论了Goos-Hanchen效应对P波正常时差所造成的影响。结果发现,对于掠入射波或当入射角在临界角附近时,反射P波的Goos-Hanchen效应较为明显,故在实际的地震资料处理中应进行横向偏移效应的误差校正。     

Plane wave spectrum scattering analysis of near-field obstacle effects on directive antenna patterns

A plane wave spectrum scattering analysis of the effects of a near-zone obstacle on the pattern performance of a directive antenna is discussed. The free-space azimuth monopulse antenna is characterized by its sum and difference mode plane wave spectra, and a computed plane wave scattering dyad is used to characterize the scattering by a metallic right circular cylinder when it is excited by the incident antenna spectra. An efficient computer algorithm has been developed to compute the far-zone scattered and total fields for the antenna/cylinder system. Extensive experimental data have been obtained, and the patterns calculated using the present analysis are in agreement with the measured data.     

Ka 波段紧缩场天线的设计

设计了一个Ka 波段紧缩场天线。馈源设计结合角锥喇叭在反射面边缘照射电平下降快与波纹喇叭波瓣宽度宽的特点,通过给角锥喇叭加扼流套的改进设计来实现对反射面的照射。反射面采用锯齿型边缘来降低边缘的绕射作用,并进一步在无法加工锯齿部位贴附三角形吸波材料来减小绕射对静区特性的影响。暗室内采用平面测量结果表明,Ka 波段紧缩场天线静区特性满足设计指标要求。     

Method for testing reflector antennas at THz frequencies

The possibilities of applying far-field, near-field, and compact-range techniques to reflector antennas in the THz frequency range are discussed. Other methods, such as defocusing and combining of the mechanical-reflector-surface measurements and the feed-horn-radiation patterns, are also discussed. A recently introduced hologram type of compact range is described. It may be concluded from the analysis of the different methods that the far-field method can be discarded due to atmospheric effects. The near-field method remains a possibility. However, an expensive, high-quality, moving stage is needed, and phase errors caused by flexible cables have to be dealt with. In some cases, a defocusing method, to bring the far field closer, may prove to be practical. Technically, the most-feasible and least-expensive method appears to be the hologram type of compact range. In this method, a planar-amplitude hologram is used to form the required plane wave. The hologram is inexpensive to manufacture, and it is also less sensitive to surface-accuracy errors than a reflector     

Impulse-radiating antenna with an offset geometry

An offset impulse-radiating antenna (IRA) is numerically analyzed and compared with a typical centered IRA. In the typical centered IRA, the transverse electromagnetic (TEM) feed arms block the aperture because they are located at the center of the aperture. This blockage causes multiple reflections inside the antenna and, thus, ripples in the tail of the radiated waveform. In the offset IRA, the TEM feed arms are removed from the aperture, lowering the tail ripples caused by multiple reflections between the TEM feed arms and the reflector. The boresight gains and the impulse amplitudes are seen to be essentially the same for both IRAs. The monostatic radar cross section of the offset IRA is significantly lower than that of the centered IRA for the plane wave incident from the boresight direction because the wave incident to the offset IRA is diverted toward the focal point of the reflector, which is away from the boresight direction. The offset IRA has a shadow behind the reflector. This feature can be useful in bistatic radar applications because the antennas can be placed in the shadows of each other.