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The holographic antenna measurement system developed for the COMSAT Labs far-field range was tested with various antennas including axis-symmetric reflector antennas, offset single and dual reflector antennas, and phased-array antennas. Numerous examples which demonstrate the value of holographic measurement as an antenna diagnostic tool are presented. Microwave holography utilizes the Fourier transform relation between the antenna radiation pattern and the antenna aperture electromagnetic field distribution. Complex far-field date are collected at sample points and a Fourier transform is performed to give amplitude and phase contours in the antenna aperture plane. These contours facilitate reflector antenna diagnosis. The feed illumination and blockage pattern are provided by the amplitude distribution. The aperture phase distribution allows simple determination of deviations in the reflector surface and feed focusing. For phased-array antennas, the contours provide a measure of the complex element excitation. Measurement system parameters including pointing accuracy, phase stability, and measurement dynamic range were studied and refinements implemented to increase speed, accuracy, and resolution of the contour plots. To prevent aliasing errors, sampling criteria were explored to determine the optimum parameter ranges. For most antenna positioners, the antenna center is displaced from the rotation center. The importance of properly accounting for this displacement is discussed in the final section.
Imaging by microwave holography was initially envisaged as a two dimensional diagnostic technique applicable to a wide variety of objects and environments [1], [2], being particularly relevant to reflector antenna metrology [3]. For electrically large paraboloidal reflectors the radiation is well collimated and can be assumed to arise from an effective aperture field at a specified plane within the antenna volume. Fresnel or far field measurements are then restricted to a small angular range around boresight so as not to violate the assumptions made for reconstruction of the aperture field. The processed image represents the aperture illumination function whose phase can be accurately related to feed position and profile error by comparison with 'a priori' knowledge of the ideal reflector shape [4]. Since the aperture field approximation imposes severe restrictions on the data window size the intrinsic depth resolution of the image is characteristically poor, and wide angle scattering from feed support struts for example is not recorded causing the struts to appear as geometric shadows on the image. Regions of the reflector surface lying beneath these blockages cannot therefore be reconstructed. Moreover, the narrow data recording bandwidth also produces inferior transverse resolution of profile perturbations on the reflector surface.
There is renewed interest in the idea of determining the near-zone and far-zone bistatic RCS of complex targets from near-field data. This paper addresses the issue of efficient acquisition and processing of the requisite scattered near-field electric field data for determining the wide-angle bistatic RCS of electrically-large targets. Toward that end, several potential combinations of target illumination and near-field scanning techniques are considered in this paper. The techniques considered encompass mechanical and electronic scanning methods using single probes, linear probe arrays, and planar probe arrays to accomplish the near-field scanning, combined with either (a) compact range illumination or (b) "synthesized" plane wave illumination employing a single probe, a one-dimensional (1-D) probe array, or a two-dimensional (2-D) probe array. A general Spherical Angular Function (SAF) integral formulation of near-field bistatic coupling/scattering is presented, and an approximate "deconvolution" technique for electrically-large targets is described.
The techniques of near-field antenna pattern measurement can be extended to near-field RCS measurement. The motivation for doing so is precisely the same as that for near-field antenna measurements; i.e., the convenience of an indoor antenna range, and an improvement in accuracy. Although the near-field measurement problem is solvable in principle in a manner analogous to the near-field antenna problem, it requires a significantly larger amount of time to take the necessary data, and to subsequently process the data to obtain useful quantities. BDM is currently involved in an on-going program to evaluate the feasibility of near-field bistatic RCS measurements. At the time of this writing, a complete set of mathematics has been formulated to handle the probe correction and data processing. The hardware has been built, software development is near completion, and the analysis of canonical scattering objects has been completed. Experimental data soon to be taken for these objects will be presented. It is hoped that the technique will prove to be a practical approach to RCS measurements.
High resolution radar imaging is becoming an increasingly important component of RCS measurement systems. The primary purpose of radar imaging as applied to RCS measurements is to locate and quantify the various scattering components that contribute to the total RCS of a model under test. The technique when properly applied by trained personnel can greatly improve the productivity of measurement programs by reducing the number of measurements needed to find defects in a model, and by rapid improvement in the understanding of the scattering phenomena itself.
This paper explores a design approach to RCS measurements as required for the radar backscatter community. Background will be provided as to the approach and the measurement system experience of the RCS system design team. This will include the approach to computer networking, multiple range configurations and data reduction schemes. The solution under development will detail some of the requirements for the controllers and peripherals needed for the task. System design goals such as CPU independent software design, real time data acquisition and status display, multiple CPU and radar front end networks, system resource control and dynamic graphics design will be explored.
An RCS measurement system based on the HP 8510 and a Compact Range reflector system has the following limitations: high clutter levels limit the maximum transmit power and therefore the system's sensitivity, the maximum number of frequency points limit the maximum resolution and/or range length, and the proper separation of clutter and test target data requires taking data describing the entire range, even for a desired CW measurement, thus increasing measurement times significantly.
For over a decade the National Bureau of Standards has utilized the Planar Near-field Method to accurately determine antenna gain, polarization and antenna patterns. Measurements of near-field amplitudes and phases over a planar surface are routinely obtained and processed to calculate these parameters. The measurement system includes using a cw source connected to an accessible antenna port and a two channel receiver to obtain both amplitude and phase of the measurement signal with respect to a fixed reference signal. Many radar systems operate in a pulsed-cw mode and it is very difficult if not impossible to inject a cw signal at a desired antenna port in order to calibrate the antenna. As a result it is highly desirable to obtain accurate near-field amplitude and phase data for an antenna in the pulsed-cw mode so that the antenna far-field parameters can be determined. Whether operating in the cw or pulsed-cw modes, one must be concerned with calibrating the measurement system by determining its linearity and phase measurement accuracy over a wide dynamic range. Tests were recently conducted at NBS for these purposes using a precision rotary vane attenuator and calibrated phase shifter. Such tests would apply not only to measurement systems for determining antenna parameters but also to systems for radar cross section (RCS) measurements. The measurement setup will be discussed and results will be presented.
A planar near-field scanner is described. It has an effective scan plane of more than 5 by 12 meter. The scanner will be used for the measurement of the Synthetic Aperture Radar (SAR) antenna of the European Remote Sensing satellite ERS-1. The requirements are discussed and the results of the first mechanical verification measurements are presented.
Near-field antenna measurements have many advantages, but also some limitations, which can be mainly attributed to the need for costly facilities or severe environmental effects. Although anechoic chambers are widely employed, absorbing material is very expensive and the whole construction becomes a considerable project, especially if it is required to accommodate various size antennas over wide frequency ranges.
A 15-meter diameter self-deployable antenna has been developed which utilizes the hoop-column structural concept with a gold-plated molybdenum mesh reflector. This antenna was developed to determine if a system could be designed and built with the dimensional tolerances necessary for in-space operational performance and for use as a test article in a ground based technology development program. One feature of the design is the provision for reflector surface shape control by cable adjustment. The antenna was deployed and tested at the Martin Marietta Denver Aerospace Near-Field Test Laboratory to measure its surface shape and its electromagnetic performance. RF test results show very good agreement between predicted and measured radiation patterns. The antenna is currently undergoing modifications which will allow automated surface adjustments and adaptive feeds to be utilized for further improvement in the electromagnetic performance. Controls, structural, and simulated thermal deformation tests will be integrated with future electromagnetic tests.
A spherical backward transform technique has been developed and applied to the determination of radome anomalies from near-field measurements. This paper reports on this technique and presents measured data for a missile radome.
In any type of electromagnetic measurements, the ideas of "precision and accuracy" and "low cost" tend to be mutually exclusive. At Scientific-Atlanta, for instance, production testing of antenna products is conducted in low cost miniature "anechoic chambers" which are fabricated in-house. These "chambers" are actually medium-sized to large (64-200 cubic feet) rectangular boxes with absorber attached to their walls. They are usually equipped with single axis positioners at one or both ends, and their usefulness is limited to the measurement of axial ratio on low gain small antennas.
The recent activity and study of the compact range has been increasing the past few years. Both radar cross section (RCS) and antenna measurements have been conducted in the compact range. Important research and analytical investigation has also been done in the design and construction of the reflectors so characteristic of these types of ranges. Edge diffraction from the reflector has been studied and characterized by methods of geometrical optics, geometrical theory of diffraction, physical optics and physical theory of diffraction. Treatment of edge diffraction effects on the reflector have included serrations, rolled edges, and absorbing materials. The primary goal is to obtain as perfect a plane wave as possible in the enclosed chamber with reduction of edge diffraction from the reflector.
As the operating frequencies of compact range antennas increase, the accuracy of the field probes used to characterize their performance must also increase. Obtaining the required accuracy through mechanical design becomes more and more difficult as the size of the area to be probed increases. This paper describes the use of a laser measurement system to sense the probe's mechanical displacements thereby allowing corrections of compact range measurement. The relatively simple laser alignment system is well-suited for compact range probing in which accuracy is much more critical in the Z direction than the X-Y direction.
The compact range reflector used these days for RCS and antenna measurements have rolled edges [1] to reduce the stray fields diffracted from the rim of the parabolic section. For optimum performance (small edge diffracted fields), blended rolled edges [2] are used. A blended rolled edge ensures that the radius of curvature of the surface is continuous at the junction between the paraboloid and the rolled edge. By selecting an appropriate blending function, one can make the first and higher derivatives of the radius of curvature continuous at the junction [3] which in turn results in a weaker diffracted field. However, the resulting reflector may be too large to be practical. Also, the minimum radius of curvature of the reflector surface in the lit region may become less than one fourth of the wavelength at the lowest operating frequency, which is not desirable. Thus, the choice of blending function and rolled edge parameters is quite important in the design of compact range reflector antennas. In this paper, a procedure to design blended rolled edges for such applications is discussed. The design procedure leads to a rolled edge that minimizes the edge diffracted fields while satisfying certain constraints regarding the reflector size and minimum operating frequency of the system. Some design examples are included.
Recently, needs have arisen for low axial ratio feed horns for prime focus fed compact ranges. The compact range environment necessitates a feed possessing low back lobes to minimize extraneous radiation. Circular polarization demands dual orthogonal linear polarizations with symmetrical radiation characteristics. An iris loaded square waveguide section was developed to produce a quadrature phase shift in one linear polarization versus the orthogonal polarization. This 90 degree phase shifter was incorporated into a corrugated horn to achieve a 1 dB axial ratio or less over a full waveguide band. Theoretical and experimental data will be presented for several of these horns. Extensions to lower axial ratios (less than .5 dB) using a double tuned circuit approach will also be presented.
The offset fed parabola is one type of reflector used in compact radar ranges. Cross-polarization problems have been noted when a parabola is used in near field applications. A good understanding of the near field cross-polarization effects was needed to evaluate this type of reflector for a compact range. We found that the polarization vector was rotated differently at each location in the "quiet zone." The polarization vector rotation is due to the parabolic curvature. In addition, a mathematical model was derived that compares well with the data. A theoretical study of how the RCS measurements of a wing are affected is presented.
We have found several crucial inconsistencies in the basic equations of radar polarimetry which are rather common in the current literature on the subject. In particular, the pertinent formulations of the polarization state definitions given in the IEEE/ANSI Standards 149-1979 are in error. These and other inconsistencies and conceptual errors are analyzed very carefully in this presentation. We provide the correct formulae for the proposed revision of the polarimetric standards together with a well-defined and consistent procedure for measuring target scattering matrices in both, mono-static and bi-static arrangements. Further, the proposed procedure can be applied to an arbitrary measurement process in any general elliptical polarization basis.
A bicone telemetry and command antenna is a stack of two physical antennas with toroidal patterns which have radiation patterns which are omnidirectional in the azimuth plane, perpendicular to the transfer orbit spin axis of the satellite, but are directional in the elevation plane. Each of the two physical antennas,, which operate at different frequencies and polarizations to avoid feedback, has two independent RF inputs (for redundancy) making it actually a four antenna configuration. Each physical antenna consists of three components, which are the feed input section with dual RF inputs, a circular polarizer and a radiation structure comprised of slots, in the circumference waveguide structure, which feed the circumferential conical horn necessary to obtain the required directivity in the elevation plane. The procedures and the problems encountered in constructing and testing each of these parts, as well as the components necessary to permit their testing as independent units is discussed. Because of the broad radiation patterns which characterized these omnidirectional C-Band and K-Band antennas, special consideration had to be given to the measurement of the antenna patterns. These problems and their solutions are highlighted in the paper.