AMTA Paper Archive

This paper describes specially constructed instrumentation and positioning systems used in evaluating RF absorber, discusses measurement techniques, and presents data and conclusions from current programs. The selected absorbers which were evaluated are typical of those used in anechoic chambers and terminated ranges for antenna, radome and RCS testing.

A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.

Reducing ripple in the aperture field of the parabolic reflector is one of the main considerations in the design of a compact range, since it determines the "usable" target zone for RCS and antenna measurements. The usable target zone is typically defined as the aperture region where the ripple is less than 0.1 dB [1]. Studies [2,3] have shown that edge diffractions and therefore ripple can be significantly reduced by using blended rolled edges such as in Figure 1. For low aperture field ripple, it is assumed that the junction between the parabolic surface and the blended rolled edge is smooth. In practice, however, the rolled edges may be machined separately and then fitted to the main reflector. If this is done, small wedge angle errors (Figure 2) or step discontinuities (Figure 3) may be mechanically introduced at the junctions. Typically, angle deviations of plus-or-minus 0.5 degrees and steps of plus-or-minus 0.005 inches may be expected. If the parabola and part of the rolled edge is machined as a unit, diffraction due to discontinuities in the mechanical junction between this surface and the rest of the rolled edge can have less effect on ripple in the aperture field. Now, the questions to be answered are: * How much of the target zone is lost due to discontinuities at the edge of the parabola? * How much of the rolled edge need to be machined with the parabola to prevent mechanical discontinuities from decreasing the usable target zone? * What range of discontinuities can be tolerated? In this paper, these questions are answered for a 12 foot radius semi-circular compact range reflector with cosine-blended rolled edges.

Harris Corporation has developed and introduced a miniature version of its shaped compact range called the Model 1603. This model is actually a scaled version of its very large compact ranges. The range features a three foot quiet zone in a very compact configuration, allowing the range to be set up in an anechoic chamber as small as a normal conference room. Performance features are equivalent to those achieved in large compact ranges by Harris, such as the Model 1640 with a forty foot quiet zone. Key features are very low quiet zone ripple, extremely low noise floor, and low cross polarization. This range can be used for the full gamut of precision RCS testing of small models or precision testing of antennas. It should also find wide application in production testing of these items. Harris can also provide turnkey compact range test systems based on the Model 1603 that use available radar instrumentation. Several of these miniature compact ranges have been delivered and are in use.

Current demands for accurate low-level radar cross section (RCS) measurements require anechoic chambers and compact ranges to have extremely low background scattering levels. Such demands place difficult requirements on the entire chamber and warrant the need to predict and mathematically model chamber performance. Accurate modeling, prior to chamber construction, also aids in chamber performance optimization through improved chamber designs.

A way has been found to utilize the reflector return in a compact range as a source of continuous drift compensation. This is performed by translating receive polarizations 45 degrees with respect to the transmit polarizations to ensure returns in co- and cross-polarizations. An added benefit is the simplicity of alignment for the polarization calibration standard.

A wide variety of precise, automatic instrumentation systems is currently available for RCS testing. These systems, either commercially available as integrated units or assembled from laboratory test instruments, can automatically measure the RCS of a target over fine frequency increments spanning wide bandwidths. When the frequency responses are measured for discrete increments of target rotation, the resulting two-dimensional (frequency-angle) data arrays can be processed to obtain two-dimensional RCS images.

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.

This paper discusses an anechoic chamber absorber evaluation which was conducted for the purpose of improving anechoic chamber and compact range performance through better absorber characterization. This study shows that performance of conventional absorber materials is dependent on selection of the material's shape, size and orientation with respect to the incident energy direction. This, demonstrates the importance of better characterization of the material. Nonhomogeneities in the material composition and physical structure were also found to significantly modify performance; in some cases even improving it. Also shown, is the need for improved evaluation techniques and procedures over conventionally used methods. An evaluation procedure using modern imaging techniques is presented. Several measured results for various absorber types and sizes are presented which show the usefulness of the evaluation technique and demonstrate relative performance characteristics for these materials. Measured reflectivity data on various absorber types, which consistently show better performance than levels specified by the vendors, are also presented.

Motorola's Government Electronics Group (GEG) located in Scottsdale, Arizona has recently completed construction of an indoor Antenna/RCS Test Facility. Motorola achieved quality construction of this new facility by utilizing local building contractors working under Motorola supervision through concept study, design, and construction phases. Motorola achieved quality chambers without turn-key costs. Three anechoic chambers and one shielded computer room were fabricated. The chambers sizes vary from 20'W x 16'H x 41'L to 36'W x 36'H x 72'L. All chambers were evaluated using techniques described by MIL-STD-285 (Attenuation measurements for enclosures, electromagnetic shielding, for electronic test purposes, Method of) and indicated shielding effectiveness, before absorber installation of -60 to -70 db at 400 MHz and -80 db from 1-18 GHz. Shielding effectiveness increased to -80 dB at 400 MHz and to greater than -115 dB from 1-18 GHz after absorber installation. In addition, the building contains eight individual security areas meeting government standards for security as prescribed in the Defense Intelligence Agency Manual (DIAM 50-3).

The indoor compact range has proven to be quite successful in measuring the radar cross section (RCS) of various targets. As the performance capabilities of the compact range have expanded, the use of larger, heavier, and more sophisticated targets has also expanded. Early target dimensions were limited by the size of the useful test area, as well as the capacities of the low RCS pedestal mount used. Today, our anechoic chamber has a large useful test area, thus the size and weight of targets dictate that a new method be employed in target handling and positioning, as well as target mounting to a low RCS pedestal. Work was recently completed here at the Ohio State University ElectroScience Laboratory to remodel our anechoic chamber to allow for the new generation of targets and the demands that they place on the anechoic chamber. This work included the addition of a one ton motorized underhung bridge crane to our anechoic chamber, the design and construction of an hydraulic assist to smoothly and precisely raise and lower the target for the final linkup of the support column and the receiving hole in the target, the design and installation of a one ton telescopic crane in the chamber annex to link with the main chamber crane, the design and installation of the necessary microwave treatments to minimize the impact of the remodeling on accurate RCS measurements, the development and installation of a sloping raised floor, the design and manufacture of a track guided rolling cart to shuttle operating personnel to and from the target area, the replacement of the existing radar absorbing material, the improvement of the ambient lighting in the chamber to facilitate film and video tape documentation, and the development of new target mounting schemes to ensure ease of handling as well as secure mounting for vector background subtraction.

Characteristics of the Harris Model 1606 Compact Range are summarized and considered for applicability to RCS measurements. Measured characteristics of quiet zone performance (amplitude and phase distributions) and standard target RCS data are presented. Of particular interest is a comparison of predicted and measured radar cross section versus aspect angle of some familiar standard targets under various conditions.

This paper presents both radar cross section and polarization scattering matrix measurements on microwave radar navigation targets. The polarization measurements are performed using a unique two-channel facility which allows for measuring the circularly polarized scattering matrix elements at X-band. For the same targets conventional RCS measurements are performed using an automated system comprising a network analyzer (HP-8510) and a desk top computer system (HP-236 or 310). This system allows wide frequency range measurements. Details of these measurement techniques, and results will be presented.

This paper describes an automated, frequency-step, pulsed/CW Radar Cross Section (RCS) measurement system using the HP 8510 network analyzer. The system has been built using the concepts developed at Lincoln Laboratory (1) and is being utilized in an operational capacity. The unique features of this system are the use of (a) a dual-probe antenna for the transmission and reception of RF signals, and (b) a pulse system for separating the target-scattered signals from the incident and background signals. The single antenna configuration provides a true monostatic backscatter measurement. A polarization control circuit makes RCS measurements for all combinations of transmit/receive polarizations possible (linear and/or circular). The pulse system uses pin-diode switches capable of generating a 7-ns pulse width and a repetition rate up to 8 MHz. The pulse system effectively eliminates unwanted signals at ranges other than the target range. Therefore, the full dynamic range of the receiver can be used for the measurement of the target.

Narrow band RCS measurements are usually presented as RCS versus target aspect angle in either a rectangular or polar format. Wide band measurements are not normally analyzed in the frequency domain. The normal procedure is to perform either a one or two-dimensional Fourier transform of wide band data or obtain high resolution information on the location of scattering sources. In this paper, we investigate the possible uses of the wide band data directly. In particular, we show that a natural coordinate system for analysis of these data is a polar format with frequency taking on the polar distance parameter and aspect angle taking on the polar angle parameter. This format is not coincidentally, an intermediate step in the production of fully focused two-dimensional radar images. The polar format frequency domain plots are shown to be effective at categorizing the nature of the physical scattering. This is especially true when combined with image domain filtering to isolate scattering regions of interest. In addition, it can be useful in determining anomalies in the radar measurement system performance, and in assisting the analyst to explain unexpected image domain results.

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.

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.