AMTA Paper Archive

This paper presents precision measurements of the RCS of a simplified aircraft geometry called the "generic aircraft". The RCS is measured over a frequency range of 2 to 18 GHz, and for incidence angle from "nose-on" through "broadside" to "tail on". This data is presented in the form of RCS contours as a function of frequency and incidence angle, and is compared with the computed RCS using wire-grid modeling. The contours show distinct patterns due to airframe resonance and due to the interference of the scattered field from the nose and from the tail of the aircraft.

A method has been developed by which the fair-field RCS of a target can be evaluated from its RCS measured in the near field. The method can compensate for the nonuniformity of the antenna pattern which can be a function of the angle, the frequency, and the target distance. A correction transform is evaluated which depends on the antenna pattern, the frequency, the target distance and the target size. The correction transform is independent of the target geometry. The RCS of a target is measured in the near field, in a band of frequencies around the frequency at which the far field RCS of the target is desired. The method can practically handle directional scattering elements, shading of the scattering elements by each other, and interactions among the scattering elements. The reconstructed RCS evaluated by this method shows excellent agreement with the actual far-field RCS.

A 4 foot by 4 foot near field planar scanner is used to evaluate the performance of a SA5751 compact range in CSIST. Using the far field patterns integrated from the scanned aperture fields, the coming directions of the clutters in the chamber can be determined. Often the clutter level is less than the side lobe level of the far field pattern, the scanned field is multiplied by a certain weighting function before integration to pop out the clutter signal. However the weighting method would broaden the main beam and hence clutters coming close along the reflected wave of the reflector are still can not be seen (sic). In this article, a method called main beam suppression, subtracting a constant filed (sic) on the scanned aperture, is introduced to solve this kind of problem and the result shows it serves well for finding those clutters hidden by the main beam and the side lobes nearer to it.

The Slotline/Bowtie Hybrid (SBH) antenna concept has been applied to develop an ultra wide bandwidth feed for compact range applications. The initial design requirements were to develop a feed with a 30 degree 1dB beamwidth from 1 to 18 GHz. It was felt that one could sacrifice the beamwidth at the lower frequencies somewhat because that would reduce the feed spill over which is normally the worst at lower frequencies. The resulting antenna has an 18" by 18" aperture and basically meets the bandwidth requirements. In the worst case, it has 2 dB variation across the desired 30" beamwidth. The phase center is relatively constant, and VSWR is basically less than 2:1 from 1 to 18 GHz. Measured and calculated results are shown to illustrate the performance of this new feed antenna. In addition, the measured amplitude and phase patterns have been input to a reflector analysis code to predict the field probe data in the simulated quiet zone. These results clearly show that this new feed performs very well from 1 to 18 GHz.

Recently, super resolution algorithm have been used in radar target imaging to increase the down range and/or the cross range resolution. In the open literature, however, the super resolution algorithms have been applied to simulated targets or very simple targets measured in a test range. In this paper, the super resolution algorithms, namely the hybrid algorithm and the 2-D linear prediction, are applied to more realistic targets. One of the targets is a flat plate model of the F-117 aircraft. The back-scattered fields of the flat plate model were measured in a compact range. The other target is a Mooney 231 aircraft. The aircraft was flown in a circular pattern approximately 10 miles from the radar. It is shown that the super resolution algorithm can be successfully applied to these targets.

In recent years, a need has arisen to measure the patterns of high gain antennas having ultra low sidelobe levels (ULSA), usually installed on aircraft platforms. The objective is to measure antenna patterns which include the platform multipath, generated by reflections off the aircraft, but which exclude the effects of range multipath. These advanced antennas require testing capabilities to about 60 dB below the maximum level. Moreover, the range multipath is at times expected to be stronger than the direct path if the mainbeam antenna is pointed below the horizon. Aggravating the problem is the fact that usually CW antenna patterns are desired. ATEK developed an innovative technique which accomplishes the required tasks using the following concept; A 20 MHz signal is used to generate desired weight taps needed to cancel the received signal. Since part of these taps are used to cancel the desired signal, it has been shown that they can be used separately, following the adaptive process to measure the antenna patterns with the multipath suppressed. The technique can be used to measure very accurately antenna patterns in response to either CW or non-CW signals.

Despite their high cost, phased array antennas are becoming popular for radar applications because of their ability to provide reliable information even in a hostile environment. Evaluation of these antennas requires parameters like gain, radiation pattern, beam width, sidelobe (both near and far off) azimuth and elevation null depth, etc. to be tested over the entire range of frequency spots and scan angles. Typically, if the number of frequency spots are 24 and the number of beam positions for which the measurement has to be done are about 100, then the total number of measurements needed to generate the required data are 7200. In addition, phased arrays with a space feed have to be initially collimated at all the spot frequencies. The outdoor testing of these many parameters may not be convenient, and at times it may even be impossible. The planar near field measurement technique provides a systematic and accurate method of measuring large array antennas for all the required parameters.

An alternate method is presented for computing far-field antenna patterns from planar near-field measurements. The method utilizes near-field data to determine equivalent magnetic current sources over a fictitious planar surface which encompasses the antenna, and these currents are used to ascertain the far-fields. An electric field integral equation (EFIE) is developed to relate the near-fields to the equivalent magnetic currents. Method of moments (MOM) procedure is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method (CGM), and in the case of a rectangular matrix, a least squares solution for the currents is found without explicitly computing the normal form of the operation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions by exploiting the block Toeplitz structure of the matrix and using CGM and Fast Fourier Transform (CGFFT) thereby drastically reducing comparison and storage requirements. Numerical results are presented by extrapolating the far-fields using experimental near-field data.

When the planar near-field method is used for antenna characterization, two probes are required to measure an antenna under test (AUT). The receiving patterns (both amplitude and phase) of these probes, obtained from planar near-field measurements, must be utilized to accurately determine the far field of the AUT. This process is commonly called planar near field probe correction. When the AUT is nominally circularly polarized (CP), the measurements are more accurate and efficient if nominally circularly-polarized probes are used. Further efficiency is obtained when only one probe which is dual-polarized is used to allow for simultaneous measurements of both components. However, when using dual-port CP probes to measure the antenna, we must apply the probe correction even for on-axis measurements.

Near field measurement techniques have become popular now a days (sic) and they are preferred for many applications to the conventional testing. In particular, for testing large planar arrays with a variety of parameters to measure, planar near field technique proves to be useful. The design of the planar near field measurement system varies with the needs of the measurements. This flexibility makes it suitable for somespecial (sic) tests on array antennas. The important design parametersinclude (sic) the type of scan, positional error limit, receiver and source instabilities, cabling, etc. It discuss (sic) in detail the type of scanning when large number of parameters like sum, azimuth and elevation difference patterns of an array antenna for various scan positions are to be measured. This paper describes a planar field system of size 2.5X2.5 metres (sic) and discusses its design details and in particular the type of scanning and the methods of correcting instability of the entire system.

A series of measurements to validate the performance of a Planar Near-Field (PNF) Antenna Test Range located at the Satellite and Aerospace Systems Division at Spar Aerospace Limited were made by Scientific-Atlanta during the month of February 1992. These measurements were made as a part of a contract to provide Spar with a Model 2095 Microwave Measurement System with planar near-field software options and related instrumentation and hardware. The range validation consisted of a series of self-tests and far-field pattern comparison tests using a planar array antenna provided by Spar that had been independently calibrated at another range facility. This paper describes the range validation tests and presents some of the results. Comparisons of far-field patterns measured on the validation antenna at both the Spar PNF facility and another antenna range are presented.

This paper presents the feasability (sic) to explore the Focal Plane (FP) of a Compact Antenna Test Range (CATR). We first introduce the interest of getting very fast the Far Field Pattern of an antenna with a 2D modulated scattering array located at the focus of a CATR. Then, we discuss the geometric, electrical and optical constraints involved when using this technique. A comparison with a classical measurement performed at ESA-ESTEC is shown and we conclude by emphasizing the potentialities of this technique.

As part of the Phalanx weapons system refurbishment program, Thorn EMI Electronics was required to perform a sequence of monopulse feed-pattern measurements at microwave frequencies and receiver-pattern measurements at IF frequencies. The feed-pattern measurements were accomplished using a HP 85301A antenna measurement system. Measurement of the receiver IF patterns, however, required a more novel approach. Because of dual downconversion within the Phalanx receiver, adaptation of the HP 85301A system was required to allow for this frequency translation characteristic. Reconfiguration time between the feed RF and the receiver IF test systems was to be kept to a minimum. The antenna measurement system is described in this paper and IF patterns are presented. The results show that the performance of the test system is not compromised in any way.

This paper describes the most recent version of the Model 200 portable RCS diagnostic radar. The Model 200 was designed to provide high-resolution RCS measurements in unprepared rooms indoors as well as on outdoor ranges. The system can provide real aperture measurements, ISAR measurements, or SAR measurements without changing system configuration.

The initial phase of METRATEK's new Model 300 Radar System has been installed at the Navy's Chesapeake Tests Range (CTR) at Patuxent River, MD. This ground-to-air Multimode, Multifrequency Instrumentation Radar System (MMIRS) is a high-throughput frequency-and-polarization agile radar that is designed to drastically reduce the cost of measuring the radar cross section of airborne targets by allowing simultaneous measurements to be made at VHF through Ku Band.

Today's requirements for dynamic Radar Cross Section (RCS) test data set new demands upon instrumentation Radar systems. Transmitters must deliver high power and operate at high data rates. Additionally, noise floor reduction of coherent spurious signals improves raw data and minimizes the need for manipulation of data.

A method is presented to qualify a compact range measurement system for low sidelobe antenna measurements. The method uses the target zone fields (probe data) of the compact range. Using the method, one can identify the angular regions around which the measurement errors can be significant. The sidelobe levels which can be measured around these angular regions with less than a 3 dB error are also defined.

Prony's method has been found useful in extracting the time domain response over extended time using data samples of limited time span. This paper describes results of studies underway to apply Prony's method to extracting the RCS time and frequency response from limited measured and computed data. The technique has been applied to characterize the RCS response of structures with inherent multiple resonances, e.g., a dielectric cube constructed using high-permittivity dielectric material. Implications of the technique to gated antenna and RCS measurements are discussed.

The radiation characteristics for a dual-frequency, dual-polarized millimeter wave antenna for a radar operating at 33 and 95-GHz were measured at the Ipswich Research Facility. On-pole and cross-pole radiation patterns were measured using the 2600 foot far field range. In this paper we'll discuss the general design of the antenna feed system and the instrumentation ensemble used to perform the far field characterization of this high performance large aperture dielectric lens antenna.

The feed support struts often cause noticeable strut lobes in the patterns of reflector antennas. For example, strut lobes are apparent in the measured and calculated patterns presented in Ref. [1] for the 8-foot diameter reflector with a prime focus feed. As pointed out in [1], the calculated strut lobes are higher than the measured ones. The reason for the difference is secondary scattering by the oppositely located strut, which was not modeled in the calculated pattern in [1]. Detailed examination showed a difference of about 2 1/2 dB caused by the secondary scattering for this reflector antenna design. The purpose of this paper is to present measured and calculated patterns which explicitly demonstrate the quantitative effect of the secondary strut scattering. This effort is shown by comparing the measured strut lobe levels with the oppositely located strut removed, i.e., by using 3 struts instead of 4 struts. Calculated patterns are also given in which the secondary scattering is modeled.