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Extracting absolute RCS levels from radar images has become a prevalent practice, but is it valid? Scattering strengths associated with pixels in radar images are derived from responses of the target averaged over frequency and aspect angle. This paper presents theoretical and experimental data for simple and complex targets with frequency-and angle-dependent scattering to illustrate differences between results of narrowband and wideband RCS measurements.
The use of electronically scanned phased array antennas in demanding rolls such as satellite communications and radar systems has led to an increasing desire to analyze the sources of error present in the boresight alignment of such systems. Not the least among these errors are those introduced by thermal effects on the various components which comprise the array structure. In an effort to understand this mechanism, this paper will discuss a technique which uses an infrared camera system to analyze the beam deflection errors caused by the effects of temperature gradients present in the antenna system.
Phased array antenna sidelobe levels are evaluated based on the statistics of the differences in element patterns. It is shown that the differences can be treated as random errors. The standard formula for predicting the average sidelobe level of an array due to random errors is valid if the interaction between the element patterns and the excitation function is taken into account. Sidelobes of a linear array with a variety of near-field perturbations are considered. The statistics indicate that for an N-element array, adaptive calibrations may lower the average sidelobe level by a factor of N.
This paper deals with the development of an approach to the design of triad steering antenna arrays which are used in anechoic chambers for hardware-in-the-loop testing of monopulse antenna seeker systems. In the design of a large array, such as those used for hardware-in-the-loop of guided weapons, it is important to optimize the array element spacing. An excessively narrow spacing results in an unreasonable number of required antennas and increased cost, while an excessively wide spacing will induce angle measurement errors in the seeker under test which can be significant. The specific objective of this effort is to quantitatively describe the monopulse discriminant efforts which result when a non-planar field, radiated by an antenna triad, illuminates a monopulse seeker under test. The approach to this problem is to calculate the triad field at the aperture of the monopulse seeker assuming various levels of triad element phase and amplitude error. Using this illumination field and the illumination function of the monopulse antenna, the resulting sum and difference patterns are calculated along with the monopulse discriminant. Software has been developed to perform these calculations. The resulting patterns are compared with the ideal far field pattern and the discriminant bias, or angle measurement error, is quantified.
With the recent use of dual-polarized transmission and reception on communications links, the capability to perform accurate polarization measurements is an important requirement of test-range systems. Satellite antennas are commonly measured in the clean, protected environment of compact and near-field ranges, and a circularly polarized feed/field probe is a primary factor in establishing their polarization properties. The feeds also provide excellent source-horn systems for tapered anechoic chambers, where their circular symmetry and decoupling of the fields from the absorber walls improve the often troublesome polarization characteristics of tapered chambers. Circularly polarized feeds are generally composed of four primary waveguide components: the orthomode transducer, quarter-wave polarizer, scalar ring horn, and circular waveguide step transformer. Linearly polarized feeds omit the quarter-wave polarizer. This paper discusses the design and performance of high-polarization-purity source feeds for evaluating the polarization properties of antennas under test. Circularly polarized feeds have been constructed which operate over 10- to 20-percent bandwidths from 1.5 to 70 GHz. Gain values are generally in the area of 12 to 18 dBi, with cross-polarization isolation in excess of 40 dB. Representative measured data are presented.
As antennas have become more sophisticated, the testing requirements have grown tremendously. Testing often adds significantly to the cost of the system. A need has developed for test equipment more advanced than the completely manual systems of the past and less expensive than the completely automated systems of today. An antenna pattern recorder which helps to minimize test time is presented. The instrument utilizes a use friendly touch screen which facilitates user interaction with the unit. The pattern recorder is capable of measuring up to five channels of data simultaneously as a function of angle, linear position, or time. The data is stored on electronic media and may be saved, retrieved, zoomed, plotted, analyzed by internal programs or exported for analysis by external programs. The user may customize the plot format for test reports, proposal information, and other data requirements.
Northrop Corporation's Business and Advanced Systems Development Group has recently completed a very successful Radar Cross Section (RCS) measurements program on the USAF/Northrop B-2 bomber. One of the capabilities spawned from the program is a measurements radar system, comprised largely of off the shelf hardware, which provides high resolution whole body two-dimensional RCS images of large targets on the ground in the near field. Its high gain antennas allow operation in a space limited area and utilizes Synthetic Aperture Radar (SAR) data collection techniques. The system, though designed for use at VHF, has been expanded to operate from 100-2000 MHz in three bands. The hardware, associated signal processing, its applications and limitations are discussed.
The utility of array feeds for compact range reflector antenna applications is discussed. The goal is to feed a circular-aperture, offset parabolic reflector such that the central illumination is uniform and the rim illumination is zero. The illumination taper results in significant reduction of edge-diffracted fields without the use of reflector edge treatment. A methodology for designing an array feed requiring only two real excitation coefficients is outlined. A simple and cost effective array implementation is presented. The array beam forming network is realized as a radial transmission line which is excited at the center from a coaxial transmission line, and terminated at the perimeter with absorber and conductive tape. Energy is probe-coupled from the radial line to balun-fed dipole array elements. The required element amplitude excitation is obtained by adjusting the probe insertion depth, and the required element phase excitation is supplied by the traveling radial wave. Construction and test of an X-band array are summarized. The measured array patterns display a flat-topped beam with a deep null at angles corresponding to the reflector rim.
The Compact Range is becoming the method of choice for indoor testing of many types and sizes of antennas. Implementation of a compact range requires a suitable parent building structure in which to house the chamber. The chamber is located within the parent building and the compact range is then installed within the chamber. In some cases an existing building may not be available for the range and it may be difficult to acquire a new building due to local or proprietary requirements. Once a building has been located, many problems still exist with coordination installation of the chamber and compact range within this building. Overcoming these problems can be both time consuming and inefficient in terms of cost. This paper describes a Compact Antenna Range conceived and designed to be totally self contained and truck transportable. The compact range consists of a complete anechoic chamber facility with self contained electrical, lighting, HVAC and fire protection systems. The compact range provides a 3 foot test zone over the 5.8 to 94 GHz frequency range. Once completed and tested at the factory, the facility is transported and set in place at the user site. Details are presented which describe the structural requirements of the chamber, the RF performance of the completed facility, and the transport and installation process. The integrated test positioner and an automatic feed changing mechanism are also described.
This paper briefly reviews existing compact range performance characterization methods showing the limitations of each technique and the need for an accepted and well understood technique which provides efficient and accurate characterization of compact range measurement accuracy. A technique known as the transverse pattern comparison method is then described which has been practiced by the author and some range users for the past several years. The method is related to the well known longitudinal pattern comparison method, however, comparisons are conducted in the transverse planes where the required span of aperture displacement is much smaller and does not exceed the dimensions of the quiet zone. This method provides several advantages for characterizing compact range performance as well as enables range users to improve achievable measurement accuracies by eliminating the impact of extraneous signal errors in the quiet zone.
Design and implementation of a large horizontal planar near-field system delivered to Space Systems/Loral for satellite antenna testing will be discussed. The 22' x 22' scan plane is 25' above the ground and employs real-time optical compensation for the X, Y, Z corrections to the probe position. The system provides high speed multiplexed near-field measurements using NSI's software and the HP-8530A microwave receiver. System throughput is enhanced through the use of a powerful and flexible test sequencer software module.
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 field present in the test zone of an antenna measurement range can be calculated from the range field measured on a spherical surface containing the test zone. Calculated test zone fields are accurate only within a spherical volume concentric to the measurement surface. This paper presents a technique for determining the probing radius necessary to create a volume of accuracy containing the test zone of the range. The volume of accuracy radium limit is caused by the spherical mode filtering property of the displaced probe. This property is demonstrated in the paper using measured field data for probes of differing displacement radii. This property is used to determine the volume of accuracy radium from the probing radius. This is demonstrated using measured far-field range data.
The Method of Cauchy has been used to extrapolate a desired parameter over a broad range of frequencies using some information about the parameter as a few frequency points. The approach is to assume that the parameter, as a function of frequency, is a ratio of two polynomials. The problem is to determine the order of the polynomials and the coefficients that define them. For theoretical extrapolation/interpolation the sampled values of the function and, optically, a few of its derivatives with respect to frequency have been used to reconstruct the function. This technique also incorporates the method of Total Least Squares to solve the resulting matrix equation.
A new multi-purpose antenna measurement facility was put into operation at the National Institute of Standards and Technology (NIST) in 1993. This facility is currently used to perform gain, pattern, and polarization measurements on probes and standard gain horns. The facility can also provide spherical and cylindrical near-field measurements. The frequency range is typically from 1 to 75 GHz. The paper discusses the capabilities of this new facility in detail. The facility has 10 m long horizontal rails for gain measurements using the NIST developed extrapolation technique. This length was chosen so that gain calibrations at 1 GHz could be performed on antennas with apertures as large as 1 meter. This facility also has a precision phi-over-theta rotator setup used to perform spherical near-field, probe pattern and polarization measurements. This setup uses a pair of 4 m long horizontal rails for positioning antennas over the center of rotation of the theta rotator. This allows antennas up to 2 m in length to be accommodated for probe pattern measurements. A set of 6 meter long vertical rails that are part of the source tower gives the facility that added capability of performing cylindrical near-field measurements. Spherical and cylindrical near-field measurements can be performed on antennas up to 3.5 m in diameter.
A large deployable multibeam antenna for communication satellites operating in the Ka band with 2.5 GHz transmit/receive bandwidth was developed and measured. The antenna is an offset Cassegain system with a 4.7 m diameter mail reflector divided into a central and 24 rigid deployable panels. One application studied in detail was the continuous illumination of the FRG with 16 beams. Spherical nearfield measurement techniques were used to validate the predicted performance. Because the gravity influence would cause inadmissible deformations, a compensation device must be used. To take into account the influence of the remaining deformations varying with the elevation position of the antenna, a special analysis software was developed which uses measured surface coordinates. Because measured and computed values agree well, it is possible to predict the performance in orbit precisely. A pointing accuracy of 0.01 degrees was achieved by adjustment of the sub reflector using a monopulse tracking system.
Antenna microwave holography is now a well established technique and has for a number of years provided a diagnostic tool for the evaluation and optimization of the electrically large reflector antennas used for satellite ground stations. Increasing interest is being shown in the use of the technique during the development of other complex antenna configurations in order to improve the design, minimize design cycles and, hence, reduce the overall cost. This contribution presents two examples of applications of the technique during the development of high performance antennas at ERA Technology LTD. For a corrugated slot-array antenna operating at 19.95 GHz, a clear improvement in the performance following design optimization based on the results obtained from microwave holography is shown for a 3 Am diamond reflector antenna for SATCOM applications operating at 14GHz, the technique provides a verification of distortions in the surface profile by mapping of the aperture phase distribution.
Anechoic chambers have difficulty in meeting the new basic standards for radiated emission and susceptibility test facilities that have come into operations by the new EMC directive of the European Economic Community. In this contribution a method first presented at the 1992 A.M.T.A. meeting is extended to compute the performance of anechoic chambers at the most critical lower MHz frequency range. Computational results are shown of a real semi-anechoic chamber with a sloped ceiling and a symmetrical reference chamber. The results are compared with measurements values obtained by scanning the chamber with a small field probe. Following this, several methods for optimizing the chamber performance are proposed and evaluated in their effectiveness. The goal of this work is to achieve an accreditation of existing as well as chambers still to be built as standardized EMC test facilities in the specified frequency range.
DASA's high precision Compact Range Program, which already was a breakthrough in new dimensions of RF measurements standards, will not be completed by a revolutionary new and one of the world's most unique types of Cylindrical Outdoor Near-Field Test Range. The most striking component of this new type facility will be its dominating fully air-conditioned, up to 50 m high diamond shaped concrete tower which is the integral part of the vertical probe scanner subsystem. Although this test range is located outdoor, it allows extremely precise characterization of all typical parameters for state of the art antenna systems.
In polarimetric RCS measurements, the cross-polarization levels which are required in the test zone, correspond closely to those which are realizable with most Compact Antenna Test Ranges (CATR). On the other hand, such a performance may not satisfy the accuracy requirements in cross-polarization measurements of high performance microwave antennas. These applications include spacecraft antennas, ground stations for satellite communications or microwave antennas for terrestrial applications, where two polarizations are used simultaneously.