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Compact ranges have found wide use in the pa rametric characterization of high performance radomes such as those found on modern military aircraft. A properly designed compact range facility provides a stable, repeatable test environment suitable for the measurement of small variations in antenna boresight position (beam deflection), antenna pattern distortion, and transmission loss. Radomes have increased in complexity from small structures housing a single antenna to multi-band, multi-system structures large enough to stand inside. Similarly, compact range reflectors have increased in commercial units available today provide quiet zone extents of 12 feet or larger. This paper describes the system design and performance of a compact range test facility designed to test a C-130 Combat Talon II nose radome measuring 7 feet in length and diameter. The facility was constructed at Robins AFB, GA, and is in operation. A description of the facility and its major subsystems is given. Sizing of the chamber and layout of equipment is described. Chamber electromagnetic design considerations are discussed. Electromagnetic design was complicated by the physical size of the structure required to mount the radome, by the fact that multiple antennas and gimbals are present inside the radome during testing, and by the need to use a broad band feed to eliminate mechanical feed changes. Absorber layout and control of spurious reflections is discussed. Electromagnetic performance data is presented.
A reconstruction method that calculates bi-dimensional equivalent magnetic currents from the tangential electric field components over a hemispherical region is presented. The method is applied for diagnosis as well as for near field to far Field (NF-FF) transformation. The method is well suited for antenna radiation pattern measurement using a near-field spherical acquisition system in anechoic chamber.
The accuracy of near field measurements have in the past largely been judged by inspection however the authors have developed an objective measure of the accuracy and repeatability of such measurements. This paper illustrates the measurement process and the techniques associated with statistical image classification used to confirm its accuracy and repeatability. The technique will be illustrated via the correlation of data sets acquired over a variety of different frequencies and scan plane areas. The examination of these measurements will demonstrate the applicability and sensitivity of the technique when the accurate assessment of highly correlated patterns is required.
The angular coverage of planar near-field measurements is limited by the size of the scan plane, and the "region of validity" is defined by the angle between the edge of the AUT and the edge of the scan plane. In some applications, results are required over a larger angular region than is possible with the available scanner. The angular coverage can be increased by rotating the antenna and repeating the measurement. The results of the two measurements are then combined. Successful combination depends on using both the coordinate system and vector components that are appropriate for the antenna rotation. In general for a single antenna orientation, any coordinate system can be used with any vector components, but when combining or comparing patterns for two antenna rotations, the axis of rotation must be the polar axis and the vector components must correspond to that coordinate system. Measurements results will be used to demonstrate the proper choice of coordinates and components and to illustrate potential problems that may arise.
Microwave Instrumentation Technologies (MI Technologies) in cooperation with Hollandse Signaalapparaten B.V. (Signaal) and the Royal Netherlands Navy has designed and produced a compact antenna test range to specifically address the unique testing requirements imposed in the testing of active phased array antennas. The compact range was built specifically to test Signaal's new Active Phased Array Radar (APAR) prior to introduction into various naval fleets throughout the world. This reversible Compact Antenna Test Range (CATR) allows antenna testing in both transmit and receive modes. The measurement hardware is capable of testing both CW and pulsed waveforms with high dynamic range. In addition to conventional antenna pattern measurements the system is capable of measuring EIRP, Gff and G/NF, as well as providing analysis software to provide aperture reconstruction. A special Antenna Interface Unit (AIU) was designed and built to communicate with the Beam Steering Computer which controls the thousands of T/R modules which make up the APAR antenna system. A special high power absorber fence and other safeguards were installed to handle the transmit energy capable of being delivered from the APAR antenna system.
Modern Satellite Antennas and Payloads are characterized by a lot of physical parameters like e.g. Radiation Pattern, Gain, EIRP, Flux Density, Gff and PIM, whereas the available time frame for measurements is getting shorter and shorter. The DSS Compensated Compact Range (CCR) allows a time efficient measurement of all payload parameters with high accuracy under controlled environmental conditions. The CCR consists of two doubly curved reflectors, which prevent inherent cross-polarization and create a very high constant amplitude and phase distribution in the quiet zone with a very good scanning performance. Most of the payload parameters can be measured directly or have to be calculated from a set of measurement values. For the G/T measurement of active antennas a new method for the noise power measurement was established. This paper describes the principle test set-ups with the corresponding measurement techniques to improve the measurement accuracy. Error budgets will be presented for pattern and gain measurement.
In order to assess the suitability of long thin metal rods as calibration devices for both co-polarized and cross-polarized (abbreviated as co-pol and x-pol) RCS measurements, we study RCS data from rods at broadside and compare them with 2D theoretical predictions. We find that the 45° tilt angle is optimum for calibration purposes. Near grazing incidence to a horizontal rod, the first traveling wave lobe in the HH pattern is a very prominent feature. Its angular location and amplitude have been measured as a function of frequency and compared with theory. A formerly unexplained error due to a contaminated calibration is identified.
The interferometric measurements for the structure change detection of a dam due to water level change and to seasonal temperature variation is presented. The instrument used is the Linear SAR (LISA) of the European Microwave Signature Laboratory, which allows two synthetic apertures, one linear of 5 meters length and another circular of about 2 meters. The microwave instrumentation, based on a vector network analyzer and on a pair of wide-band antenna, allows a dual polarized measurement in a frequency band, ranging from 500 MHz to 6 GHz. In this particular context, fully polarimetric measurements have been performed in the frequency band from 5.2 to 6 GHz. From the selected measurements parameters a spatial resolution on the structure of about 30 by 30-cm is achieved. Measurements have been repeated at 7 different dates in the period from June to September. From the set of obtained images a large number of differential interferograms was been formed corresponding to different deformation conditions of the barrage. Results showing the deformation pattern, clearly visible on the whole imaged portion of the structure, are presented. The comparison between measured displacements by D-InSAR and those from the barrage monitoring system in the selected points where traditional tools are installed are in good agreement.
Plasmas inside the TJ-II Stellerator are generated by heating the electron cyclotron resonance waves with a high-power millimeter-wave beam from gyrotron generators and through two transmission lines. Both lines have been tested at nominal power level and they are currently in operation. This paper is devoted to the low-power testing of the transmission lines performed before their operation at high power level. A corrugated horn antenna was designed to generate a pattern that simulates the gyrotron output. In order to evaluate the set up, a twofold approach was taken. On one hand, the antenna pattern was measured and compared with the predicted one. On the other hand, the beam propagation through the mirror line was measured and simulated using Huygens diffraction theory. The comparison of the theoretical and experimental results from both the corrugated antenna and propagation through the transmission line are presented in this paper.
The automobile antenna industry is facing two rapidly growing trends: (1) the incorporation of effective, low cost, AM/FM conformal antenna designs and (2) the antenna capability to handle diversity FM radio receivers. The development of techniques for testing automotive conformal diversity antennas' performance becomes necessary to evaluate and compare them. Testing techniques to obtain the antenna Input Impedance (Zin), Standing Wave Ratio (SWR) and Mismatch Loss (MML) as well as the azimuth gain patterns and the combined diversity signal (maximum of the diversity signals) are described. Experimental results for the Annular Slot Windshield Diversity Antenna using polarization diversity are shown. It is demonstrated that the Annular Slot Windshield Diversity Antenna can be used effectively to reduce multipath fading.
Omnidirectional antennas are typically used as Tracking, Telemetry and Command (TT&C) antennas for satellites. However, the omnidirectional patterns of TT&C antennas located on satellite structures are susceptible to substantial scattering and polarization mismatch loss, especially at the initial mission stage. Consequently, it is very important to properly evaluate the extent of these effects for each of the initial mission configurations. In this paper, measurement techniques to achieve proper evaluation of scattering level and polarization mismatch loss for TT&C antennas of NASA's Tracking and Data Relay Satellite (TDRS) are presented. The paper encompasses a test approach, a test procedure and test results. Application of these test techniques is essential to the TDRS TT&C antenna qualification program.
This paper presents an approximate, practical technique for the compensation of antenna pattern amplitude taper effects that occur in near field RCS data. The technique uses inverse synthetic aperture radar (ISAR) data sets. Complete pattern determination uses an iterative approach over target rotation angle and frequency bandwidth, with a series of near field ISAR images as input to obtain the corresponding corrected, near field, frequency/azimuth pattern data. Assumed is direct target illumination using a source with a known angular illumination pattern. The technique and its application environment in the Boeing Near Field Test Facility is described. It is then demonstrated using a near field data collection range of 100 feet from the target center of rotation. The approach is shown to be effective for target sizes with cross range extents extending to the one-way 3 dB points of the illumination taper (two-way 6 dB points). Demonstration of compensation performance and a study of accuracy achievable versus the near field image parameters used is presented.
We present innovations based on pattern recognition technology that significantly reduce the level of human intervention and increase data throughput when processing radar images of airborne targets. Time consuming operator intervention is normally required to insure that images are centered and non-aliased and wireframe overlay drawings are properly registered with the target image. We have developed techniques that produce high-quality images without operator intervention. These include a template registration algorithm that can reliably orient the outline drawing with a radar image even in the presence of image artifacts such as jet engine modulation (JEM). In addition, we have developed methods that remove the average Doppler responsible for crossrange image displacement or aliasing and methods that resolve downrange ambiguities. Examples are shown which illustrate these processes applied to images of a jet aircraft in flight.
Compact antenna test ranges intended for low cross polarization antenna measurements require the use of feeds with polarization ratios typically greater than 40 dB across the included angle of the quiet zone as well as across the frequency band of interest. The design for a series of circular corrugated aperture feeds to meet these requirements is presented. The feeds are based on a circular waveguide OMT covering a full waveguide frequency band with interchangeable corrugated apertures to cover three sub-bands. In order to validate the design of this series of scalar feeds, high accuracy cross-polarization data was collected. The primary limiting factor in the measurement of the polarization ratio was the finite polarization ratio of the source antennas. A technique for correcting for the polarization ratio of the source is presented along with measured data on the feeds. The technique begins with the accurate characterization of the linear polarization ratio of the standard gain horns using a three antenna technique, followed by pattern measurements of the feeds, and ends with the removal of the polarization error due to the source antenna from the measured data. Measured data on these feeds is presented before and after data correction along with data predicted using the CHAMP® moment method software.
Compact Antenna Test Ranges are eminently suitable for obtaining the far-field patterns of various types of antennas provided that the frequency is not too low. Typically, a low frequency limit of 1 or 2 GHz is realizable. There are, however a number of important applications between 500 and 1000 MHz for antenna diameters between 1 and 3 meters. The far field distance R = 2D2/l is just too large for an indoor far field range. It is generally accepted at present that a good solution for an indoor range for these kind of measurements is very difficult to realize. In this paper the low frequency performance of single and dual-reflector Compact Antenna Test Ranges will be investigated. It will be shown that with carefully designed serrations and feeds, excellent antenna measurements can be carried out at frequencies below 1.0 GHz for a large number of applications. For purposes of comparison, low frequency performance of a compact range with so called blended rolled edges will be presented as well.
Proper operation of a planar NFR (near field range) includes probe correction as part of the processing of the measured data to result in accurate far field angle patterns, particularly for low cross polarized patterns. The far field transform of the near field data produces the angular spectrum which is the product of the plane wave transmission coefficient pattern of the AUT (antenna under test) with the plane wave receiving coefficient pattern of the probe. Probe correction consists of dividing the angular spectrum by the complex probe angle pattern resulting in the pure far field pattern of the AUT [1]. For best accuracy of co and cross polarized AUT patterns one needs to use accurately measured probe complex co and cross polarized patterns in probe correction for each NFR test frequency. The most accurate probe measurements are usually obtained from specialized test laboratories. However, if the number of frequencies is large, this may create problems due to cost or schedule. Because of this it is typical to procure probe calibration at only a few frequencies spanning the test band for each AUT even though pattern measurements are needed at several additional frequencies falling between the calibration frequencies. A typical strategy at any given test frequency is to perform probe correction using the nearest-neighbor-frequency probe calibration data. This strategy produces some unknown error in the processed probe corrected far field patterns of the AUT at each non-calibrated frequency. Inthis paper we will show a method for estimating the non-calibrated frequency probe correction error for co and cross polarized patterns with examples.
A bias error in planar near-field measurement data comes from receiver crosstalk or leakage effects [1, 2, 3]. The bias error is a complex constant added to every near-field data sample. After transformation from the near-field to the far-field, the bias error becomes an easily identifiable spike located at the center of k-space. If one is measuring a horn, then the bias error produces a small bump or spike at the center of the far-zone pattern (i.e. at the center of k-space). If one is measuring a highgain antenna with the antenna beam pointed away from the center of k-space, then the bias error causes an erroneous sidelobe spike at the center of k-space. The bias constant is difficult to estimate be cause it may be more than 60 dB below the peak near-field level. Nevertheless, if the effect of the bias error can be seen in the far zone pattern of the test antenna, then it can be estimated and removed from the measured data. An algorithm is presented that is used to estimate the bias constant directly from the near-field data, then the bias constant is simply subtracted from the data. Examples using measured data are used to illustrate how the algorithm works and to show its effectiveness.
Traditional measurement methods assume that very accurate antenna to range alignment of the antenna under test (AUT) is convenient or possible. It has recently been shown that the use of non-rectilinear co-ordinate systems are of particular use for the purpose of correcting antenna to range misalignment. Additionally, this misalignment correction can be used to construct an extended composite measurement plane from a series of mis-aligned scans that themselves can be considered as constituting a polyhedral measurement surface. This paper describes the additional processing that is required to yield corrected near and far field data from an acquisition of a mis-aligned AUT. This technique is then illustrated with example results. The agreement of the corrected results is determined via the application of image classification techniques which correlate antenna patterns in a reduced vector pattern space in terms of their overall global features.
A compact cassegrain antenna has been designed for dual-band satellite communications, operating at 20GHz and 30GHz. The antenna consists of a parabolic reflector, a hyperbolic sub-reflector, and a dual-band choke feed. The cassegrain structure has been optimized considering theoretical and measured feed patterns using different software packages, for maximum antenna efficiency with minimum sidelobe levels for a compact design objective. Experimental studies have been carried out in the near-field chamber of the University of Manitoba. The knowledgenof the near-field is helpful in order to adjust different components of the cassegrain antenna. After adjustment, results in terms of gain and radiation patterns are computed by Fourier transform using near-field data, and compared to the measurements realized in the compact range of the University of Manitoba. Comparisons are also made with the results obtained by simulation.
A near-field antenna range will often utilize a flexible microwave cable assembly as a means to transport the sampled signal from the moving sample antenna to a receiver as part of the measurement system. The performance of that cable directly impacts the quality of the final far-field pattern. It has been observed that the cable had been exhibiting a flex life much shorter than anticipated. Analysis of a failed cable revealed that the problem was the result of non-uniformities in the extruded jacket, which produced sites of high stress. These sites ultimately caused the cable conductors to work harden and fracture. A cable which utiized a woven expanded Polytetrafluoroethylene (ePTFE) fiber as an outer jacket was substituted, resulting in a threefold improvement in flex life to date, with the cable still in operation at this writing.