Test-zone field quality in planar near-field measurements
This paper reports on the results of computer simulations of planar near-field scanning and its ability to achieve an high accuracy test-zone field over a wide range of pattern angles. An quality test-zone field was defined for this study to have less than 0.2 dB peak-to-peak amplitude variation and less than 1.5 peak-topeak phase variation. This investigation sought the minimum scan length, for a given critical angle, ec and separation, S. The minimum scan length determined from this investigation is given by: L = D + 2S(tan(0c)) + 20/cos(0c). This scan length is approximately 60),, larger, for a critical angle of 70 degrees, than previously accepted. It is suggested that the maximum practical value of Sc is between 60 and 70 degrees. The use of raised cosine amplitude and/or quadratic phase windows to the edges of the measurement plane is shown to provide test-zone field quality improvement and/or allow scan lengths approximately 10),, smaller.
Enhancement of efficiency and accuracy of near-field measurement
This paper examines the possibility of increasing the speed of Near-Field measurement of an Antenna, by reducing the number of measurement points and by determining the degree of truncation permissible while maintaining a prescribed degree of precision of the reconstructed far-field.
The Near-Field of a planar radiating array is analysed in depth. A formulation and a procedure to correct the spectral domain of the field are established.
It is shown that correction in the spectral domain can improve the accuracy of the Far-Field while using the same amount of Near-Field data.
The technique has a good potential to be applied to Near Field data of large radiating Antennas leading to new information about the accuracy and speed of measurement achievable.
General order N analytic correction of probe-position errors in planar near-field measurements
An analytic technique recently developed at NIST   to correct for probe position errors in planar near-field measurements has been implemented to arbitrary accuracy. The nth-order correction scheme is composed of an mth-order ordered expansion and an n - m higher-order approximation, where both n and m are arbitrary. The technique successfully removes very large probe position errors in the near-field, so the residual near-field probe position errors are substantially below levels that can be measured on a near-field range. Only the error-contaminated near-field measurements and an accurate probe position error function are needed for implementation of the correction technique. The method also requires the ability to obtain derivatives of the error-contaminated near field defined on an error-free regular grid with respect to the coordi nates. In planar geometry the derivatives are obtained using FFTs , giving an approximate operation count of (3 • 2=- 1 - 1 + (n - m)) N log N, where N is the number of data points. Efficient computer codes have been developed to demonstrate the technique. The results of simulations are more accurate than those obtained us ing the well-known k correction [3), which can correct for position errors in some direction in k space, but further contaminates the sidelobe levels.
Hologram accuracy determination
Hologram measurements are becoming more and more popular as a reliable method for identifying bad elements and the tuning of active phased array antennas. Relying on holographic data to adjust phase shifters and attenuators in these antennas can give undesired results if the accuracy of the data is poor. Often measurements can be improved if the error sources can be isolated and quantified. This paper presents an approach to producing a hologram accuracy budget based on the NIST 18-term error budget created for near-field measurements. A set of hologram accuracy terms is identified and data is presented showing the typical hologram accuracy that can be expected from a near-field scanner.
Phase-stationary high performance antenna test body, A
Modern low profile and conformal antennas are fre quently evaluated in the presence of a conducting surface. Antenna designers usually predict the an tenna scattering and radiation performance over an infinitely conducting ground plane. To bridge the gap between a (possibly curved) antenna host surface and the designer's infinite ground plane model, an antenna testbody is required.
This testbody must possess a variety of demanding attributes, such as a very close approximation to an infinite ground plane, low testbody signature, ability to provide positional accuracy (in both azimuth and elevation), physical stability (for repeatability and background subtraction), just to name a few. The most widely regarded testbody has been the "almond" testbody [1, 2], which boasts a very low signature, and excellent fidelity when compared to an infinite ground plane.
This paper addresses a new variation from the traditional "almond" testbody, in which a unique positioning design provides a phase-stationary antenna aperture center under rotation of both azimuth and elevation. This testbody will be used for a variety of antenna tests at Wright Laboratory's Radiation and Scattering Compact Antenna Laboratory (RASCAL).
Performance evaluation of serrated edge and blended rolled edge compact range reflectors
Evaluation of serrated edge and blended rolled edge compact range reflectors is presented in this paper. An interactive approach is used to design the serrated reflectors. Several issues associated with the serrated reflectors are also discussed in this paper. Quiet zone fields for various serrated edge with an optimally designed blended rolled reflector are presented for comparison. In addition, simulations of a low sidelobe phased array measurement using serrated and blended rolled edge reflectors are shown to investigate their impact on the measurement accuracy.
Accuracy of RCS measurements
Some precautions necessary for accurate RCS measurements using a short model range are discussed. Sources of error in these measurements such as non ideal range geometry, misalignment of the target and inappropriate time domain gating are discussed. A simple technique to estimate possible errors in RCS measurements due to factors such as bistatic angle due to finite separation of source and receive horns and finite length of the measurement range, is presented. The range of RCS values that can be measured within defined error bonds is identified.
Accurate determination of main beam position and beamwidth from near field measurements
For narrow beam antennas or track antennas some parameters like main beam or null position and 3 dB beamwidth need to be determined with an accuracy of less than a mill or mrad. With Near Field measurements, the Far Field is normally calculated by FFT-processing. This does, however, not provide the required accuracy. Nevertheless, the measured Near Field data contains information about any Far Field point. An iterative approach is presented to determine the Far Field antenna characteristics with high accuracy.
Practical issues in advanced antenna pattern comparison
This paper addresses some of the practical considerations and numerical consequences of using the Advanced Antenna Pattern Comparison (AAPC) method to improve the accuracy of antenna measurements in compact ranges. Two main issues are of particular importance: 1. Appropriateness of circle-fitting algorithm results to the measured data.
2. Ambiguous circles due to the crowding of data.
These issues deal specifically with Kasa’s circle-fitting procedure—an essential part of the AAPC method—and provides useful checks for conditions commonly met with the use of this technique. In addition, we consider the problem of data distribution along the fitted circle, another important element of the AAPC method. Simulation results are submitted in support of the proposed methods.
Accurate gain calibration procedure for large antennas
Gain calibration of circular horns and radiation pattern integration applying patterns in two principle planes only is accurate and does not require large computational or measurement effort. This technique is thus more practical than the integration over the entire angular domain, required in case of rectangular horns. However, for many types of AUT’s, additional errors may occur due to the differences in aperture size of the AUT and standard gain horn. The AUT will in many cases have physically larger aperture dimensions. Consequently, unknown test-zone field variations across this aperture can result in additional errors in gain determination. The new method uses a flat plate as a reference target. An RCS measurement of the flat plate is used to derive test-zone field characteristics over the same physical area as the AUT. Combined with the accurate gain calibration described above, field information is available over the entire area of interest and the accuracy in gain determination is increased. In this paper, experimental results and practical considerations of the method will be presented.
Windows 96 for planar near-field measurements
This paper reports on the results of computer simulations of planar near-field test-zone-fields. Techniques for the improvement of the quality of the test fields are presented and demonstrated. These techniques include the use of larger scan areas and the use of window functions applied to the measured near-field data. Test-zone-field quality is measured by the angular spectrum of the error of the test-zone-field as compared to an ideal plane wave test-zone-field. This investigation sought the minimum scan length, L, for a given critical angle, ?c and separation, S. It is shown that significant improvements in test-zone-field quality can be realized if the test zone is extended from the standard length, Ls=D+2S(tan(?c)) by an amount 20?/cos(?c). This scan length is approximately 30? larger, for a critical angle of 50 degree and 60? larger, for a critical angle of 70 degrees, than the standard length. A raised cosine amplitude/quadratic phase window applied to the measured near-field data can significantly reduce scan length requirement while maintaining the increased accuracy of the extended scan length. The recommended scan length with window is given by Lw=D+2S(tan(?c))+2W, where W is the length of the window applied to each end of the scan measurements. The window description and required length are presented.
A Small-size, heavy-duty RCS AZ/EL rotator pylon tip
So far, Azimuth-over-Elevation rotators on RCS pylon tips were of large size (typically 10” for 500 lb. load, over 2’ for a 6000 lb. load). Therefore, RCS measurements of small but heavy targets were very difficult if not impossible to perform. The new design supports loads of 5,000 lb. with an Azimuth turntable diameter of only 136 mm, close to the pylon’s maximum width.
The Azimuth and Elevation axes mechanisms are hidden inside the pylon body. The Azimmuth rotator is mounted on the top surface of the elevation main plate. The Elevation plate is attached to the pylon tip on one side and on the other side to the actuator, which is attached to the base of the tip. The actuator drives the Elevation plate to the required rotation angle.
Even with its small size, the new design does not compromise on performance. The Azimuth axis moves a full 360° continuous motion at 22 deg/min with 0.03° accuracy, 0.03° backlash and 0.01° repeatability. The Elevation axis moves in a 0°-40° sector at 1.5 deg/min with 0.05° accuracy, 0.05° backlash and 0.01° repeatability.
Single-plane collimators for measurements on large antennas
For indoor antenna measurements, compact ranges or near-field/far-field techniques are most frequently used. One of the major problems is the handling of physically large antennas. Compact ranges will in general provide test-zone sizes up to approximately 5 meters in diameter. Applying the planar NF/FF technique, even larger test-zone sizes can be realized for certain applications.
On the other hand, requirement of real-time capability, for instance in production testing, will exclude NF/FF techniques. It has been shown previously that single-plane collimators have a pseudo real-time capability which makes these devices comparable to compact ranges. Furthermore, the physical test-zone sizes which can be realized when compared to compact ranges are approximately 2-3 times larger for the same size of the anechoic chamber. Finally, it will be shown that the accuracy in sidelobe level determination, gain and cross polarization is considerable higher than with other indoor techniques, even at frequencies below 1 GHz.
Radar cross section range characterization
Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG).
We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.
Radar target scatter (RATSCAT) division low frequency range characterization
The RATSCAT Radar Cross Section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and program office desires for certifies RCS data. The first step is to characterize the Low Frequency portion of the RATSCAT Mainsite Integrated Radar Measurement System (IRMS). This step is critical to identifying error budgets, background levels, and calibration procedures to support various test programs with certified data. This paper addresses characterization results in the 150 – 250 MHz frequency range. System noise, clutter, background and generic target measurements are presented and discussed. The use of background subtraction on an outdoor range is reviewed and results are presented. Computer predictions of generic targets are used to help determine measurement accuracy.
Acceptance of the Sanders Merrimack 23 compact range for RCS measurements
In 1993, we presented the newly completed compact range and tapered chamber facility . As part of this presentation, the issue of “range certification” was presented. This paper will discuss the work that we have done with the compact range for radar cross section (RCS) measurement acceptance.
For customer acceptance, we had to “prove” that the compact range made acceptable measurements for the fixtures and apertures involved. Schedule and funding did not permit the full exploitation of the uncertainty analysis of the chambers, not was it felt to be necessary . The determination of our range capabilities and accuracy was based on system parameters and target measurements. Targets that were calculable either in closed form solutions (spheres) or by numerical methods (cylinders and rods) were used. Finally, range to range comparisons with the Rye Canyon Facility  of a standard target was used. The range to range comparison proved especially difficult due to customer exceptions, feed differences, and target mounting. This paper will discuss the “success” criteria applied, the procedures used, and the results. The paper will close with a discuss of RCS standards and the range certification process.
Parametric signal history editing techniques for removal of additive support contamination in narrowband RCS measurements
ERIM has developed techniques, based on parametric spectral estimators, for removing additive target support contamination from narrowband RCS measurements . These techniques allow target and support returns to be extracted from frequency sweep data with much greater accuracy and resolution than that afforded by conventional Fourier techniques. These algorithms have recently been enhanced to incorporate scattering mechanism frequency dependence in the underlying signal model. Specifically, damped exponential and power-of-frequency sweep data with much greater accuracy and resolution than that afforded by conventional Fourier techniques. These algorithms have recently been enhanced to incorporate scattering mechanism frequency dependence in the underlying signal model. Specifically, damped exponential and power-of-frequency signal models have been used. The modification substantially improves algorithm performance in measurement situations where there is small absolute bandwidth, but relatively large fractional bandwidth, which can lead to appreciable variation in scattering mechanism amplitude.
The paper will demonstrate the technique’s ability to remove target support contamination using numerical simulations and compact range measurements of canonical targets mounted on pylon supports. It will be shown that the algorithm can remove the additive pylon contamination even for situations where the pylon return dominates the target return and cannot be resolved from the target in conventional Fourier range profiles.
Compact range antenna measurement error model
A compact range antenna measurement error model is presented which shows that the ripple in the quiet zone can only be caused by stray radiation from the edges of the reflector, presuming a perfectly shaped (serrated) reflector.
This is proven by defining an equivalent system which gives significant intuitive insight in the behavior of a compact range. For a simple example this model is shown to be consistent with PO.
The model intuitively explans many antenna measurement accuracy observations made in a compact range without the need for extensive knowledge of antenna or diffraction theory. These observations include the relation between quiet zone ripple characteristics and antenna measurement accuracy, especially for boresight, narrow angle and wide angle measurements.
It also explains why new correction techniques like AAPC work so well in spite of their presumable simplified modeling.
The Use of pattern comparison methods for satellite antenna testing
Nowadays, the standard facility for accurate satellite antenna testing is the Compensated Compact Range (CCR). In order to increase measurement accuracy several techniques can be applied, which are based on antenna pattern comparison. The theory of these techniques together with experimental results have been described in several papers in the past . This paper presents how pattern comparison techniques are applied for space programs and is another step to official qualification of the Advanced Antenna Pattern Comparison (AAPC) method at Dornier Satellitensysteme (DSS).
Cross polarization measurement accuracy improvement on a single reflector compact range
Scientific-Atlanta has developed a new algorithm for obtaining high accuracy cross-polarization measurements from prime focus, single reflector, compact ranges. The algorithm reduced cross-polarization extraneous signals to levels that rival or exceed much more expensive dual reflector systems, but with the associated cost and simplicity of a single reflector system. This paper provides an overview of the new algorithm. It explains the limitations on conventional polarization measurements in single reflector systems and the methods for overcoming these limitations without error correction for some antennas. A method for determining if error correction is needed for a particular antenna is reviewed and the fundamentals of the error correction algorithm are explained. Preliminary test results are provided.