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While probe arrays are now recognized to allow rapid and accurate near-field measurements, the interaction with the Antenna Under Test (AUT) is still sometimes considered as a potential limitation, especially for electrically large directive antennas [1]. Based on numerical simulations, this paper reports the results of a thorough investigation of the interaction mechanism and analyses its impact on the far-field pattern accuracy. The most often, interaction effects can be maintained at an acceptable level, thanks to an appropriate design of the probe array element and structure. However, the efficiency of a posteriori compensation schemes has also been investigated. The Pattern Coherent Averaging Technique (PCAT) [2], which is well known for compensating plane wave deviations in the quiet zone of antenna far-field test ranges or interactions from single probe near-field facilities, also proved very efficient to reduce the interaction effects with a probe array.
In this paper I demonstrate how our current technology now very readily permits a standard of accuracy and utility to be realized, that was formerly available only in research laboratories. This is accomplished with standardly available positioning equipment and standardly available software. Accurate alignment of the range is enabled by a tracking laser interferometer. This composite nearfield scanning antenna range has afforded us the opportunity to compare readily, far-field results from the classic planar, cylindrical and spherical coodinate systems. Comparison data are presented.
Tapered chambers have long been used for far-field antenna and RCS measurements. Conventional taper chambers used commercial antennas such as horns or log-period dipoles as wave launchers. One problem of this approach is the movement of the phase center associated with the antenna design. The positioning of the antenna inside the chamber is also critical. Undesired target-zone amplitude and phase distortion are caused by the scattering from the absorber walls. A novel feed antenna design for a tapered chamber is proposed here to provide broadband and dual polarization capabilities. This design integrates the absorber and the conducting walls behind the absorbers into to ensure a stationary phase center over a wider frequency range. In such a design, the dielectric constant of the absorber is utilized to maintain a clean phase front and a single incident wave at high frequencies. The conductivity of the absorber is also utilized to shape the field distribution at low frequencies. As a result, a wider frequency range can achievable for a given chamber size. One trade-off of this design is its reduced efficiency could be associated with the absorber absorption. Some simulation results from a 3-D FDTD model of a prototype design will be presented.
Single-beam and composite-beam performance of a 4-port X-band waveguide Butler matrix was measured on the Georgia Tech Research Institute planar near-field range for wideband frequency performance. The techniques necessary to perform accurate measurements on a broad-beamed antenna in a near-field range will be discussed, and measured far-field pattern data collected at the design frequency of 9.3 GHz are presented and compared with predicted results of the Butler matrix. In cases where the measured data and the expected results do not compare well, aperture amplitude and phase data, transformed from the near-field data, are shown as a diagnostic tool for corrections. After correction, new data at 9.3 GHz are presented for comparison with predicted results, and selected farfield pattern data collected at 8.6 GHz and 11.0 GHz are presented.
This paper describes two methods that can be used to measure the leakage signals in quadrature detectors, predict the effect on the far-field pattern, and correct the measured data for leakage bias errors without additional near-field measurements. One method is an extension and addition to the work previously reported by Rousseau1. An alternative method will be discussed to determine the leakage signal by summing the near-field data at the edges of the scan rather than summing below a threshold level. Examples for both broad-beam horns and narrowbeam antennas will be used to illustrate the techniques.
The free-space VSWR technique, which involves scanning a field probe through the quiet zone area and plotting the amplitude and phase ripples over this region, is generally used for evaluating the performance of a farfield range. In this paper, this free-space VSWR technique is simulated by the finite-difference time-domain (FDTD) method to demonstrate the relationship between the ripple amplitude and the absorber reflectivity. The commercial package named “FIDELITYTM”, based on FDTD algorithm released by Zeland Software, Inc., is used for the simulations. The pyramidal absorbers on the walls of the far-field range are modeled by using effective layer model. That is, in the FIDELITYTM simulation setup, the absorbers are replaced with several homogeneous but uniaxially anisotropic layers. The amplitude ripples for both cases of 12-in-pyramid chamber and 18-in-pyramid chamber are presented and discussed.
The free-space VSWR technique, which involves scanning a field probe through the quiet zone area and plotting the amplitude and phase ripples over this region, is generally used for evaluating the performance of a farfield range. In this paper, this free-space VSWR technique is simulated by the finite-difference time-domain (FDTD) method to demonstrate the relationship between the ripple amplitude and the absorber reflectivity. The commercial package named “FIDELITYTM”, based on FDTD algorithm released by Zeland Software, Inc., is used for the simulations. The pyramidal absorbers on the walls of the far-field range are modeled by using effective layer model. That is, in the FIDELITYTM simulation setup, the absorbers are replaced with several homogeneous but uniaxially anisotropic layers. The amplitude ripples for both cases of 12-in-pyramid chamber and 18-in-pyramid chamber are presented and discussed.
The bistatic radar signature of military systems is of interest for various applications including performance evaluation of semi-active missile systems, surveillance systems, and survivability assessment. While bistatic radar cross section (RCS) measurements have been made for high frequencies at several U.S facilities, there has been little reported work in low frequency bistatic RCS measurements. This paper presents the results of recent low frequency coherent bistatic RCS measurements from 210 MHz to 1.99 GHz at bistatic receiver angles of 0°, 35°, 70°, 120° and 145°. These measurements were successfully completed at the Naval Air Systems Command Weapons Division Etcheron Valley Range (EVR), formerly known as Junction Ranch (JR), China Lake, California This paper describes the process and provides results of low frequency bistatic RCS measurements on a hemisphere-capped cylinder target. Comparisons are presented of measured data to predicted results from moment method models of the calibration object and the cylinder target. Methodologies used in optimizing RCS data quality are also provided.
Accurate UHF phased array antenna patterns are difficult to achieve due to high level multipath present in the far field measurement test range. Special range geometry’s and source arrangements have been devised over the years to mitigate the measurement errors produced by test range multipath. In this paper we will describe new measurement results achieved using Aperture Synthesis illumination method designed to optimize and control the influence of ground reflections and in turn reduce quietzone amplitude ripple. Measured phased array patterns at 418, 434, 449, and 464 MHz will be shown for a 64- element array.
We have measured the amplitude and the phase of the electric field on a planar area very near (about 0.3 wavelengths) to the aperture of a X-band standard gain horn antenna using a photonic sensor and transformed the aperture field distribution to the far field pattern. The measured aperture field distributions and antenna patterns agreed well with those calculated by the method of moments. Comparing the far field patterns by the photonic sensor and the conventional open-ended rectangular waveguide probe reveals that the antenna measurement using the photonic sensor has advantages over the conventional probe.
Choosing the proper antenna range configuration is important in making accurate measurements and verifying antenna performance. This paper will describe the steps involved so the antenna engineer can select and specify the best antenna range configuration for a given antenna. It will describe the factors involved in choosing between near-field systems versus far-field systems, and the different scan types involved. It will explain the advantages of each type of antenna range and how the choices are affected by such factors as aperture size, frequency range, gain, beamwidth, polarization, field of view, sidelobe levels, and backlobe characterization desires. This paper will help the antenna engineer identify, understand, and evaluate the applicable characteristics and will help him in specifying the proper antenna range for testing the antenna.
The radiation properties of an antenna are defined in the far field, since this is the environment that they will operate. Creating far field conditions when testing a large aperture antenna is quite challenging. This is particularly true if testing occurs within the confines of an anechoic chamber, or if other complicating field characteristics (like angle-of-arrival simulation) are desired. Rather than attempt to generate a true planewave in the usual manner, we propose an instrument that creates a field distribution in the near field of a transmit array that is planewave-like in nature only over specified regions of interest (a region occupied by an antenna under test, for example); we do not require that the incident field be a true planewave at other locations. In these other locations the field is free to assume any value demanded by the governing equations of electromagnetics. By relaxing the requirement on the electromagnetic field in the test volume, we considerably reduce the complexity of the problem and define a tractable problem with a potential engineering solution.
This paper presents a 'mid-range' calibration technique, now being developed for a 60 ft. diameter reflector site. With this technique, near-field amplitude and phase is collected at a calibration tower as the reflector scans across it. The mid-range 'near-field' data is then transformed to a far-field pattern using a Fourier transform technique. Information on far-field EIRP, directivity, pointing, axial ratio and tilt, as well as encoder timing is obtained with accuracies comparable to standard measurement techniques. A particular advantage is that the system, once set-up, can be used on a regular basis without impacting site operations.
The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The copolar and cross-polar directivity patterns are compared at different levels down from the peak directivity.
We at MI Technologies have employed the Hansen error analysis [1] developed at the Technical University of Denmark (TUD), as a starting point for new system layouts. Here I expand it in two ways: the approach to mechanical errors, and the approach to system design. I offer an alternative approach to the analysis of mechanical uncertainties. This alternative approach is based upon an earlier treatment of spherical coordinate positioning analysis for far-field ranges [2]. The result is an appropriate extension of the TUD uncertainty analysis. Also, the TUD error analysis restricts its attention to three categories of errors: mechanical inaccuracies and receiver inaccuracies and truncation effects. An error analysis for a spherical measurement system should desirably contain entries equivalent to the 18-term NIST table for planar near-field [5]. In this paper, I offer such an extended tabulation for spherical measurements.
Calibrated probe complex pattern data is used in planar NFR (near field range) data processing to remove the effects of the probe on the measurement. In a prior paper [1] we proposed a procedure to estimate the measurement error (uncertainty) introduced into a near field antenna radiation pattern measurement due to test frequencies that do not coincide with available calibration frequencies of the range probe. Our prior paper resulted in a “19th term” which was added to the well known NIST NFR 18 Term Error Table used to evaluate the unavoidable uncertainty of far-field radiation patterns derived from a near field scan of a given AUT (antenna under test). A limitation of this procedure, pointed out in our prior paper, is that it was most accurate for a test frequency falling midway between two nearest neighbor probe calibration frequencies. The estimated uncertainty became overly pessimistic as the test frequency of interest moved closer to one of the neighboring calibrated frequencies. The procedure is improved in the present paper by the inclusion of a new term that is a function of the test frequency and the two nearest neighbor probe calibration frequencies. Examples are shown of the use of the new procedure to obtain an improved estimate of this measurement uncertainty and to create the 19th term for use with the standard 18 Term Error Table.
In previous AMTA presentations, we developed and evaluated an image-based near field-to-far field transformation (IB NFFFT) algorithm for monostatic RCS measurements. We showed that the algorithm’s far field RCS pattern prediction performance was quite good for a variety of frequencies, near field measurement distances, and target geometries. In this paper, we quantify the statistical RCS prediction performance of the IB NFFFT using simulated data from a generalized point scatterer model and method of moments (MoM) code, both of which allow modeling of targets with single and multiple interactions. It is shown that the predicted RCS statistics remain quite accurate under conditions where the predicted far field patterns have significantly degraded due to multiple interactions and other effects.
This paper will deal with basic rectangular chamber design and the choices that most affect the performance characteristics of a typical Rectangular Anechoic Chamber. The first and foremost criterion that needs to be addressed is “What is the chamber for”. The answer to this question is the primary driving factor regulating the overall chamber design. Is the chamber to be used to evaluate low gain, low frequency antennas? Is the chamber going to be used for RCS measurements of unique test bodies? Is the chamber going to be used to test high gain high frequency antennas? Is the chamber going to be used for far field measurements? Is the chamber going to be used for near field measurements? On and on. The answers to these very basic questions have a dramatic effect on the overall design of the anechoic chamber. Since there are so many preliminary criteria that have to be decided before we can even attempt a design I will make the following assumptions: 1) The chamber is to be a far field antenna measurement facility 2) The chamber is to operate from 2.0 Ghz to 18.0 Ghz 3) The chamber is to be of a rectangular design 4) The quiet zone is to be a 4’ diameter sphere 5) The range length is to be 20’ 6) The desired Quiet Zone performance is a. –30 dB @ 2.0 Ghz b. –40 dB @ 4.0 Ghz c. –50 dB @ 10.0 Ghz d. –50 dB @ 18.0 Ghz With these parameters we will first look at the effect that source antenna selection has on the chamber deign. The first design example will be with a low gain broadband antenna chosen as the source and the second case will be with a high gain antenna chosen as the source. This paper will detail the different design approaches that this choice has on the overall size and absorber placement in the chamber. These will have a dramatic effect on overall chamber size and cost.
NPL has recently commissioned a new indoor test range. This test range has been designed to offer Extrapolation Gain Measurements, Far-Field Probe Calibrations, and eventually, a Spherical Near-Field Test Capability. This paper describes this new range and the results of the initial validation measurements. It also compares the gains of a standard gain horn calibrated in NPL’s old Extrapolation Range with those from the new one.
An empirical study on Planar Near-Field Scan Plane Truncation applied to the measurement of a large phased array radar antenna saves test time per antenna. Lockheed Martin has been manufacturing, aligning, and verifying the AEGIS SPY-1B/D phased array radar antenna for the past 17 yrs . A custom built planar nearfield scanner system (ANFAST II) was designed and built specifically for this purpose. Existing raw near-field measured data sets were cropped in both the X and Y scan planes, processed to the far field, and compared with the un-truncated data to determine the error sensitivity vs near-field amplitude level truncated. Near-field measurements were then acquired at the truncated scan plane dimensions and compared. It was demonstrated that 100 hrs of test time could be saved by applying this technique without adversely effecting the antenna measurement uncertainty. This paper discusses the application of the truncation technique, results of the experiments, and practical limitations.