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Far Field
APERTURE FIELD INVESTIGATION OF PLANAR 2D LEAKYWAVE
ANTENNA DESIGNS USING THE INVERSE FOURIER TRANSFORM
The nearfield aperture distribution excited on the guiding surface of various planar leakywave antenna designs is examined. The investigated antennas (for millimeter wave applications) are realized by circular, straight and elliptical metallic strip gratings on a high permittivity dielectric substrate. With such straight and curvilinear grating configurations, analytical determination of the nearfield, and hence the leakywave phase and attenuation constants along the guiding surface, can be mathematically intensive. To assist in such complex characterizations, the nearfield/farfield extrapolation techniques can provide insight and thus illustrate such 2 D aperture field distributions. Specifically, by taking the inverse Fourier transform of measured 2D farfield beam patterns, the nearfield distribution along the aperture can be estimated.
TEST ZONE PERFORMANCE IN LOW FREQUENCY ANECHOIC CHAMBERS
Advantages of FarField (FF) anechoic chambers utilized for antenna measurements, as compared to conventional outdoor ranges, such as security, interferencefree radiation, and immunity to weather conditions allowing broadband antenna measurements on a 24/7 basis, are well known. The dimensions of an anechoic chamber are primarily determined by the lowest operating frequency and are, therefore, significantly increased if operation is required down to VHF and UHF frequency bands. As a result, the advantages of indoor chambers are often disputed when considering low frequency applications. The main counterargument is the real estate required for chamber construction. In addition, such chambers require the use of high performance absorbing materials, and consequently, chamber certification is always a challenging task. Therefore, rigorous and accurate 3D EM analysis of the chamber is an important procedure to increase confidence, reduce the risk associated with achieving the required test zone performance, and to make the design more efficient. Thus, an accurate simulation of the chamber is even more important these days due to a dramatically growing number of antenna manufacturers supplying products at VHF and UHF bands. Such analysis is a standard procedure at ORBIT/FR, and is described below for the example of a chamber with dimensions of 6m (W) x 6m (H) x 10m (L), operating down to 150 MHz.
A Measurement Setup for Characterizing Antenna on an Infinite Ground Plane from 1 to 18 GHz
Currently there is a lack of facilities capable of measuring the full upper hemisphere radiation patterns of antennas mounted on an infinite ground plane. Measurements performed with a finite ground plane suffer diffraction interference from the truncated edges. To circumvent this problem, a new measurement setup was developed at the Ohio State University ElectroScience Laboratory (ESL) for fully characterizing upper hemisphere radiation gain patterns and polarization for antennas up to 4” in diameter from 118 GHz. A probe antenna is positioned 46” away from the antenna under test (AUT). The ground plane end diffractions are removed using timedomain gating. The key design consideration is to position the probe antenna in the farfield region and yet shorter than the radius of the ground plane. This paper will present the calibration procedure necessary for the measurement system and it’s limitations due to ground plane probe antenna coupling at low elevation angles. In addition, the complete radiation pattern of a 4” monopole measured from 15.5GHz to demonstrate the systems capability for the lower third of the systems operating frequency range.
Mathematical Absorber Reflection Suppression (MARS) to Extend the Frequency Range of an Anechoic Chamber
NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been used to improve performance in anechoic chambers and has been demonstrated over a wide range of frequencies on numerous antenna types. MARS is a postprocessing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical or farfield range or Compact Antenna Test Range (CATR). It has also been applied to extend the useful frequency range of microwave absorber to both lower and higher frequencies than its normal operating band. This paper will demonstrate the use of the MARS capability in evaluating the performance of anechoic chambers used for spherical nearfield measurements, as well as in improving chamber performance.
A Novel Phaseless Spherical NearField Antenna Measurement Including the Issue of Robustness
The radiation characteristics of antennas can be determined by measuring amplitude and phase data in the radiating nearfield followed by a transformation to the farfield. Accurate phase measurements especially at high frequencies are very demanding in terms of the required measurement equipment and tolerances. Phaseless measurement techniques have been proposed, which often deal with a second set of amplitude only measurement data in order to compensate the lack of phase information. In this paper the concept of phaseless spherical nearfield measurements will be addressed by presenting a phaseless nearfield transformation algorithm for spherical antenna measurements, working with amplitude only data on two spheres. In particular the measurement of a patch antenna is considered to demonstrate the utility of the technique for low gain antennas. To address the issue of robustness, inaccurate measurement distances as well as spherical rotation angles are considered in order to evaluate the accuracy of the method against probe positioning errors. Furthermore noise contributions are introduced to emulate measurement inaccuracies in general.
Generalization of the Circular and Linear Near FieldtoFar Field RCS Transformations to OffWaterline Collection Geometries
In previous papers [1][4] we have presented formulations for the circular and linear near fieldtofar field RCS transformations (CNFFFT and LNFFFT, respectively). These formulations assumed that the target did not have significant extent above or below a central (waterline) plane, and that the circular or linear near field scans lied in this waterline plane. In this paper, the CNFFFT and LNFFFT formulations are generalized to scans that lie in a plane parallel to and above or below the waterline plane. These scans correspond to conical or great circle RCS cuts, respectively, in the far field at elevation angles other than 90°. We will show that the generalization can be accomplished by modifying just the frequency domain processing steps that are common to both algorithms, while leaving the spatial processing portions (apart from a minor variable redefinition) unchanged. The paper focuses on the mathematical derivation and numerical implementation of the algorithms; examples of numerical and experiment results are deferred to future papers.
NearlyOrthogonal Hierarchical Vector Basis Functions Employed for the Discretization of Inverse Equivalent Surface Currents
Inverse equivalent current method has recently gained popularity in the applications of nearfield farfield (NFFF) transformations especially when nearfield (NF) measurements are carried out on irregular measurement grids around the arbitrarily shaped object under test. Usually low order (LO) RaoWiltonGlisson (RWG) basis functions or even point based low order basis functions are used for the discretization of the unknown surface current densities on the triangular discretization elements. Better accuracies are achievable when equal number of higher order (HO) basis functions is employed to represent unknown surface current densities. Nearlyorthogonal hierarchical vector basis functions complete to full first order with respect to the curl space are therefore utilized for the discretization of inverse equivalent surface currents defined on flat triangular domains. Various numerical examples are presented and comparison is made with the results of LO discretization.
Single Antenna Method for Determining the Gain of NearField Waveguide Probes
Accurate calibration of nearfield measurements requires the probe used for the measurement be well characterized. The determination of the absolute gain of rectangular openended waveguide probes is difficult due to the broad beamwidth in both the Eplane and Hplane which increase the likelihood of multipath affecting the accuracy of the measurement. Multipath may be minimized by reducing the separation distance, but at the price that farfield conditions may no longer apply. A variation of the two matched antenna method is to use a large reflecting plate to form an image of the probe. Use of the entire bandwidth of the probe, and timegating the results to isolate the signal reflected from the plate allows the gain to be determined. The procedure also allows the determination of the aperture reflection coefficient used by theoretical probe models used for pattern compensation in the neartofarfield transformation.
Fault Monitoring of Patch Antenna Arrays Using Neural Networks
A technique to diagnose faulty elements present in patch antenna array from either measured far field radiation pattern or return loss characteristic is suggested. A linear array consisting of eight square patch elements with uniform excitation and ./2 spacing between them is considered. A method is developed using Artificial Neural Networks to detect one or two faulty elements present in the array. A neural network is trained with one third of the possible faulty radiation patterns and tested with two thirds of faulty patterns. ANN is implemented with Radial Basis Function neural network (RBF) and Probabilistic neural network and their performance is compared.
An accurate and efficient error predictor tool for CATR measurements
An accurate and efficient numerical model is developed to simulate the far field of an antenna under test (AUT) measured in a Compact Antenna Test Range (CATR), on the basis of the known quiet zone field and the theoretical aperture field distribution of the AUT. The comparison with the theoretical farfield pattern of the AUT shows the expected measurement accuracy. The numerical model takes into account the relative movement of the AUT within the quiet zone and is valid for any CATR and AUT of which the quiet zone and aperture field, respectively, are known. The antenna under test is the Validation Standard Antenna (VAST12), especially designed in the past for antenna test ranges validations. Simulated results as well as real measurements data are provided.
An accurate and efficient error predictor tool for CATR measurements
An accurate and efficient numerical model is developed to simulate the far field of an antenna under test (AUT) measured in a Compact Antenna Test Range (CATR), on the basis of the known quiet zone field and the theoretical aperture field distribution of the AUT. The comparison with the theoretical farfield pattern of the AUT shows the expected measurement accuracy. The numerical model takes into account the relative movement of the AUT within the quiet zone and is valid for any CATR and AUT of which the quiet zone and aperture field, respectively, are known. The antenna under test is the Validation Standard Antenna (VAST12), especially designed in the past for antenna test ranges validations. Simulated results as well as real measurements data are provided.
A NONREDUNDANT NFâ€“FF TRANSFORMATION WITH SPHERICAL SPIRAL SCANNING USING A FLEXIBLE AUT MODEL
ABSTRACT In this work, a probe compensated nearfield – farfield transformation technique with spherical spiral scanning suitable to deal with elongated antennas is developed by properly applying the unified theory of spiral scans for nonspherical antennas. A very flexible source modelling, formed by a cylinder ended in two halfspheres, is considered as surface enclosing the antenna under test. It is so possible to obtain a remarkable reduction of the number of data to be acquired, thus significantly reducing the required measurement time. Some numerical tests, assessing the accuracy of the technique and its stability with respect to random errors affecting the data, are reported.
The Effect of the Absorber Collar on Open Ended Waveguide Probes
This paper describes measurements performed at the National Physical Laboratory (NPL) and Near Field Systems Inc (NSI) on Open Ended Waveguide (OEWG) probes that are typically used for nearfield measurements. The effect of the size and location of the absorber collar placed behind the probe was studied. It was found that for some configurations, the absorber collar could cause noticeable ripples in the farfield patterns of the probe and this in turn could affect the probe correction process when the probe was used in nearfield measurements. General guidelines were developed to select an absorber configuration that would have minimal effect on the patterns, polarization and gain of the probes.
Interferometer antenna calibration by centered element diagram measurements
A broad band interferometer antenna was designed and manufactured by Saab Avitronics. Saab Aerotech has installed a test facility for calibration of the interferometer antenna. The main purpose of the facility is to measure the interferometric function of the antenna. The interferometric function of the antenna can be measured directly but this method puts very high demands on the test range performance. An alternative method where each element is centered on a short farfield range is evaluated and compared by measurement with a large compact range at Saab Microwave Systems. The paper also describes the design aspects when measuring broad band, broad beam interferometer elements together with the actual design of critical components such as positioners, RFsystem and absorber treatment.
Improvement of the Signal to Noise in spherical near field antenna measurement through mode truncation and spatial filtering
This paper analyzes the reduction of the noise effect in spherical nearfield antenna measurements. Two techniques have been evaluated: the first one is based on the mode truncation and the second one consists of a spatial filtering after a diagnosis process. The antenna under test (AUT) used for this evaluation is the 12 GHz Validation Standard antenna (VAST12). The VAST12 measurements have been performed in the Spherical NearField Antenna Test Facility of the Technical University of Madrid (UPM). These measurements have been corrupted adding a White Gaussian Noise (WGN) with different levels. First, the effect of the number of spherical modes considered in the neartofarfield transformation has been evaluated, analyzing also the error due to the mode truncation versus the reduction of the noise uncertainty associated to each spherical mode. Second, a diagnosis process based on a holographic technique has been carried out. A spatial filtering including the AUT aperture has been applied and then, the farfield is reconstructed and compared with the uncorrupted farfield. Several results illustrate the signal In this paper, the effect of White Gaussian Noise (WGN) in spherical nearfield measurements and the improvement of the signal to noise ratio (SNR) through mode truncation and spatial filtering are evaluated employing simulations and measurements. Section 2 and 3 respectively explain the mode truncation and spatial filtering to minimize the noise. In section 4 and 5 the results achieved when applying both techniques are illustrated. Finally, section 6 summarizes the conclusions drawn.
An Evaluation of the Aperture Backprojection Technique Using Measurements Made on a Flat Plate Array with a Spherical NearField Arch
We describe two theoretical bases for an algorithm for backprojection. The first is (1) Fourier inversion of the mathematical expression for the far electric field components in terms of the aperture electric field. The second is (2) Fourier inversion of the complete vectorial transmitting characteristic of Kerns' scattering matrix. It is this characteristic that results from the standard process of planar nearfield (PNF) scanning and the ensuing reduction of the PNF transmission equation. We demonstrate that the theoretical approaches (1) and (2) yield identical backprojection algorithms. We report on backprojection measurements of an 18 inch Xband flat plate phased array using the farfield obtained from both planar and spherical nearfield scanning. The spherical measurements were made on a large arch range.
Wband Antenna Gain Calibration in Extrapolation Range Using TimeDomain Gating
A new simple approach is presented to calibrate the gain of standard gain horn antennas operating in the millimeterwave frequency band. In terms of calibration, it is difficult to accurately measure the gain of standard gain horn antennas in the farfield region due to the space limitation. Therefore, nearfield measurement methods are generally used to calibrate the gain of standard horn antennas. The extrapolation range method is one of the most accurate measurement methods in the nearfield region. In the conventional extrapolation range method, a moving average process is applied to remove multiple reflections between antennas. Moving average can only remove multiple reflections between antennas. Therefore, electromagnetic absorbers are required to remove other reflections increasing measurement uncertainties. The timedomain gating method in extrapolation range allows us to remove all reflection waves, and achieve accurate antenna gain calibration without absorbers. The timedomain gating also reduces the number of measurement positions in the extrapolation ranges and obtains the gain of antennas in wide frequency ranges. In this paper, we compare the theoretical value with the timedomain gating method without absorbers by measuring three Wband standard gain horn antennas.
RANDOM NOISE IN SPHERICAL NEAR FIELD SYSTEMS
Thermal and other random noise sources give rise to an error contribution in spherical near field measurement systems [12]. With modern receivers and sufficient amplification in the system this term often give an insignificant contribution in the overall measurement uncertainty. However, in special cases the uncertainty linked to random noise may be more significant and the proper treatment of this term is needed to evaluate the impact on overall measurement uncertainty. The motivation for this paper comes from observations on spherical near field (SNF) measurement of relatively small antennas using a high degree of oversampling. In a multiprobe system this is generally the case particularly in measurements of small antennas like dipoles as shown in Figure 1. In these cases the near field to far field (NF/FF) processing is performed with data collected from all probes and some truncation of the spherical mode spectrum depending on the antenna size. The term modal filtering is often used to describe the deliberate truncation of the mode spectrum. What can be observed is that the effective signal to noise ratio (S/N) in small antenna measurements in which modal filtering is applied during NF/FF processing are often much better than the apparent S/N in the “raw” near field (NF) data. Parseval’s theorem, which states that power computed in either domains equals the power in the other explains this difference. The “noise power” is spread out on all available spherical modes and therefore reduced when the mode spectrum is truncated by modal filtering at the appropriate order/distance depending on the size of the antenna. In this paper we present a formal discussion on how the residual noise power after NF/FF processing is affected by the processing parameters. It will be shown that the “effective S/N” can be calculated directly from simple formulas from the applied sampling and filtering. The formulas will be validated by an experimental setup.
Improving and Extending the MARS Technique to Reduce Scattering Errors
The Mathematical Absorber Reflection Suppression (MARS) technique is a method to reduce scattering errors in nearfield and farfield antenna measurement systems. Previous tests by the authors had indicated that NSI's MARS technique was not as effective for directive antennas. A recent development of a scattering reduction technique for cylindrical nearfield measurements has demonstrated that it can also work well for directive antennas. These measurements showed that the AUT shouldbeoffsetfromtheorigin byadistanceatleastequal to the largest dimension of the AUT rather than only 13 wavelengthswhich hadbeenusedfor smallerantennasin the earlier MARS measurements. Spherical nearfield measurementshaverecently beenconcludedwhich confirm that with the larger offsets, the MARS technique can be applied to directive antennaswith excellent results. The MARS processing has recently been modified to produce significantly improved results. This improvement isespeciallyusefulfor antennaswherethephasecenterof the horns is located inside the horn and varies with frequency like pyramidal Standard Gain Horns (SGH). Fewermodesarerequired for thetranslatedpatternandthe filtering is more effective at reducing the effect of scattering. The improvement is very apparent for pyramidal horns.
DIELECTRIC PROBES, COMPACT NEARFIELD AND ADVANCED NEARFIELD FARFIELD TECHNIQUES
We present an innovative NearField test range, named Compact NearField (CNF) test range, using photonic probes and advanced NearField FarField transformations (NFFF). The photonic probe allows distances of one wavelength or less between AUT and probe, drastically reducing test range and scanner dimensions, improving the Signal to Clutter Ratio and the Signal to Noise Ratio, and reducing the scanning area and time. The NFFF, properly formulated as a linear inverse problem, further improves the rejection to clutter, noise and truncation error. The advantages of CNF test ranges are numerically foreseen and experimental results are presented under both, planar and cylindrical scanning geometries.

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