Object Under Test Research Articles

A Linear-Scan-Based Wideband Single-Cut Near-Field RCS Measurement Technique With Wide Effective Angle Coverage

A linear-scan-based wideband near-field radar cross section (RCS) measurement technique for achieving wide effective-angle-coverage is proposed in this letter. Both the data preprocessing and linear near-field to far-field transformation (LNF2FFT) techniques are used to ensure the accurate measurement of wideband far-field RCS in a wide azimuthal angle range. With the linear-scan-based near-field sampling strategy, the object under test (OUT) keeps stationary during the measurement process, and the equipment of turntable is not required. Moreover, based on the 2D plane wave expansion, LNF2FFT is proposed to translate the linearly sampled near-field data into far-field RCS pattern. An adaptive time gating strategy is also proposed to deal with the varied background property in liner samplings. Serval important practical factors relating to the measurement accuracy, such as phase center calibration, background subtraction and calibration, are also discussed in detail. Two illustrative test examples are presented, in which RCS measurement with wide effective-angle-coverage of ±35° and the average wideband measurement errors less than 0.4 dB are achieved in 1 m limited measurement range.

The Performance of Sensitivity-Maps Method in Reconstructing Low Contrast and Multi-Contrast Objects for Microwave Imaging Applications

The microwave imaging system for breast tumor/cancer detection requires high sensitivity to detect abnormal tissue that has little contrast in high-density breasts. This paper proposes a qualitative microwave imaging system simulation based on inverse scattering using the sensitivity-maps method. This method utilizes two measurement types for system calibration: a reference object as a scatterer-free background and a calibration object to obtain the system's impulse response. The object under test (OUT) consists of an object with low dielectric contrast and a phantom with multiple low dielectric contrasts (multi-contrast). Reconstruction is carried out on three types of S-parameter measurement data, namely S_11,〖 S〗_21, and a combination of both. S-parameters are measured at several frequencies, which are 3, 10, 14, 15, 16, 20 GHz, and the combination of all those frequencies (multifrequency). Reconstructed images show that the system is capable of reconstructing dielectric objects accurately. Quantitatively, the results show that the multifrequency S_21 measurement yields the best image quality with relative root mean squared error (RRMSE) values of 0.1272 and structural similarity index (SSIM) of 0.9076. The designed imaging system also successfully reconstructs multi-contrast phantom accurately with RRMSE of 0.1434 and SSIM of 0.4609.

Toward Computerized Tomography With Microwaves

Microwave imaging holds the potential to bring an alternative to ionizing and expensive imaging modalities, such as mammography, magnetic resonance imaging (MRI), and computerized tomography (CT) scan. The absence of a high-resolution and non-model-based image reconstruction technique is one of the reasons that limited microwaves-based imaging. We introduce a microwaves-based imaging modality relying on the Radon transform technique adopted to create an imaging system identical in concept to CT-scanning methods that employ X-rays. This system uses low-frequency microwaves and highly localized radiators and receivers. While low-frequency microwaves are not perceived to have the ray-like behavior of X-rays, our system hinges on the principle that if the object under test (OUT) is electrically small (smaller than the wavelength), then the impression/projection due to an electrically very small localized source will be highly similar to a projection generated by a ray-like source. This phenomenon is due to the fact that a receiver closest to the highly localized transmitter will capture the highest energy. Several simulation studies and measurements demonstrate the ability of this technique to provide cross-sectional images of a 3-D object with high resolution.

Low-Cost Millimeter Wave Frequency Scanning Based Synthesis Aperture Imaging System for Concealed Weapon Detection

In this article, a novel low-cost frequency scanning-based synthetic aperture radar (F-SAR) system is proposed for high-resolution imaging. F-SAR scheme is demonstrated for personal screening for the first time. A hybrid scanning model is presented to illuminate the human body, namely, frequency control beam steering in cross-track and mechanical scanning in along-track. A frequency scanning-based tomographic SAR focusing algorithm is proposed. Accordingly, a power spectrum estimation algorithm in cross-track and synthetical aperture processing in along-track are compounded in the signal processing framework. Unlike a variety of multiple input multiple output (MIMO) array imaging which illuminates the object by switching different channels sequentially over separate time intervals, the F-SAR concept sweeps across all swaths over each chirp by employing a frequency scanned beam. Herein imaging time and hardware cost can be significantly cut down by dimensionality reduction from the MIMO system to a single input single output (SISO) system. Furthermore, to fulfill the particular scheme of the F-SAR system, a novel combo antenna module, leaky-wave slotted waveguide combined with a cylindrical lens, is proposed and achieved at 78–92-GHz band, by which a steering fan-beam focused at the object under test (OUT) plane is radiated. The radiation beam is focalized by the cylindrical lens accompanied by frequency sweeping, while is diffused in along-track. The frequency-controlled beam-steering concept and the fan-beam focusing mechanism are verified by the chamber measurements. The measured scanning range is 23°. Beamwidth of 11–16 mm is obtained at the focal plane. Beam steering in cross-track together with mechanical scanning in along-track can attain a field of view (FoV) of 0.54 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{2}$ </tex-math></inline-formula> . Through-clothes imaging of person-borne concealed objects and the real-time capability are demonstrated by experiments. Resolutions in both along-track and cross-track can reach 5 mm.

Application of Adaptive Pulse Compression in Cluttered Radar Cross Section Measurements

This article discusses a post-processing method to achieve highly accurate radar cross section (RCS) measurements in challenging test environments. In the ideal RCS measurement setup, the only received signal is the reflection from the object under test (OUT). However, this scenario is often not realizable in practice as outside measurements suffer from a multitude of scattering objects beyond the OUT, and even anechoic chambers have nonnegligible multipath clutter. Time-gating has been shown to be a powerful post-processing clutter suppression tool by temporally filtering any undesired return signals. However, this technique is only sufficient when there is ample spacing between the OUT and undesired return signals or when the sidelobes of one scatterer do not mask the return of a smaller nearby scatterer in the time domain. In the latter scenario, traditional time-gating techniques cannot separate the sidelobes from a nearby dominant scatterer overlapping with the main lobe of the OUT, and the RCS of the OUT cannot be extracted accurately. By applying the proposed stepped frequency continuous wave (SFCW) adaptive pulse compression (APC) algorithm, a range-dependent filter is created to iteratively estimate the range profile and suppress sidelobes allowing for temporal separation of the scatterers and accurate RCS estimation. To validate the concept, the RCS of a 15.2 cm sphere is extracted in two measurement configurations (with and without a large nearby dominating scatterer). The traditional matched filter data processing accuracy is compared to the proposed APC technique. Implementation of APC in post-processing yields a 71.2% improvement in average RCS error compared to traditional processing in the sphere-only measurement and an 88.2% improvement in the presence of a metal plate.

Methodology and Techniques for Highly-Precise Radar Cross Section Measurements at W-Band

This paper explores the challenges of radar cross section (RCS) measurements at W-band and proposes several techniques to overcome these challenges to obtain an accurate RCS measurement. This paper combines <i>S</i>₁₁ measurements collected using traditional radio frequency (RF) hardware with post-processing algorithms to extract RCS values with a single antenna. Vector background subtraction and time-gating suppress unwanted reflections while increasing the system&#x2019;s dynamic range capability. Experiments are conducted at the University of Oklahoma&#x2019;s mm-Wave Laboratory with a Vector Network Analyzer (VNA) and frequency extender to evaluate the proposed measurement techniques. The setup is calibrated with a metal sphere, and the RCS of a smaller metal sphere and an arrangement of four spheres is extracted. The single sphere&#x2019;s RCS is measured with a 0.304 dBsm average error, and the distributed object under test (OUT) RCS is measured with a 1.60 dBsm average error from 80-90 GHz. Each OUT is placed on a 3D-printed pedestal, and the remainder of the test hardware is part of an existing mm-Wave antenna measurement configuration. The post-processing clutter cancellation techniques enable a flexible test configuration to isolate the OUT RCS without purchasing specialized hardware.

Fast Eye-in-Hand 3-D Scanner-Robot Calibration for Low Stitching Errors

This article proposes a new eye-in-hand 3-D scanner-robot calibration approach to realize low data stitching errors during long-term continuous measurement. Eye-in-hand 3-D scanner-robot systems are commonly used for the complete measurement of an object under test (OUT) from multiple fields of view (FOVs). To align the multiple FOVs into a single coordinate system, marker-free stitching assisted by robot's positioning is attractive since it bypasses the cumbersome traditional fiducial marker-based method. Based on periodically capturing calibration images from a 2-D calibration target, scanner's and robot kinematic model's parameters are optimized. The challenges overcome in this article include: how to compensate for the center-detection error in scanner calibration; how to avoid the dependence of hand-eye calibration on DH parameters; and how to calculate an accurate world-to-robot transformation. These challenges were tackled by several new techniques including accurate scanner calibration with iterative refinement of control points, virtual arm-based scanner-robot kinematic modeling, and trajectory-based world-to-robot transformation calculation. Experimental results demonstrated a low-initial stitching error similar to the fiducial marker-based method (0.0446 mm versus 0.0542 mm) was achieved. The mean stitching error was effectively maintained to be $<; $0.1 mm during the continuous measurement with an average intermittent downtime of 78 s for recalibration.

Item response function in antagonistic situations

AbstractThe main characteristic of a binary test is the item response function (IRF) expressing the probability P (d, a) of an object under test (OUT), possessing ability a, to successfully overcome the test item (TI) of difficulty d. Each specific test requires its own definitions of TI difficulty and OUT ability and has its own P (d, a) describing the probability of “success” mentioned above. This is demonstrated on the basis of several examples taken from different areas of statistical engineering. A common feature is that they all relate to “antagonistic” situations, in which the “success” of one side may formally be considered as a “loss” to the opposite side. For such situations ability and difficulty are two interchangeable sides of the same coin and the corresponding IRFs are complementary, that is, P (d, a) = 1 − P(a, d), with all consequences and restrictions imposed by this property. A study shows that the family of feasible IRFs is limited and has a number of interesting properties, which are discussed in the article. The analysis provided should facilitate avoiding errors in decisions about an IRF adequately describing the studied test.

Millimetre‐wave multi‐view near‐field scattering tomography system

In a previous study by Shahir et al., the permittivity profile of object under test (OUT) from electric fields measured outside OUT is accurately estimated by considering the region of interest confined within the boundary of the OUT. This study proposes a continuous-wave multi-view near-field scattering tomography system at the millimetre-wave frequency range and reformulates the sub-space approach for reconstructing the continuous-wave tomographic images while considering the region of interest beyond the OUT boundary. The OUT reconstructed tomographic images enable one to localise the OUT boundaries. The proposed system also eliminates the need for fully systemic isolation (i.e. anechoic chamber) or water as a background medium by reducing multipath effects substantially at millimetre wave. The system performance is evaluated by reconstructing the tomographic images of various samples, and the experimental results are presented.

Inverse Scattering for Monochromatic Phaseless Measurements

An inverse method and measurement setup for profile and constitutive parameters reconstruction from monochromatic phaseless information is presented. The method is based on the minimization of a cost function that relates the measured field with the one scattered by a model of the object under test (OUT), where the position, contour, and constitutive parameters are the unknowns. As a result, phaseless information is directly related to the inverse problem unknowns, thus avoiding the need of an intermediate phase retrieval step. Due to the nonlinear nature of the cost function, global optimization techniques, such as the particle swarm optimization and differential evolution algorithms, have been considered for cost function minimization. An exhaustive analysis of the cost function behavior as a function of the electric size of the OUT is presented, discussing the optimal OUT size where the proposed methodology provides accurate profile and constitutive parameters reconstruction. The proposed methodology is conceived to use it together with a simple, low-cost measurement setup for fast characterization of perfect electric conductor and dielectric objects. Measurement examples are presented aiming to prove the feasibility of the described measurement setup.

An Improved Technique for Profile Reconstruction of PEC Bodies

This letter describes an improved technique for model-based profile reconstruction of perfect electric conductor (PEC) objects. The technique takes advantage of the fact that the illuminated part of the object-under-test (OUT) is, in general, accurately reconstructed when considering independent incident angles. The knowledge of the illuminated part is used to weight the reconstructed profile for each incidence. A comparison to the extended methodology that takes into account all the incident angles simultaneously is presented, proving that the proposed technique provides faster convergence to an accurate recovery of the OUT. Application examples considering one and two OUTs are presented.

Average Absorption Coefficient Measurement of Arbitrarily Shaped Electrically Large Objects in a Reverberation Chamber

It is shown in this letter that the average absorption coefficient (AAC) of an object under test (OUT) can be obtained by comparing the measured absorption cross section (ACS) of the OUT with the ACS value of a perfect absorber of the same shape. The ACS of an arbitrarily shaped electrically large perfect absorber can be obtained numerically. Measurements are performed to demonstrate the proposed method. Results show that the AAC is not sensitive (but not independent) to the shape and size of an electrically large absorber. The procedure proposed in this letter can be used to characterize the electromagnetic wave absorptive and scattering ability of arbitrarily shaped objects. A system-level AAC could be very useful for many applications.

Fourier-Based Imaging for Subsampled Multistatic Arrays

This contribution focuses on the limitations of Fourier-based imaging when applied to subsampled arrays used in multistatic millimeter-wave radar systems. The aim is to determine the relationship between the size of the object under test (OUT), its position with respect to the radar aperture, and the sampling requirements on the aperture so as to recover aliasing-free images. Based on the analysis results, a method for recovering spectral information is proposed, the idea of which is to replicate the plane wave spectrum, and then apply a band-pass filter defined by a priori knowledge of the OUT size. For simplicity, the analysis is done in two-dimensional (2-D; range and cross-range dimensions). A simulation-based application is presented for validation purposes.

Solicitation for Measurements Corner

Articles for future installments of the Measurements Comer are being actively solicited. The column provides for quick turn­ around of new contributions to the science and art of RF measure­ ments. These can range from short notes to full-length papers on all topics related to RF measurement technology and its applica­ tions, including antennas, propagation, materials, scattering, and radar cross section. New or unique measurement techniques are of particular interest. Articles should be sent to Brian Fischer and Ivan LaHaie at the above address (e-mail preferred), and they will be considered for publication as quickly as possible. Abstract A radar cross section (RCS) measurement setup for the characterization and measurement of periodic structures is presented. The idea is to modify an existing spherical range in an anechoic-chamber setup, allowing scattered-field measurements of PEC and artificial magnetic conductor prototypes. With the proposed setup, the scattered field of the object under test (OUT) can be measured for different azimuth and roll angles, as well as for any incident wave polarization. The measurement setup's performance is validated by comparing simulations and measurements. Finally, a combined PEC­ artificial magnetic conductor surface, intended for RCS reduction, was measured with the proposed setup.

Susceptibility phantom for compatibility testing of SPECT components for a SPECT/MR hybrid system.

A susceptibility phantom was designed and built from an acrylic glass cylinder (inner diameter: 170 mm, length: 250 mm). A reference structure consisting of 32 rods (diameter: 3 mm) was placed inside the phantom (Figure ​(Figure1A)1A) together with a plastic scale with marks every 20 mm. The phantom was filled with agarose gel (er = 75, σ = 0.73 S/m) to mimic human tissue [2]. Figure 1 A) Susceptibility imaging phantom without the agarose gel B) Reference phase image of the phantom at d = 30 mm from the front end of the phantom C) Reference magnitude image of the phantom at d= 30 mm from the front end of the phantom Amplitude and phase images were acquired with a gradient echo technique at 7.0 T (5 echoes TE=5/10/15/20/25/30 ms). A reference image was acquired with the phantom placed longitudinally in the magnets isocenter without the object under test (OUT). Subsequently, the OUT was positioned at the front end of the phantom (Figure ​(Figure2A)2A) and a series of tranversal images covering the entire phantom was acquired. A post-processing algorithm was used to evaluate image. This setup was employed to examine the MR compatibility of a copper cooling block (52 x 74 x 8 mm³). Figure 2 A) Illustration of the orientation of the copper cooling block in respect to the phantom B) Susceptibility effects in the phase image caused by the cooling block in orientation III, d=30 mm C) Distortion in the magnitude image caused by the cooling block ... No distortion was found to be at d = 30 mm, d = 30 mm and d = 200 mm for three different orientations of a copper cooling block. The proposed setup supports the assessment of susceptibility effects induced by SPECT components. Our results demonstrate that the orientation and position of SPECT components in an MR environment need to be carefully taken into account during the design process of an integrated SPECT/MR device to avoid image distortion.

Measurement Setup for Imaging Applications Using Frequency Scanning Illumination

A measurement setup for imaging applications is presented. The idea is to reuse an existing antenna measurement setup to develop the imaging system by using a frequency scanning antenna array that focuses the incident field on the object under test (OUT) at different angular positions by means of a frequency sweep. From the measured total field data, an inverse technique based on source reconstruction is proposed to recover the OUT profile. In order to check the feasibility of the proposed setup and for the sake of simplicity, the OUTs to be dealt with present translation symmetry along one dimension (2-D problems). Measurement results and full-wave method-of-moment simulations are presented to test the proposed setup as well as to point out the frequency scanning capabilities.

RCS Measurement Setup for Periodic-Structure Prototype Characterization

A radar cross section (RCS) measurement setup for the characterization and measurement of periodic structures is presented. The idea is to modify an existing spherical range in an anechoic-chamber setup, allowing scattered-field measurements of PEC and artificial magnetic conductor prototypes. With the proposed setup, the scattered field of the object under test (OUT) can be measured for different azimuth and roll angles, as well as for any incident wave polarization. The measurement setup's performance is validated by comparing simulations and measurements. Finally, a combined PEC-artificial magnetic conductor surface, intended for RCS reduction, was measured with the proposed setup.

Simulation and design of a superconducting eddy current probe for nondestructive testing

Numerical modeling studies were carried out for the design and performance evaluation of an eddy current probe for superconducting quantum interference device (SQUID) based noncontacting electromagnetic nondestructive testing. The probe utilizes high-Tc SQUID from electromagnetic interference and to focus magnetic flux due to the eddy current excitation coil into the object under test (OUT). This is very important in order to achieve “self-nulling,” where the probe responds only to the defects in the OUT. Simulations are carried out with the eddy current probe located a finite distance above a defect free conducting half-space of infinite thickness. Eddy current densities in the OUT are then calculated for various probe designs. These eddy current densities give an indication of the flux focusing, and hence increased spatial resolution for combinations of HTS and μ-metal shielding. Shielding factors are also calculated for the screening of the primary excitation field and the “defect-free” eddy current field. Agreement with the analytical solution of a simple solenoid above a conducting half-space verifies the numerical method.