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Sampling of the probe signal first-order spatial derivative, in addition to the probe signal itself, enables the sampling step to be increased to twice that of the standard sampling criterion. In this work, we investigate – theoretically, numerically, and experimentally - the potential of using probe signal derivatives for spherical near-field antenna measurements with the aim of reducing the measurement time. We present a closed-form Fourier coefficient formula and a closed-form interpolation formula based on signal and signal derivative samples. We validate these new formulas using experimental measurement data and thus demonstrate the feasibility of doubling the sampling step in practice. We discuss different principles for determining the probe signal derivative; and we demonstrate the use of probe signal derivatives, in addition to probe signals themselves, for a full-sphere near-field antenna measurement skipping every second full-circle scan.
Compact Ranges are widely used for antenna measurements across wide frequency ranges spanning frequencies as low as 350MHz to as high as 60GHz and above. Advances in electromagnetic (EM) simulations have significantly improved the design process for compact ranges, resulting in reduced costs. Characterizing compact range including the anechoic chamber is computationally very challenging in terms of computer memory and time. In this paper, we will present the full wave method, MLFMM for characterizing compact range without the chamber and application of asymptotic method, RL-GO to characterize the compact range inside the anechoic chamber.
Wireless devices supporting global navigation satellite systems (GNSS) services have become an essential tool in different areas of technology such as agriculture, construction, automotive, etc. Therefore the performance and reliability of such devices are important aspects that need to be addressed in the testing stage during the development of the units. The integration of the Over-the-Air (OTA) testing method with the 3D Wave Field Synthesis (3DWFS) technique offer not only the benefit of having tests under controllable and repeatable conditions but also the ability to recreate complex and realistic scenarios in a controlled environment with full polarimetric support for the testing of wireless devices. This contribution applies this technology to emulate a GNSS scenario within an anechoic chamber. For the results validation, a realistic GNSS outdoor scenario was recorded and compared with the emulated scenario where 3DWFS was applied for each individual satellite. This represents a significant step for the GNSS community and also for the future development and testing of wireless devices.
The plane-polar approach for near-field antenna measurements has attracted a great deal of interest in the open literature during the past four decades [1, 2, 3, 4, 5, 6, 7]. The measurement system is formed from the intersection of a linear translation stage and a rotation stage with the combination of the axes enabling the scanning probe to trace out a radial vector in two-dimensions facilitating the acquisition of samples across the surface of a planar disk, typically being tabulated on a set of concentric rings. In its classical form, the probe moves in a fixed radial direction and the AUT rotates axially. However, with the ever more prevalent utilization of industrial multi-axis robots and uninhabited air vehicles (UAV), i.e. drones, being harnessed for the task of mechanical probe positioning, such systems offer the possibility of acquisitions being taken across non-planar surfaces. In this paper an accelerated, rigorous, near-field to far-field transform for data that was sampled using a polar acquisition scheme that is based on a Fourier-Bessel expansion [4] is developed and presented that can be employed in the above circumstances. This highly efficient, robust, transform enables near-field data acquired on planar, and non-planar, surfaces to be transformed to the far-field providing the acquisition surface is rotationally symmetric about some fixed point in the x,y-plane with z being purely a function of the radial displacement. The utility of the non-planar acquisition interval stemming from the ability to minimize truncation effects without needing to increase the measurement size. The transform efficiency stems from the utilization of the fast Fourier transform (FFT) algorithm with the rigor and robustness deriving from the avoidance of recourse to approximation, e.g. piecewise polynomial interpolation cf. [7]. Numerical results are presented and used to verify the accuracy and efficiency of the novel transformation, as well as to confirm convergence of the requisite Bessel series expansion and sampling theorem.
Inverse Synthetic Aperture Radar (ISAR) image gating for RCS extraction using backprojection is compared with image gating using smoothed reweighted L1-optimization in this study. The RCS of an object is measured by placing the object placed on a turntable which is rotated in an angular range while sweeping the frequency in the desired frequency range. A common model with isotropic point scatterers fixed in the object coordinate system is used in the ISAR imaging process. This model is used to define a forward operator. The ISAR image can be formed by operating with the backpropagation operator (i.e. backprojection), the adjoint of the forward operator, on the measured RCS. This robust method to solve the inverse problem gives an image with a resolution limited by the frequency bandwidth and the angular range. The RCS for a scattering feature is commonly determined by using the forward operator on the point scatterers in the image that are determined to belong to the scattering feature in ISAR image gating. L1-optimization is a method that can be used to get images with higher resolution and hence better separation of the different scattering features than backprojection. L1-optimization is well suited for naturally sparse ISAR images. One method to mitigate that the scatterers are restricted to a fixed grid is to use smoothed reweighting [1]. L1-optimizations are performed consecutively in a few steps where a smoothed version of the previous solution is used to determine a weighting matrix for the next step. Smoothed reweighted L1-optimization gives images with better separation of the scattering features in the ISAR image. Simulated and measured RCS data are used to compare image gating using backprojection with gating using smoothed reweighted L1-optimization. The main conclusion of this study is that the RCS can be extracted for scattering features, not resolved in backprojection images, using the smoothed reweighted L1-optimization. [1] D. Pinchera and M. D. Migliore, “Accurate reconstruction of the radiation of sparse sources from a small set of near-field measurements by means of a smooth-weighted norm for cluster-sparsity problems,” Electronics, vol. 10, no. 22, p. 2854, 2021.
The RCS of a target can be estimated using electromagnetic modeling if accurate geometries and material descriptions are available. An exact numerical calculation often requires prohibitive processing times. Moreover, numerical predictions with approximate techniques are difficult as it is challenging to consider all the physical phenomena. Therefore, a suitable RCS measurement facility adapted to the target size and specifications is required to estimate the RCS of a given target and to validate the numerical predictions. In general, the measurement of RCS takes place in anechoic chambers that simulate free-space and far-field conditions and where the unwanted reflections (walls, target mount, objects in the range, and the target interactions) are reduced. This paper presents a broadband measurement and validation of the RCS of a metallic trihedral corner reflector of 30 cm sides when fully anechoic conditions are not available, and consequently, some undesirable echoes are present in the measurements. Firstly, the measurement facility calibration and the target calibration are outlined. A single target reference approach is performed using a sphere as a reference, and its scattering response is shortly described. Then, the measurement of the target is performed. After these steps, a processing procedure is applied to isolate the target response from the background and the close responses due to unwanted reflections. The post-processing technique and the acquisition system are presented and discussed. The measurements are performed at X band as a function of the viewing angle for vertical transmit and receive polarization. To validate the technique, the RCS of the trihedral corner reflector is numerically simulated using the Integral Solver (I-Solver) of CST, with a Gaussian excitation, for vertical transmit and receive polarization. Measurements are compared with results obtained from CST software and show a good agreement with the numerical simulations. This setup will be used for RCS measurement of different complex targets and compared with measurements from other facilities to analyze and evaluate the RCS measurement uncertainty.
The characterization of antenna radiation pattern is a costly but a mandatory step for the development of any radiating system. Nowadays, these assessments commonly call for the estimation of the whole 3D radiated far field. Such evaluation is a time consuming task and many research works aim to minimize its impact on the whole prototyping process. Of course, the expected accuracy on the results has to be preserved in the meantime. To this end, two main approaches can be cited: reducing the field acquisition time or decreasing the cost of the measurement system itself. The first point can be achieved by more efficient sampling strategies (number of field samples as well as their spatial distribution) and the second one by magnitude-only, or phaseless, measurements. Unfortunately, phase retrieval problems are notoriously hard to solve and these numerical difficulties may be mitigated by considering large field samples, which goes against the reduction of the field acquisition time. We propose a phaseless measurement procedure in far field that amounts to solve a convex optimization problem to overcome the numerical difficulties arising in characterization of antenna radiation patterns from magnitude-only samples. More specifically, a regularization is applied to the spherical wave expansion of the radiation pattern to promote physical solutions. The proposed approach enables an accurate characterization of the far-field magnitude pattern from a small number of samples with respect to usual phaseless procedures. The proposed approach is confirmed by several experimental validations done at IETR that will be shown at the conference.
Digital array technology has advanced over the past few years to where it is now possible to build a large, wideband, multi-element digital array that can produce multiple steerable beams from the same aperture. These types of systems often have the analog-to-digital and digital-to-analog converters (ADC/DAC) and radio-frequency (RF) front end mated directly to the antenna, requiring a new measurement technique as the antenna under test (AUT) cannot be connected to traditional RF measurement equipment. Several years ago the Air Force Research Laboratory (AFRL) Sensors Directorate developed a custom 32 element uniform linear array using commercial off-the-shelf (COTS) low-noise amplifiers (LNAs) and a multi-channel digital receiver. Custom software was developed to perform automated element and digital beam pattern measurements using the compact range position controller. Hand tuned calibration parameters were calculated to form the digital beams. This work demonstrated feasibility of digital array measurement within a compact range environment. Recently AFRL has developed a highly integrated digital array with 1024 elements, LNAs, high-power amplifiers (HPAs), attenuators, phase shifters, transmit/receive switches, polarization switches, and eight multi-channel digital receiver/exciters. Measurements were required to not only test the functionality of the digital array, but also to build a calibration table for each element across the full frequency range of the array. The hand-tuned calibration method developed for the 32 element array was automated in order to build all of the necessary calibration tables for the 1024 element array. Calibrated beam patterns were also collected to analyze beam shape and pointing angle metrics. Following compact range measurement, the 1024 element array was relocated to a 100ft tower for outdoor RADAR experimentation where the collected calibration tables were successfully applied on the system. This paper will discuss challenges and successes in measurement of digital arrays within the compact range environment.
Abstract— A 2021 AMTA paper[1] introduced a 3D holographic filtering algorithm optimized for the planar near-field (PNF) geometry. This filter has been shown to have an excellent combination of AUT-signal preservation, stray-signal rejection, and processing speed. It requires only the sampling of a conventional PNF measurement, along with a specified 3D boundary surrounding all of the AUT’s possible radiating sources. The 2021 paper[1] suggested some topics for further investigation, specifically the optimal Z spacing through the 3D hologram and the X- and Y-widths of the blanking window’s tapered extension, and those are investigated here. This paper also explores the combination of filtering and probe correction, since the measured convolution of probe and AUT spatial distributions will be wider than that of the AUT by itself. Finally, additional comparisons are made to the more traditional spherical-mode-truncation approach with different synthesized constellations of stray-signal radiators. Keywords: modal filtering, spatial filtering, holographic filtering, stray signals, planar near field [1] S.T. McBride, P.N. Betjes, “Holographic PNF filtering based on known volumetric AUT bounds,” AMTA 2021, Daytona Beach, FL.
During the last few years, the increasing use of wireless equipment has raised the quantity of radiation energy to which human bodies are exposed. For this motivation, an evaluation of the Specific Absorption Rate (SAR) for persons is fundamental to determine the amount of radiation that human tissue absorbs and to comply with human safety regulations. Standard testing methodology consists of measurements with robot-based scalar/vector near-field probes and post-processing. The probe acquires the field level inside a phantom filled with liquid to ensure compliancy with certification standards. Although accurate, this technique could be extremely time-consuming, especially with the arrival of new frequency bands, new standards (5G, Wi-Fi 7), and the requirement to test different beams directions for beam-forming MIMO configuration. Another testing methodology, used especially for pre-assessment, consists of a full simulation of the radiator in the presence of the phantom, but this implies that the full wave model of the device is available, and this is rarely the case. To overcome the above-mentioned limitations, an alternative technique presented in this paper can be applied. This is based on a standalone measurement of the radiating device, that is post-processed with the method of the equivalent currents to generate NF source (in the form of a Huygens box). The SAR values inside the phantom are assessed using a non-invasive procedure with the assistance of a numerical simulation tool. Such method represents a fast procedure for pre-analysis of device prototypes, allowing to perform the conclusive testing only on the final device to verify the compliance with the regulations. The methodology is here experimentally validated on a dipole radiating in a presence of a phantom model by comparison of numerical simulated data and a reference measured data by a MVG ComoSAR V5 system.
Motivation and background: The increasing sophistication of wireless communication systems necessitates accurately designed test environments such as anechoic chambers. The minimum achievable level of noise and interference in such test environments is essentially determined by the reflectivity of the absorbers installed, emphasizing the importance of characterizing their scattering behavior under realistic test conditions. In order to improve the modeling of absorber-lined anechoic chambers e.g., based on ray-tracing methods, a profound understanding of the relationships between the geometrical (e.g., pyramidal or convoluted shapes) and material properties (complex-valued dielectric permittivity) and the frequency- and angle-dependent reflectivity of the absorbers is needed. Objectives and methods: The angle-dependent scattering off convoluted microwave absorbers at normal and oblique incidence was investigated at frequencies between 2 GHz and 18 GHz. Based on measured permittivity values, a unit-cell model was constructed to compute the angle-dependent reflectivity of absorbers of different shapes. To verify the model, the scattering off such absorbers was measured in a bi-static setup at different angles-of-incidence up to 60 degrees, and compared to the numerical results. In addition to the convoluted absorber geometry, pyramidal and wedge-shaped absorbers were studied, in order to analyze the influence of the absorber geometry on the reflectivity while maintaining the same material properties. Results and conclusions: The numerical results of the convoluted absorbers agreed well with the measured reflectivity, thus validating the numerical model. The results revealed an increase of the reflectivity at angles-of-incidence above 45 degrees, in accordance with expectation. Compared to the convoluted geometry, the pyramidal and wedge absorber shapes showed reflectivity values about 10 dB lower, for frequencies at which the electrical size of the absorbers exceeded unity. Together with the results of previous studies, these findings provide important ingredients for a comprehensive database of the angle- and frequency-dependent absorber reflectivity, from which a consistent ray-tracing modelling of anechoic test environments can be derived. This research has been funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under the grants HE3642/14-1 and BO4990/1-1 (Electromagnetic modeling of microwave absorbers - EMMA; Project-No. 418894892).
We propose a new Fresnel-field to far-field transformation to measure the absolute gain patterns of an antenna on a strip region between some elevation angles when the long axis of the antenna is placed in the horizontal plane. The measurements are done on multi circles of the same radius on the multi-cut planes (parallel to each other) at the Fresnel distance. The number of the multi circles depends on the antenna height and the radius of the circles. The number reduces to one, that is, the single-cut near-field to far-field transformation if the measurement radius is larger than the far-field distance determined by the height of the antenna. In our method, the number of the circles or the cut planes is proportional to the square root of the antenna height, whereas the conventional cylindrical scanning needs the number proportional to the antenna height because the height interval is a constant less than the half wavelength. Therefore, the measurement time by our method can be much less than the one by the cylindrical scanning. The proposed transformation is an extension of a single-cut near-field to far-field transformation combined with the Fresnel approximation along the z (height) direction. In our method, we can obtain the absolute gain pattern in the stirp region within the elevation angles spanned by the cut planes where the measurements are done. Whereas the elevation angles are limited by the angles where the Fresnel approximation holds, the azimuth angle range is only limited by the measured one and can be 360 degrees. In the presentation at the Symposium, we will show the simulation results and demonstrate the measurement results for a standard horn antenna at 70 GHz band using the new type of a photonic sensor. Our method has a possibility to extend the measurements on the circles to arbitrary curves on the multi-cut planes. This means that our method is most suitable to the measurement system using a robotic arm and a RoF (Radio on optical Fiber) technique.
Large deployable reflectors are critical for future Earth observation missions, science missions and in telecommunication, where an enhanced footprint and increased resolution are required and ensured by electrically very large reflector antennas. To accurately correlate simulations and measurements of such large and complicated antenna structures is a crucial step in improving the technology readiness level of these innovative antenna designs. A useful tool in this process is equivalent current reconstruction methods for antenna diagnostics, to allow comparisons between expected and realized performance. By finding the equivalent currents in the extreme near-field region that radiate a given/measured electromagnetic field, the user can accurately characterize the electromagnetic behaviour of the antenna under test. In this work, we present an antenna diagnostics investigation of an electrically large reflector antenna from the European Large Deployable Reflector project [1]. The antenna consists of a 5.1 m diameter deployable offset reflector in lightweight mesh technology. The antenna is an offset parabolic reflector with f/D equal to one and it has been measured at 10.65 GHz and 18.7 GHz. At such electrical sizes, an equivalent current investigation has previously been out-of-scope for the computational solvers in the market. In a recent ESA study, an accelerated equivalent current reconstruction solver based on [2] has been carefully implemented and then applied [3] to perform source reconstruction of the full reflector antenna based on measured and simulated data. Comparing the two sets of reconstructed currents gives the possibility to highlight potential deviations and pinpoint problematic aspects of the antenna design. [1] C. Cappellin, M. Lori, A. Geise, C. Hunscher, and L. Datashvili, “Predicted and Measured Antenna Patterns of the European Large Deployable Reflector,” Proceedings of EuCAP, 2022. [2] J. Kornprobst, R. A. M. Mauermayer, E. Kılıç and T. F. Eibert, "An Inverse Equivalent Surface Current Solver with Zero-Field Enforcement by Left-Hand Side Calderón Projection," Proceedings of EuCAP, 2019. [3] O. Borries, M. H. Gaede, P. Meincke, A. Ericsson, E. Jørgensen, D. Schobert, and E. Gandini, “A Fast Source Reconstruction Method for Radiating Structures on Large Scattering Platforms,” Proceedings of AMTA 2021.
The Field Strength Metrology Project at the National Institute of Standards and Technology (NIST) in Boulder, CO has restarted field probe calibration services from 10 kHz to 40 GHz, after a renovation of ouranechoic chamber. WhileNISThas long served as the nation’s link to the SI for radiated field measurements, in 2014, the anechoic chamber used for generating standard electromagnetic fields from 0.5 – 40 GHz was renovated. The positioning system was upgraded with a new rail, motion control, and a robotic arm. New absorber was installed in the main section of the chamber. During the renovation, Field Strength services were unavailable. In order to resume operation in the chamber, several tests were done to validate the chamber. We show the results of a thorough comparison of three facilities, the anechoic chamber, a TEM cell and a GTEM cell. Measurements of electric field probes in the new chamber were also compared with past measurements in the chamber before renovation. The Electromagnetic Field Strength special test services are now operational.
The mission of the Naval Surface Warfare Center, Indian Head, Maryland, EOD Department, is to utilize the latest available technology in the advancement of Explosive Ordnance Disposal (EOD) equipment and techniques. This mission includes the test and evaluation of current and developmental systems, which will be discussed in this paper. EOD exploits multiple physical phenomena in its task of ordnance detection, including chemical and electromagnetic. Electromagnetics include RF fields, light (including laser, infrared and ultraviolet), and nuclear radiation. For each phenomena, there may be several different technologies used to provide multi-mode detection capability. This study focuses on the electromagnetic subset of detection RADAR, and specifically Ground Penetrating Radar (GPR), which is distinguished by its earth surface domain and generally downward field of view. The paper will give a very brief overview of GPR theory and equipment, its use in EOD, and then will focus on the RF test and measurement of electromagnetic fields generated by GPR systems and antennas. An RF antenna/system test plan will be detailed, along with the design and development of antenna gain and radiation pattern measurement techniques. The measured data from GPR technology will be graphically displayed, analyzed and compared in terms of the potential for GPR effectiveness.
When using the gain substitution method with a pyramidal standard gain horn (SGH), it is common practice to use the on-axis NRL gain curves derived by Schelkunoff and Slayton [1]. Due to approximations in this formulation, Slayton assessed an uncertainty of ±0.3 dB for typical SGHs operating above 2.6 GHz. Since this uncertainty term is often one of the largest terms in the range measurement uncertainty budget for AUT gain, it is highly desirable to reduce it. Many studies in the past have attempted to improve upon Slayton’s expressions for SGH gain, but none have achieved widespread use. With the advent of high-performance computing (HPC), antenna simulations with computational electromagnetic (CEM) full-wave solvers are now capable of solving complex, electrically large models with high accuracy. This paper investigates the use of several commercially available solvers, including HFSS, CST, and FEKO to model the on-axis directivity and gain of a commercial off-the-shelf (COTS) X-band SGH. Relevant modeling techniques are described in detail and are shown to employ best practices as well as conformance with the IEEE 1597.1 and 1597.2 “Standards for Validation of CEM Modeling and Simulations” and “Recommended Practice for Validation of CEM Computer Modeling and Simulations,” respectively. The challenges and trade-offs of each CEM solving technique used, as well as their limitations, are discussed. Simulation errors are quantified via the IEEE standards, and other practical limitations of SGH manufacturability and measurement are discussed. Finally, the results from the CEM simulations are compared with the NRL gain curve and measured on-axis directivity and gain of the COTS SGH. Based on the compiled results of multiple simulations and measurements, this simulation methodology could be applied to other models of COTS SGH antennas to provide more accurate on-axis gain predictions. [1] W. T. Slayton, “Design and calibration of microwave antenna gain standards,” US Naval Res. Lab., Washington, DC, Rep. 4433, Nov. 1954.
Novel metamaterial and metasurface realizations provide unique control of the wave dispersion but present many challenges for accurate constitutive parameter extraction. Practical material measurement approaches for characterizing complex materials, such as biaxial anisotropic and gyrotropic media, rely on either waveguide or focus beam techniques with trade-offs in sample size and bandwidth. Multi-static focus beam setups offer many advantages for complex material measurements by enabling wider sampling bandwidths, measurement degrees of freedom and larger sample sizes. However, rotational misalignment of the principal crystal and measurement coordinates of the biaxial anisotropic media sample results in cross-polarized scattered field contributions. Likewise, interrogating gyrotropic or general bianisotropic media with a dual-polarized focus beam measurement setup produces cross-polarized scattering matrix components. These non-diagonalized S-parameters for the complex sample under test must be measured to successfully extract the constitutive parameters. A time-domain gated response isolation calibration scheme is one common approach to establish the measurement reference plane and minimize fixture uncertainties for samples with limited cross-polarization scattering. This paper extends the time-domain gated response isolation methodology for free-space focus beam systems to account for cross-polarization terms present in both the sample under test as well as the measurement setup. The featured analysis leverages a 4x4 S-parameter matrix notation to capture the polarimetric scattering at each cascaded stage. An equivalent line standard procedure is developed where four unique, linearly independent calibration standard measurements are shown to account for all unknown terms. Finally, a sensitivity analysis of the calibration standards is performed via numerical simulations to show the potential trade-off and limitations of the cross-polarization extended time-domain response isolation calibration scheme performance.
In a previous presentation[1], the author has reported that nearfield antenna measurements taken in the presence of a lossy half space such as the ocean can be accomplished by the use of Complex Image Theory. The approach allows the user to collect data over the upper hemisphere of space and employ Complex Image Theory to “fill in” the field information in the lower hemisphere. The approach has been limited, though, in application due to the limited applicability of the Complex Image approach. In this paper, a fresh look is taken at Complex Image Theory and a new field expansion proposed that allows the fields due to a source operating above a lossy half space to be expressed in terms of the fields due to an infinite sequence of equivalent Huygens sources located in complex space. The new expansion has advantages over previous work in that it properly predicts the formation of a surface wave along the interface between the two half-spaces in addition to properly accounting for the space wave field.
There has been much discussion in the last few decades regarding redundancy in conventional near-field sampling, and that redundancy is most pronounced in the planar geometry. There has also been much discussion regarding modal filtering of near-field data to attenuate the effects of stray signals. Both discussions revolve around the limited local spatial bandwidth that can be produced at each probe location when the antenna under test’s (AUT’s) radiating sources are all contained within a known geometric boundary. This paper discusses a novel filtering technique that exploits the inherent sampling redundancy in conventional planar near-field acquisitions. The filtering is based solely on the known location and shape in the scanner’s coordinate system of a closed 3D boundary around the radiators of interest. The paper describes the algorithm and presents results from both measured and synthesized input. The new filter is also compared to other available filters in terms of speed, attenuation of stray signals, and preservation of AUT signals.
Antenna performance plays a significant role in synthetic aperture radar (SAR) image quality, particularly for nondestructive evaluation (NDE) applications. To obtain high image quality and target detectability, SAR imaging systems should possess good resolution (cross- and along-range), and a relatively large penetration depth. Consequently, the antenna used must be wideband with a relatively wide beamwidth for high resolution and operate at low starting frequency for sufficient penetration depth. Meanwhile, antenna aperture size should be small rendering it sufficiently portable for scanning purposes or when employed within imaging arrays. However, increasing frequency bandwidth, reducing minimum frequency of operation while maintaining small aperture size (resulting in wide beamwidth), all at the same time is difficult. To this end, double-ridged horn (DRH) antenna, with flared aperture for improved radiation efficiency and performance is found to provide a good compromise among these parameters. Therefore, an improved modified design of DRH is proposed. The dimensions of its geometry are optimized to provide low unwanted reflections. Curved surfaces are attached at the end of the two ridged walls for better aperture matching. The final aperture size of the antenna is 230 ? 140 mm2, operating in the 0.5-4.0 GHz frequency range, and with a relatively wide beamwidth in its near-field region where most NDE imaging measurements are conducted. Measured reflection coefficient by using the fabricated antenna is used to verify the simulation results. Comparisons are also made with similar designs of DRH found in the literature showing that the proposed antenna has smaller electrical length with respect to the lowest operating frequency for designs without using absorbing material. Moreover, to conduct wideband SAR imaging, a new phase calibration method, using a small electric field monopole probe, to measure the phase change between the antenna aperture center and the input feed port for each frequency component is developed. Imaging results over a large concrete slab with delamination and voids simulated by foam and plastic sheets show that the proposed calibration approach works well, and the proposed antenna can effectively detect all of these defects with different scattering properties.