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For near-field antenna measurement it is sometimes desirable or necessary to measure only the magnitude of the near-field - to perform so-called phaseless (or amplitude-only or magnitude-only) near-field antenna measurements [1]. It is desirable when the phase measurements are unreliable due to probe positioning inaccuracy or measurement equipment inaccuracy, and it is necessary when the phase reference of the source is not available or the measurement equipment cannot provide phase. In particular, as the frequency increases near-field phase measurements become increasingly inaccurate or even impossible. However, for the near-field to far-field transformation it is necessary to obtain the missing phase information in some other way than through direct measurement; this process is generally referred to as the phase retrieval. The combined process of first measuring the magnitudes of the field and subsequently retrieving the phase is referred to as a phaseless near-field antenna measurement technique. Phaseless near-field antenna measurements have been the subject of significant research interest for many years and numerous reports are found in the literature. Today, there is still no single generally accepted and valid phaseless measurement technique, but several different techniques have been suggested and tested to different extents. These can be divided into three categories: Category 1 – Four magnitudes techniques, Category 2 – Indirect holography techniques, and Category 3 -Two scans techniques. This paper provides an overview of the different phaseless near-field antenna measurement techniques and their respective advantages and disadvantages for different near-field measurement setups. In particular, it will address new aspects such as probe correction and determination of cross-polarization in phaseless near-field antenna measurements. [1] OM. Bucci et al. “Far-field pattern determination by amplitude only near-field measurements”, Proceedings of the 11’th ESTEC Workshop on Antenna Measurements, Gothenburg, Sweden, June 1988.
A novel system architecture has been developed which makes measurements at N times the analyzer’s frequency, yet requires no communication with the analyzer. Millitech’s Spartan Test Modules, STMs, splits the input signal from an analyzer, multiplies this by N for the source, and by N-1 for the LO of the receiver mixer. The mixer downconverts to the original signal, while maintaining its phase integrity, and sends this back to the analyzer. This scheme is straightforward for narrow bandwidth requirements, but becomes more difficult for wideband ones. The filtering and temperature compensation requirements are high, but have been solved for these bands resulting in a dynamic range of 70 to 80 dB across 54-69 GHz for V-Band and across 69-90 GHz for E-Band, which directly relates to the side lobe resolution in an antenna pattern measurement. The wide dynamic range doesn’t come at a cost of slowing the sweep, as in other frequency extension solutions. This puts the Spartan system performance at the same or higher level as other mixer based systems that have much higher hardware requirements. STMs can be used to convert any make, model or vintage of vector network, scalar network or spectrum analyzer into a millimeter-wave test station. The small size of the STMs allows them to be mounted directly onto the back of the antennas. Therefore, readily available, < 10 GHz cables can be used for the long run back to the analyzer. The Spartan enables state-of-the-art antenna measurements either directly, in compact ranges, or in near-field ranges, examples will be shown.
Spherical near-field (SNF) antenna test systems offer unique advantages over other types of measurement configurations and have become increasingly popular over the years as a result. To yield high accuracy far-field radiation patterns, it is critical that the rotators of the SNF scanner are properly aligned. Many techniques using optical instruments, laser trackers, low cost devices or even electrical measurements [1 - 3] have been developed to align these systems. While these alignment procedures have been used in practice with great success, some residual alignment errors always remain. These errors can sometimes be quantified with high accuracy and low uncertainty (known error) or with large uncertainties (unknown error). In both cases, it is important to understand the impact that these SNF alignment errors will have on the far-field pattern calculated using near-field data acquired on an SNF scanner. The sensitivity to various alignment errors has been studied in the past [4 - 6]. These investigations concluded that altering the spherical acquisition sampling grid can drastically change the sensitivity to certain alignment errors. However, these investigations were limited in scope to a single type of measurement system. This paper will expand upon this work by analyzing the effects of spherical alignment errors for a variety of different measurement grids and for different SNF implementations (phi-over-theta, theta-over-phi) [7]. Results will be presented using a combination of physical alignment perturbations and errors induced via simulation in an attempt to better understand the sensitivity to SNF alignment errors for a variety of antenna types and orientations within the measurement sphere. Keywords: Spherical Near-Field, Alignment, Uncertainty, Errors. References [1] J. Demas, “Low cost and high accuracy alignment methods for cylindrical and spherical near-field measurement systems”, in the proceedings of the 27th annual Meeting and Symposium, Newport, RI, USA, 2005. [2] S. W. Zieg, “A precision optical range alignment tecnique”, in the proceedings of the 4th annual AMTA meeting and symposium, 1982. [3] A.C. Newell and G. Hindman, “The alignment of a spherical near-field rotator using electrical measurements”, in the proceedings of the 19th annual AMTA meeting and symposium, Boston, MA, USA, 1997. [4] A.C. Newell and G. Hindman, “Quantifying the effect of position errors in spherical near-field measurements”, in the proceedings of the 20th annual AMTA meeting and symposium, Montreal, Canada, 1998. [5] A.C. Newell, G. Hindman and C. Stubenrauch, “The effect of measurement geometry on alignment errors in spherical near-field measurements”, in the proceedings of the 21st annual AMTA meeting and symposium, Monterey, CA, USA, 1999. [6] G. Hindman, P. Pelland and G. Masters, “Spherical geometry selection used for error evaluation”, in the proceedings of the 37th annual AMTA meeting and symposium, Long Beach, CA, USA, 2015. [7] C. Parini, S. Gregson, J. McCormick and D. Janse van Rensburg, Theory and Practice of Modern Antenna Range Measurements. London, UK: The Institute of Engineering and Technology, 2015
Installing a large compact range reflector and electromagnetically qualifying the quiet zone is a major undertaking, especially for very large panelized reflectors. The approach taken to design the required rolled-edge reflector geometry for achieving a 5 meter quiet zone across a frequency range of 350 MHz to 40 GHz was previously presented [1]. The segmentation scheme, fabrication methodology, and intermediate qualification of panels using an NSI-MI developed microwave holography tool were also presented. This reflector has since been installed and the compact range qualified by direct measurement of the electromagnetic fields in the quiet zone using a large field probe. This paper presents the comparison and correlation between the holography predictions and the field probe measurements of the quiet zone. Installation and alignment techniques used for the multiple panel reflector are presented. Available metrology tools have inherent accuracy limitations leading to residual misalignment between the panels. NSI-MI has overcome this limitation by using its holography tool along with existing metrology techniques to predict the field quality in the quiet zone based on surface measurements of the panels. The tool was used to establish go/no-go criteria for panel alignment accuracy achieved on site. Correlation of the holography predictions with actual field probe measurements of the installed reflector validates the application of the holography tool for performance prediction of large, multiple-panel, rolled-edge reflectors. Keywords: Rolled-Edge Reflector, Compact Range, Field-Probing, Quiet Zone, Microwave Holography
The Compensated Compact Range (CCR) 75/60 of Airbus DS GmbH is the state-of-the art indoor test facility for real-time RF measurements of satellite antennas within a frequency range from 1 to 200 GHz. The CCR is composed of a two reflector system, a main reflector and a sub-reflector, to create a cross-polar-compensated plane wave in the test zone. However, even such a sophisticated design has residual cross-polar components due to the contribution of the range feed, edge diffraction from the reflector system, as well as from the serrations and imperfect absorbers. To improve and optimize the RF performance of the CCR, detailed EM simulation models are developed in order to solve the related forward scattering problem [1, 2, 3]. In spite of this it is also of great importance to analyze the CCR in a different perspective to gain insight into the CCR. To this aim, an approach based on plane wave spectrum analysis combined with inverse scattering and imaging techniques is proposed. The proposed approach firstly computes the plane wave spectrum of the measured or simulated data taken in the quite zone by using 2D Fast Fourier Transform (FFT). Then, the measured or simulated field is back-propagated by using an inverse scattering approach. By considering the geometrical shape information of the main reflector, the current distribution on the reflector is imaged. The reconstructed images help to clearly identify the effects of. Appropriate windowing is applied to the computed plane wave (angular) spectrum in order to locate and image the echoes. Based on the investigation carried out with the proposed approach, it turns out that the area of the main reflector should be increased to reduce the disturbing impact of the serrations. This investigation also shows that increasing the size of the sub-reflector does not help to improve the plane wave uniformity of the fields in the test zone. In order to test the proposed method against the experimental data, which is not in a suitable format for FFT, the measured data is interpolated to equally spaced data in a Cartesian coordinate system. The experimental results, which are obtained by processing both co and cross polar measurements, show very good agreement with the results obtained by using synthetic data. References [1] A. Geise, J. Migl, J. Hartmann, H-J. Steiner, “Full Wave Simulation of Compensated Compact Ranges at Lower Frequencies”, AMTA 33th Annual Symposium, 16 – 21 October 2011 in Englewood Colorado, USA. [2] C. H. Schmidt, A. Geise, J. Migl, H-J. Steiner, H.-H. Viskum, “A Detailed PO/ PTD GRASP Simulation Model for Compensated Compact Range Analysis with Arbitrarily Shaped Serrations”, AMTA 35th Annual Symphosium, 6 – 11 October 2013 in Colombus Ohio, USA. [3] O. Borries, P. Meincke, E. Jorgensen, C. H. Schmidt, “Design and Validation of Compact Antenna Test Ranges using Computational EM”, AMTA 37th Annual Symphosium , 11 – 16 October 2015 in Long Beach, CA, USA.
The miniaturization of modern electronics has led to the development of a new class of small satellites called CubeSats. The small size facilitates launching the CubeSats as secondary payloads, significantly reducing launch costs. The scientific community is actively investigating the potential of deployable reflectors, reflectarrays and membrane antennas to accommodate the high data rate and resolution requirements for future CubeSat missions. The development of such deployable high gain antennas significantly broadens the horizons for advanced CubeSat missions at low costs. Our goal is to develop novel, practical antenna concepts that can support these emerging applications. Horn antennas are frequently used as feeds for deployable reflector antennas. With the reflector itself occupying significant space within the CubeSat, it is critical that the feed occupies minimal volume. The horn aperture dimensions are usually fixed in satisfying the -10dB edge illumination requirements set by the reflector design. For pyramidal or conical horns, the length is limited by the quadratic phase error at its aperture. Special techniques must be used to achieve desired performance when horn length is a major constraint. Potter horns use a stepped profile to create a dual-mode distribution to provide low cross polarization at the cost of reduced bandwidth and complexity of prototyping. Corrugated horns are also capable of providing low sidelobes and cross polarization, but are expensive to fabricate and are typically heavier. Optimization techniques offer the possibilities of handling multiple design parameters, while allowing the designer to put more emphasis on critical constraints. We employ a novel spline-profiled smooth walled horn design that strikes a balance between ease of fabrication, desired radiation characteristics and overall volume. Particle Swarm Optimization (PSO) was used to optimize the horn profile for the desired beamwidth, length, cross polarization level and backlobe level. Detailed study of the aperture field distributions further illustrate the novelty of our design. The performance of the designed horn is validated using UCLA’s tabletop bipolar planar near field measurement facility. Thus, the power of optimization and elegance of monotonic splines was used to design a key component for future deployable reflector systems in CubeSats.
Near field Inverse Synthetic Aperture Radar (ISAR) Radar Cross Section (RCS) measurements are used in this study to obtain geometrically correct images of full scale objects placed on a turntable. The images of the targets are processed using a method common in the compressive sensing field, Basis Pursuit Denoise (BPDN). A near field model based on isotropic point scatterers is set up. This target model is naturally sparse and the L1-minimization method BPDN works well to solve the inverse problem. The point scatterer solution is then used to obtain far field RCS data. The methods and the developed algorithms required for the imaging and the RCS extraction are described and evaluated in terms of performance in this paper. A comparison to image based near to far field methods utilizing conventional back projection is also made. The main advantage of the method presented in this paper is the absence of noise and side lobes in the solution of the inverse problem. Most of the RCS measurements on full scale objects that are performed at our measurement ranges are set up at distances shorter than those given by the far field criterion. The reasons for this are, to mention some examples, constraints in terms of available equipment and considerations such as maximizing the signal to noise in the measurements. The calibrated near-field data can often be used as recorded for diagnostic measurements but in many cases the far field RCS is also required. Data processing is then needed to transform the near field data to far field RCS in those cases. Separate features in the images containing the point scatterers can be selected using the method presented here and a processing step can be performed to obtain the far field RCS of the full target or selected parts of the target, as a function of angle and frequency. Examples of images and far field RCS extracted from measurements on full scale targets using the method described in this paper will be given.
Indoor antenna ranges must have the walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials, however, the bi-static or off angle performance must also be known. Some manufacturers provide factors at discrete electrical thickness for a discrete range of incident angles. This approximation is based on the curves presented in [1]. In reference [2], a polynomial approximation was introduced. In this paper, a more accurate approximation is introduced. Pyramidal RF absorber is modeled using CST’s frequency domain solver. The numerical results are compared to results from other numerical methods. The highest reflectivity of the two principal polarizations for a given angle of incidence and thickness of material is calculated. Different physical thickness pyramids are modeled. Once the worst case reflectivity is calculated, a polynomial curve fit is done to get a set of equations that provide the bi-static performance for absorber as a function of angle of incidence and thickness of material. The equations can be used to predict the necessary RF absorber to treat the walls of an indoor range.
In this case study, an existing spherical near-field test facility that was used productively and effectively for many years had become a bottleneck. Recent needs for more extensive antenna characterization had driven test times to an extreme, approaching 300 hours of data acquisition for a single antenna and 160 hours for additional processing. The size of collected data files had also become extremely large, exceeding the 2 GB capacity of the commercial database used to store acquisition files. A system measurement timing assessment was conducted for this test facility to determine the most effective means of reducing the data acquisition time. A timing model was created to optimize the current system resulting in an immediate reduction of data acquisition time by 45%. A sensitivity study was conducted to show the tradeoffs between additional test time improvements that could be achieved. Results showed that by using a combination of several of these improvements along with a modest investment in new equipment, total acquisition time could be further reduced to 16 hours, achieving a 95% reduction in acquisition time as compared to the baseline. In addition to acquisition time, post-acquisition processing time was also improved. Some of the additional processing time was caused by the data file size limitation, which had been addressed by creating multiple files during the acquisition and combining the result afterward. By implementing an alternate file structure to support data acquisitions greater than 2GB, approximately 25% of the additional processing time was eliminated. This study illustrates that periodic evaluation and optimization of system test processes and measurement timing can sometimes pay large immediate dividends in range throughput and productivity. In addition, by creating an accurate system measurement timing model, sensitivity studies can easily be conducted to provide guidance in selecting the most effective alternative test plans or incremental investments in new equipment.
This paper presents a study on the low-frequency coaxial reflectometer measurement procedure. A time domain gating algorithm is developed by ETS-Lindgren and the results are validated after comparing to the Keysight 8753-time domain algorithm. The in-house time gating algorithm is then applied to the simulated reflectivity results of absorbers in reflectometer to the simulation results of the same absorbers with plane wave excitation using finite element method numerical computation. Based on the simulation results, the operable upper frequency limit and the minimum length of the straight coaxial section for the reflectometer are suggested. The errors introduced during measurement due to higher order modes are studied and the permissible limit for the errors is analyzed. The different higher order modes and their effects on field distribution are studied. The impact of the non-uniform field distribution on the absorber reflectivity measurement is also discussed.
We have developed a 60 GHz chip antenna designed for use as a gain and pattern verification tool in the calibration process of a millimeter wave antenna test chamber. The antenna is designed to interface with ground-signal-ground (GSG) micro-probes that have a probe pitch of 150 um to 250 um. This low temperature cofired ceramic (LTCC) chip antenna is fabricated using DuPont’s 9K7 GreenTapeTM material system with gold conductors. Features include a wafer-probe transition, a shielded stripline corporate feed network, aperture coupled patch elements, and an integrated Sievenpiper electromagnetic bandgap (EBG) structure for surface wave mode suppression. The use of the EBG structure enables main beam gain enhancement and side lobe level suppression. This 2x2 antenna array is directive such that it offers a nominal gain of 12 dBi at broadside over 58-62 GHz with an antenna efficiency of at least 60%. The entire antenna package has a nominal size of only 10.9 mm x 12.2 mm x 0.71 mm. Since this antenna package material is hermetic, it has stable performance under varying humidity and temperature which is highly desirable as a reference antenna.
The higher the frequency is, the greater the influence of the precision and the realism of the CAD models on electromagnetic (EM) scattering characteristics are. In the terahertz (THz) regime, surfaces of most objects can’t be taken as smooth according to Rayleigh criterion. The interaction of EM waves and the surface presents a coherent part in the specular direction and a scattering part in the other directions. Unfortunately, the roughness of surface can’t be represented by the CAD geometry. Based on statistics theory, the rough surface height profile is fully determined by the height probability density function (pdf) and its autocorrelation functions. Without loss of generality, the height pdf of surface is assumed to be Gaussian. Under the assumption, the random Gaussian rough surface is correspondingly generated. The original CAD geometry and the random Gaussian rough surface are superposed as the input of EM computation. To demonstrate the roughness impact on RCS, EM scattering characteristics of simple canonical objects such as plate, dihedral and trihedral in the THz regime are investigated. Taking into account the statistical surface roughness, the ray-based high-frequency EM method, shooting and bouncing rays (SBR), is utilized to compute the RCS of the above objects in the THz regime. Furthermore, the inverse synthetic aperture radar (ISAR) images are also carried out via filtered back projection (FBP) method. The EM scattering characteristics of the above objects in the THz regime are analyzed. Great differences of the objects EM scattering characteristics between the smooth and rough ones are observed and discussed.
Near-field antenna test systems are typically designed to optimize measurement results for a specific type of antenna. The measurement system is selected and sized based on the antenna aperture dimensions, directivity, weight and operating frequency, among other parameters. These factors are used to select either a planar, cylindrical, or spherical near-field test system for the given antenna test requirements. Antennas with different characteristics may not be compatible with the selected range and often require costly upgrades to the existing range or a different range altogether. One solution to test a wide variety of antenna types is a combination planar-cylindrical-spherical (PCS) test system. These systems usually require some level of facility re-configuration and present drawbacks when switching between the various modes of operation. The adaptation of a six-axis robotic test system is an attractive solution in these situations, as the system’s flexibility allows for rapid reconfiguration that is inherent to the system. This allows the user to select the optimal test solution for the antenna under test with little effort. This paper presents the performance of a six-axis robotic near-field measurement system showing near-field modes of operation and the system’s performance in antenna measurements when compared to a traditional spherical near-field range
“RTCA DO-213 Minimal Operation Performance Standards For Nose-Mounted Radomes” is a document frequently referenced in nose-radome testing requirements for commercial aircraft. This document was produced and is maintained by the Radio Technical Commission for Aeronautics (RTCA). The specifications of weather-radar systems have recently changed within RTCA’s DO-220A, and as a result DO-213 was updated to DO-213A in March, 2016, to ensure that radome requirements are consistent with those of the weather radar. In addition to the new requirements for radome evaluation, several existing requirements were clarified. These clarifications addressed such things as suitability of near-field measurements, proper procedures and processing, and appropriate measurement geometries. RTCA coordinated the document revision, with the bulk of the technical inputs coming from a broad-based working group. This working group had representatives from radar, aircraft, and radome manufacturers, government agencies, and providers and users of radome-testing systems. When requirements were added or when common practice conflicted with existing requirements, there was considerable effort and analysis employed to ensure that each change or clarification was truly required. Nevertheless, DO-213A has some significant impacts to many existing radome-testing facilities. This paper discusses the significant changes in DO-213A and their implications for radome test facilities, concentrating on after-repair radome electrical testing.
At AMTA 2006, we introduced the world to a system and method for over-the-air (OTA) testing of MIMO wireless devices with the concept of the boundary array technique, whereby the far-field over the air RF propagation environment is emulated to produce the realistic near field multi-path propagation conditions necessary for MIMO communication. Last year, the CTIA released Version 1.0 of their "Test Plan for 2x2 Downlink MIMO and Transmit Diversity Over-the-Air Performance," which standardizes on the boundary array technique (commonly referred to as the Multi-Probe Anechoic Chamber technique to differentiate it from the use of a reverberation chamber) for MIMO OTA testing. As the wireless industry just now prepares to perform certification testing for MIMO OTA performance for existing 4G LTE devices, the rest of the community is looking forward to the development of 5G. The corresponding future releases of the 3GPP wireless standard are expected to standardize the use of Massive MIMO in existing cellular communication bands. Massive MIMO is similar to the concept of mulit-user MIMO in IEEE 802.11ac Wi-Fi radios, but is taken to the extreme, with potentially hundreds of antennas and radios per cellular base station. This high level of radio to antenna integration at the base station will for the first time drive the industry beyond just antenna pattern measurements of base stations and OTA performance testing of handsets to full OTA performance testing of these integrated systems. At the same time, handset design is evolving to use adaptive antenna systems that will pose additional testing challenges. Likewise, manufacturers are looking to evaluate real-world usage scenarios that aren't necessarily represented by the test cases used for mobile device certification testing. This paper will discuss a number of these advances and illustrate ways that the MIMO OTA test systems must evolve to address them.
At AMTA 2006, we introduced the world to a system and method for over-the-air (OTA) testing of MIMO wireless devices with the concept of the boundary array technique, whereby the far-field over the air RF propagation environment is emulated to produce the realistic near field multi-path propagation conditions necessary for MIMO communication. Last year, the CTIA released Version 1.0 of their "Test Plan for 2x2 Downlink MIMO and Transmit Diversity Over-the-Air Performance," which standardizes on the boundary array technique (commonly referred to as the Multi-Probe Anechoic Chamber technique to differentiate it from the use of a reverberation chamber) for MIMO OTA testing. As the wireless industry just now prepares to perform certification testing for MIMO OTA performance for existing 4G LTE devices, the rest of the community is looking forward to the development of 5G. In the search for ever more communication bandwidth, the wireless industry has set its sights on broad swaths of unused spectrum in the millimeter wave (mmWave) region above 20 GHz. The first steps into this area have already been standardized as 802.11ad by the members of the WiGig Alliance for short range communication applications in the unlicensed 60 GHz band, with four 2.16 GHz wide channels defined from 58.32-65.88 GHz. With the potential for phenomenal bandwidths like this, the entire telecommunications industry is looking at the potential of using portions of this spectrum for both cellular backhaul (mmWave links from tower to tower) as well as with the hopes of developing the necessary technology for mobile communication with handsets. The complexity of these new radio systems and differences in the OTA channel model at these frequencies, not to mention limitations in both the frequency capabilities and resolution requirements involved, imply the need for a considerably different environment simulation and testing scenarios to those used for current OTA testing below 6 GHz. The traditional antenna pattern measurement techniques used for existing cellular radios are already deemed insufficient for evaluating modern device performance, and will be even less suitable for the adaptive beamforming arrays envisioned for mmWave wireless devices. Likewise, the array resolution and path loss limitations required for a boundary array system to function at these frequencies make the idea of traditional OTA spatial channel emulation impractical. However, as we move to technologies that will have the radio so heavily integrated with the antenna system that the two cannot be tested separately, the importance of OTA testing cannot be understated. This paper will discuss the potential pitfalls we face and introduce some concepts to attempt to address some of the concerns noted here.
Antennas utilized as probes, sources, and for gain comparison are typically specified to have excellent cross polarization levels, often on the order of 50 dB below the primary polarization component. In many cases, these antennas are fed with a waveguide-to-coaxial adapter, which can be sourced from a multitude of vendors. Depending on the design and construction of the adapter, and the distance from the excitation probe to antenna aperture, the adapter itself can contribute significantly to the degradation of the polarization purity of the antenna. These adapters typically use one of several methods to achieve a good impedance match across their bandwidths, including tuning screws, posts and stubs. These tuning elements may be arranged asymmetrically and can cause the waveguide to be overmoded locally. Additionally, there is wide variance in the separation of the adapter excitation probe and waveguide electrical flanges, which may not be long enough to suppress the higher order modal content. In this paper, we study the effects of adapter to antenna aperture coupling, including the coupling of fields local to the current probe as well as those that are induced by design asymmetries. The results of the analysis lead to a number of rules of thumb which can be used to ensure that the antenna polarization purity is optimized.
We present a channel sounder that can operate without a tether and still maintain an absolute time reference between the source and receiver. Based on a sliding correlator, with synchronized rubidium clocks to generate phase references for the up- and down- converted RF carriers, and a synchronous trigger, the system generates locked signals in the short term – 10’s of hours. The system has an operational range of 10 MHz to 6 GHz with an instantaneous channel bandwidth of up to 200 MHz. We start with a discussion on processing measurements for oversampled band-limited signals. Spectral truncation is compared with transmit spectrum filtering; DC bias removal and referencing to remove systematic effects are discussed. We conclude with channel sounding results, power delay profile, RMS delay spread, and time of arrival versus position for an electromagnetically complex environment.
The evaluation of RF Sensors often requires a test capability where various RF scenes are presented to the Unit Under Test (UUT). These scenes may need to be dynamic, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Scene Generator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Scene Generator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. This paper describes the system concept for an RF Scene Generator that simultaneously represents 4 targets, in highly dynamic motion, with no occlusion, over a wide range of power, frequency, and Field of View (FOV). It presents the test results from a prototype that was built and tested over a limited FOV, while being scalable to the total FOV and full system capability. The RF Scene Generator employs a wall populated with an array of emitters that enables virtually unlimited velocity and acceleration of targets and employs beam steering to provide high angular resolution and accuracy of the presented target positions across the FOV. Key words: RF Target Simulator, RF Scene Generator, Multiple Targets, Beam Steering Wall of Emitters, Steering Array Calibration, Plane-Wave Generator, Radar Environment Simulator.
In near-field antenna measurements, the pattern effect of the measuring probe represents a systematic error and thus probe pattern correction is a constitutive part of the existing processing algorithms. However, as it was shown in [1], in spherical near-field measurements, for typically used measurement distances, not exceeding two to four diameters of the measured antenna, the probe pattern effect is relatively small, and in many situations the probe pattern can be taken as that of a Hertzian dipole with the resulting effect on the measured antenna pattern being either very small or even negligible. On the other hand, for shorter measurement distances, the probe pattern effect becomes significant and omitting the probe pattern causes noticeable changes in the measured antenna pattern. It was shown in [2] by approximate simulations that in these cases applying the correction using the probe pattern not at the measured frequency, but at the center frequency of a standard waveguide band provides negligible error for even very small measurement distances, not exceeding one or two diameters of the measured antenna, depending on the probe type. Since an approximate model was used for the simulations, the obtained results show only preliminary picture and can only be used as tentative guidelines. In this paper, in order to prove the results of the simulations and the derived conclusions, experimental validation of the simplified probe pattern correction was carried out by processing measured results of several electrically large antennas including probe pattern correction at the measured frequency and at the center frequency of the waveguide band, and comparing the difference. The measured results of a center-fed parabolic reflector, an offset reflector, and a base-station antenna were used for the validation. The obtained results generally confirm the simulations and prove the conclusions that just a single probe pattern can be used for all frequencies over a standard waveguide band for majority of spherical near-field measurement scenarios. [1] S. Pivnenko, J.L. Besada, A. Ruiz, C. Rizzo: On the probe pattern correction in spherical near-field antenna measurements. Proc. 37th AMTA Symposium, Long Beach, CA, USA, October 2015 [2] S. Pivnenko, E. Venero, C. Rizzo: Application of single probe correction file for multi-frequency spherical near-field antenna measurements. Proc. 10th EuCAP, Davos, Switzerland, April 2016