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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.
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.
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
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.
Although In the 1970’s and 1980’s the near-field technology was proven to work properly for antenna characterization. It was until the late 1990’s that antenna communities begun to trust this technology and depend heavily on it. This same scenario could happen to the phase-less near-field technologies. It is true that there is still much to be done in the sense of reliability of these techniques. Nevertheless there are still situations where these techniques must be applied. This paper will be dealing with the phase-less near-field antenna measurement technique. The well-known iterative Fourier transformation (IFT) technique is used. The amplitude of the field distribution on concentric spheres surrounding the antenna under test (AUT) is used to reconstruct the phase information necessary for the spherical near-field to far-field transformation (SNFFF). It will be shown that despite its geometrical and computational complexity this technique can be applied on the spherical case achieving very good accuracy. In addition this paper makes use of global optimization techniques especially genetic algorithm (GA) to establish an initial estimate of the phase distribution necessary for the algorithm which is later on fine-tuned using the local optimization i.e. IFT to retrieve a closer estimate of the solution. It will be shown that except for the null positions the far-field accuracy can be enhanced. The implementation of the GA will be shortly given and the concept of masks, which simplifies the implementation, will be discussed.
Electro-optic (EO) probes provide an ultra-wideband, high-resolution, non-invasive technique for polarimetric near-field scanning of antennas and phased arrays. Unlike conventional near field scanning systems which typically involve metallic components, the small footprint all-dielectric EO probes can get extremely close to an RF device under test (DUT) without perturbing its fields. In this paper, we discuss and present measurement results for EO field mapping of a dual-band circularly polarized active phased array that operates at two different S and C bands: 2.1GHz and 4.8GHz. The array uses probe-fed, cross-shaped, patch antenna elements at the S-band and dual-slot-fed rectangular patch elements at the C-band. At each frequency band, the array works both as transmitting and receiving antennas. The antenna elements have been configured as scalable array tiles that are arranged together to create larger apertures. Near-field scan maps and far-field radiation patterns of the dual-band active phased array will be presented at the bore sight and at different scan angles and the results will be validated with simulation data and measurement results from an anechoic chamber.
This paper describes an mm-wave extrapolation range installed at KRISS, which may be used for testing standard gain antennas by using the three-antenna extrapolation technique in the frequency range from 110 GHz to 325 GHz. It consists of a precision linear slide and an mm-wave S-parameters measurement system. The precision linear slide for changing the separation distance between transmitting and receiving antennas is realized with a linear motor with 1.6 meter long on a precision stone surface plate. The mm-wave measurement system for measuring S-parameters at extrapolation antenna measurements consists of a 67 GHz vector network analyzer used as a main frame and three frequency extenders which are operating at three frequency bands (D-band (110 -170 GHz), G-band (140-220 GHz) and J-band (220-325 GHz)). The S-parameters measurement system is calibrated with TRL/LRL method. The general procedure of the extrapolation technique is as follows; 1) The effect of multiple reflections between transmitting and receiving antennas is removed from data measured at a reduced distance. 2) A polynomial is determined for curve-fitting the data removed the effect of multiple reflections. 3) Finally, far-field antenna properties are calculated from the polynomial. In this paper, a method using measured S-parameters for reducing multiple reflections between transmitting and receiving antennas is used. Power gain of D-band standard gain horn antennas is measured with the mm-wave extrapolation range. Description of detailed measurement system and measurement result will be presented at the symposium.
RCS and Antenna measurement accuracy critically depends on the quality of the incident field. Both compact and far field ranges can suffer from a variety of contaminating factors including phenomena such as atmospheric perturbation, clutter, multi-path, as well as Radio Frequency Interference (RFI). Each of these can play a role in distorting the incident field from the ideal plane wave necessary for an accurate measurement. Methods exist to mitigate or at least estimate the measurement uncertainty caused by these effects. However, many of these methods rely on knowledge of the incident field amplitude and phase over the test region. Traditionally the incident field quality is measured directly using an electromagnetic probe antenna which is scanned through the test region. Alternately, a scattering object such as a sphere or corner reflector is used and the scattered field measured as the object is moved through the field. In both cases the probe/scatterer must be mounted on a structure to move and report the position in the field. This support structure itself acts as a moving clutter source that perturbs the incident field being measured. Researchers at the Air Force Institute of Technology (AFIT) have recently investigated a concept that aims to eliminate this clutter source entirely. The idea is to leverage the advances in drone technology to create a free flying field probe that doesn’t require any support structure. We explore this concept in our paper, detailing the design, hardware, and software developments required to perform a concept demonstration measurement in AFIT’s RCS measurement facility. Measured data from several characterization tests will be presented to validate the method. The analysis will include an estimate of the applicability of the technique to a large outdoor RCS measurement facility.
Direction Finding (DF) Antennas are usually designed and tested in controlled environments. However, antenna far field response may change significantly in its operational environment. In such perturbing or not -controlled close context, the antennas calibration validity becomes a major issue which can lead to DF performance degradation and to a costly re-calibration process. Even if in-situ re-calibration is still complicated; the DF antenna response can be monitored, during the mission, in order to ensure the DOA accuracy. This paper presents an innovative design and the performance of a low-disturbing solution to detect the near field antenna response deviations from a nominal case. The proposed system is based on an array of transmitting miniature dipoles deployed all around the DF antennas. These probes are optically fed through a non-biased photodiode that carries the direct conversion into a RF signal at the desired frequency. The detection re-used the DF receiving RF chains to analyze any deviation (complex values) of the antennas array manifold. Compared to the Optically Modulated Scatterer (OMS) technique, the benefits of the proposed approach are demonstrated experimentally over a frequency decade (UHF band). First a better sensitivity is shown (higher than 80 dB on the monitored link), and secondly the phase detection is made really simple compared to the OMS technique. Finally, a relation between this in-situ diagnosis mode and the DF angular direction accuracy is established. Thus the capacity to detect, on the near field response, the presence of various types of closed obstacles (open trap on the carrier, additional antenna…) which perturb significantly the far field antenna response, is evaluated.
The radiated efficiency is a key performance indicator for multi-standards frequency agile electrically small antennas (ESA) that are mounted on wireless IoT sensors. One of the techniques to estimate it, consists to integrate, over all the angular directions, the gain measured in the far field condition. The gain-comparison method is usually implemented in the CEA LETI testbench ; which requires an accurate knowledge of the standard horn gain. The introduction of a new RF-optical link to remove coaxial cable perturbation on ESA radiation, in our test bench has raised the opportunity to proceed to an error budget analysis. This paper delivers the main results of this study where the impact of several parameters such as the optical fiber movement, the horn position, the received power level, chamber imperfection… have been evaluated. We have carried on the three antennas method (one Vivaldi and two TEM standard horns) to estimate the complex transfer function of the three antennas. The overall goal is to estimate the detailed uncertainty analysis of the ESA efficiency measurement over a large band of frequencies. This work aims to identify the most impacting effects on uncertainty and to initiate the discussion with the AMTA community how to decrease them.
A specific set-up for probe-fed antenna with an articulated arm has been developed by NSI with a 500mm AUT-probe distance. This paper will give an example of far-field measurement and highlight its limitations. A near field approach to filter the probe effect is investigated. First measurement results, including amplitude and phase, will be presented. Phase data will be leveraged to develop post-processing technique to filter probe and environmental effect.
Recently, the Rapid Scatterometer (RapidScat) instrument was developed to sense ocean winds while being housed onboard the International Space Station (ISS). This latest addition to the ISS, launched and mounted in September 2014, significantly improves the detection and sensing capabilities of the current satellite constellation. The dual-beam Ku-band reflector antenna autonomously rotates at 18 rpm and acquires scientific data over a circular scan during typical ISS operations. Mounting such an antenna on the ISS, however, gives rise to many engineering challenges. An important consideration for any antenna onboard the ISS is the interference generated towards nearby ISS systems, space vehicles and humans due to the possible exposure to high RF power. To avoid this issue, this work aimed to characterize the antenna's absolute near-field distribution, whose knowledge was required for a blanker circuit design to shut off the RF power for certain time slots during the scan period. Computation of these absolute near-fields is not a straightforward task and can require extensive computational resources. The initial computation of those fields was done using GRASP; however, an independent validation of the GRASP results was necessary because of safety concerns. A customized plane wave spectrum back projection method was developed to recover the absolute electric field magnitudes from the knowledge of the measured far-field patterns. The customized technique exploits the rapid computation of the Fast Fourier Transform alongside the proper normalization. The procedure starts by scaling the normalized (measured or simulated) far-field patterns appropriately to manifest the desired total radiated power. This was followed by transforming the vectors into the desired rectangular coordinate system and interpolating those components onto a regularized spectral grid. The FFT of the resulting Plane Wave spectrum was properly scaled using the sampling lengths to determine the absolute near-field distributions. The procedure was initially validated by comparing the results with analytical aperture distributions with known far-field patterns. The properly normalized PWS approach was subsequently applied to the RapidScat Antenna using measured patterns from JPL’s cylindrical near-field range. The resulting near-fields compare quite well between the plane wave spectrum technique and GRASP, thus validating the calculations. This work provided significant enabling guidelines for the safe operation of the ISS-RapidScat instrument.
The design of active electronically steered arrays (AESA) is a challenging, time-consuming and costly endeavor. The design process becomes much more sophisticated in the case of dual-band circularly polarized active phased arrays, in which CP radiating elements at two different frequency bands occupy a common shared aperture. A design process that takes into account various inter-element and intra-element coupling effects at different frequency bands currently relies solely on computer simulations. The conventional near-field scanning systems have serious limitations for quantifying these coupling effects mainly due to the invasive nature of their metallic probes, which indeed act as receiving antennas and have to be placed far enough from the antenna under test (AUT) to avoid perturbing the latter’s near fields. In recent years, a unique, versatile, near-field mapping/scanning technique has been introduced that circumvents most of such measurement limitations thanks to the non-invasive nature of the optical probes. This technique uses the linear Pockels effect in certain electro-optic crystals to modulate the polarization state of a propagating optical beam with the RF electric field penetrating and present inside the crystal. In this paper, we will present near-field and far-field measurement data for a dual-band circularly polarized active phased array that operates at two different S and C bands: 2.1GHz and 4.8GHz. The array uses probe-fed, cross-shaped, patch antenna elements at the S-band and dual-slot-fed rectangular patch elements at the C-band. At each frequency band, the array works both as transmitting and receiving antennas. The antenna elements have been configured as scalable array tiles that are patched together to create larger apertures.
In planar and cylindrical near-field antenna measurements the probe pattern correction is essential, since the used angular sector of the probe pattern extends over large part of the forward hemisphere. But in spherical near-field measurements, the probe is always looking towards an antenna under test (AUT) and the used angular sector of the probe pattern is relatively small: it usually does not exceed some ±30deg, but typically is much smaller, depending on the size of the AUT and the distance to the probe. For this reason, for low-directive probes with little pattern variation in the used angular sector, it is often said that the probe pattern correction can be omitted without introducing significant error in the calculated far-field AUT pattern. However, no specific guidelines on the value of the introduced error have been presented so far in the literature. In this paper, the error in the calculated far-field AUT pattern due to omitted probe pattern correction is investigated by simulations and confirmed by selected measurements. The investigation is carried out for two typical probes, an open-ended waveguide and a small conical horn, and for aperture-type AUTs of different electrical size with different distance to the probe. The obtained results allow making a justified choice on including or omitting the probe pattern correction in practical situations based on the estimated error at different levels of the AUT pattern.
This paper discusses the application of modern NF measurements and statistical analysis techniques to efficiently characterize the FF radiation pattern statistics of antennas and other EM emitters whose radiated EM fields vary erratically in a seemingly random manner. Such randomly-varying radiation has been encountered, for example, in measurements involving array antenna elements and reflector feed horn(s) containing active or passive devices that affect the relative phases and/or amplitudes of the pertinent RF signals in a non-deterministic manner [1-2]. In-Band (IB) as well as Out-Of-Band (OB) signals may be involved in some cases. Other possible randomly varying EM radiations include leakage from imperfectly-shielded equipment, connectors, cables, and waveguide runs [2- 4] Previous work at GTRI [5-7] has shown that computations of key FF radiation pattern statistics can be made based on NFFF transformations involving a) the sample average value of the complex electric field at each NF measurement point, b) the sample average value (a real number) of the standard deviation of the complex electric field at each NF measurement point, and c) the measured complex cross-covariance functions at all different NF measurement points. The key FF radiation pattern statistics of most interest are typically a) the statistical average FF radiation pattern, b) the standard deviation, c) the probability density function (p.d.f.), and d) the cumulative probability distribution (C.P.D.). Simulated data measurement protocols and the requisite statistical processing of the NF measured data will be presented and discussed in detail at the symposium. The NF cross-covariance functions introduce a new level of complexity in NF measurements and analysis that is absent for “deterministic” EM field measurements because the cross covariance functions must be measured and processed for all different NF measurement points on the NF surface to compute valid Pattern FF statistics. However, pairs of linear or circular probe arrays can be used to great advantage to achieve tolerable NF measurement times for the cross covariance functions and the aforementioned NF statistical quantities, thereby enabling valid computations of the FF pattern statistics. The use of dual probe arrays will be presented and discussed in detail and compared with mechanical scanning of two “single” probes over two NF measurement surfaces. A technique for estimating the cross-covariance functions will be presented and compared with exact values.
MIL STD 461 is the Department of Defense standard that states the requirements for the control of electromagnetic interference (EMI) in subsystems and equipment used by the armed forces. The standard requires users to measure the unintentional radiated emissions from equipment by placing a measuring antenna at one meter distance from the equipment under test (EUT). The performance of the antenna at 1m distance must be known for the antenna to measure objects located at this close proximity. MIL STD 461 requires the antennas to be calibrated at 1 m distance using the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 958. This SAE ARP 958 document describes a standard calibration method where two identical antennas are used at 1m distance to obtain the gain at 1m for each antenna. In this paper the authors show using simulations that the SAE ARP 958 approach introduces errors as high at 2 dB to the measured gain and AF. To eliminate this problem the authors introduce a new method for calibrating EMC antennas for MIL STD 461. The Method is based on the well-known extrapolation range technique. The process is to obtain the polynomial curve that is used to get the far field gain in the extrapolation gain procedure, and to perform an interpolation to get the gain at 1 m. The results show that some data in the far field must be collected during the extrapolation scan. When the polynomial is calculated the antenna performance values at shorter distances will be free of near field coupling. Measured results for a typical antenna required for emissions testing per the MIL STD 461 match well with the numerical results for the computed gain at 1 m distance. Future work is required to study the use of this technique for other short test distances used in other electromagnetic compatibility standards, such as the 3 m test distance used by the CISPR 16 standard. Keywords: Antenna Calibrations, EMC Measurements, Extrapolation Range Techniques
Spherical spiral scanning involves coordinating the motion of two simultaneous axes to accomplish near-field antenna measurements along a line on a sphere that does not cross itself. The line would ideally start near a pole and trace a path along the sphere to the other pole. An RF probe is moved along this path in order to collect RF measurements at predefined locations. The data collected from these measurements is used along with a near-field to far-field transformation algorithm to determine the radiated far-field antenna pattern. The method for transforming data collected along spherical spiral scan has been previously presented [1]. Later laboratory measurement studies have shown the validity of the spherical spiral scanning technique [2]. Here, the authors present a review of the spherical scanning technique and present recent advances and the applicability of the method to testing antennas mounted on automobiles. The method has the advantages of a reduction of the overall number of data points required in order to meet a minimum sampling requirement determined using non-redundant sampling techniques. This reduction in the number of data points and the advantage of moving two axes simultaneously result in a significant reduction in the time required to collect a set of measured data. Keywords: Spherical Near-Field, Telematics, Automotive References: [1] F. D’Agostino, C. Gennarelli, G. Riccio, and C. Savarese, “Theoretical Foundations of near-field to far-field transformations with spiral scannings,” Prog. In Electromagn. Res., vol. PIER 61, pp 193-214, 2006. [2] F. D’Agostino, F. Ferrara, J. Fordham, C. Gennarelli, R. Guerriero, and M. Migliozzi, “An Experimental Validation of the Near-Field to Far-Field Transformation with Spherical Spiral Scan,” Proc. Of the Antenna Measurement Techniques Association, 2012.
We previously demonstrated that 60 GHz planar near-field antenna measurements without external frequency conversion can provide far-field radiation patterns in good agreement with spherical near-field antenna measurements in spite of the cable flexing and thermal drift effects [P.I.Popa, S.Pivnenko, J.M.Nielsen, O.Breinbjerg, ”60 GHz Antenna Measurement Setup Using a VNA without External Frequency Conversion ”36thAnnual Meeting and Symposium of the Antenna Measurements Techniques Association, 12-17 October, 2014]. In this work we extend the validation of this 60 GHz planar near-field set-up to antenna diagnostics and perform a detailed systematic study of the extreme near-field of a standard gain horn at 60 GHz from planar and spherical near-field measurement data. The magnitude and phase of all three rectangular components of the electric and the magnetic aperture fields are calculated, as is the main component of the Poynting vector showing the power flow over the aperture. While the magnitude of the co-polar electric field may seem the obvious object for antenna diagnostics, we demonstrate that there is much additional information in those additional quantities that combine to give the full picture of the aperture field. The usefulness of the complete information is illustrated with an example where the horn aperture is disturbed by a fault. We compare the results of the planar and spherical near-field measurements to each other and to simulation results.
As millimeter-wave applications become more widely available technologies, there is a demand to know material properties for design and application purposes. However, many mass produced materials are either not specified at these frequencies or the price materials can be costly. Therefore the easiest method for characterization is by measurement. Traditional methods of this measurement type involve the reflectivity of a fabric sample placed on a flat metallic reference plate. However, this method has some major difficulties at these high frequencies. For example, the surface of the reference plate must be very flat and smooth and must be carefully oriented such that their surface is precisely facing the transmitting and receive and antennas. Furthermore the electrically large size of the reference plate of this setup makes it difficult to measure in far-field and anechoic range time is expensive. Resistive and conductive fabrics have applications such as shielding, anti-static, and radio wave absorption. Radio wave absorption and radar cross section engineering is currently of high interest to the automotive industry for testing newly emerging automotive radar systems. Such fabric measurement has already been utilized to accurately characterize artificial skin for radar mannequins to recreate the backscattering of human targets at 77 GHz. This paper presents a new and convenient method for measuring the reflective properties of conductive and resistive materials at millimeter wave frequencies by wrapping fabrics around a metallic reference cylinder. This new approach to fabric characterization method is able to obtain higher accuracy and repeatability despite the difficulties of measuring at high frequency.
During the last years, new algorithms, based on time filtering, spatial or modal filtering, have been designed for echo reduction techniques applied to antenna measurements. These algorithms have been used for different applications where the effect of the echoes is important, as far field system, VHF or UHF applications, automotive systems, small antennas, etc. The authors, in previous papers, have analysed the effect of different algorithms: time filtering (fft, non uniform dft or matrix pencial), modal filtering based on Spherical modes (MV-Echo) and spatial filtering based on Integral Equations (Insight) and holographic techniques (fft and dft) to cancel the effect of the reflections. This comparison has been applied to the measurements of a dipole antenna (SD1900) using a StarLab system. It is observed that each of the algorithms is better for different situations, depending on the source of the echo. For instance, time filtering techniques are good for reflections coming from different distances with respect the direct ray, but not so good for close reflections. In addition hey need a large frequency band to work properly. Spatial algorithms can correct the effect of positioners or other structures close to the antenna under test, but they are better for planar near field acquisitions and worse for classical single probe spherical near field where the antenna is rotated and probe is fixed (e.g. roll-over-azimuths systems). Moreover, they require extra information of the AUT geometry. This paper presents first a comparison of each algorithm and then, a combination of time and spatial techniques based on uniform or non-uniform DFT to take advantage of the benefits of each algorithm for different origins of the reflections.