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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.
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
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.
Precise determination of antenna phase centers is crucial to reduce the uncertainty in gain when employing the three-antenna method, particularly operated over a short range-such as a 3-m radio anechoic chamber, where the distance between the phase centers and the open ends of an aperture antenna (the most commonly-used reference) is not negligible, compared with the propagation distance. An automatic system to determine the phase centers of aperture antennas in a radio anechoic chamber has been developed and the absolute gain of horn antennas have been thereby evaluated with the three-antenna method. The phase center of an X-band horn was found to migrate up to 55 mm from the open end. Uncertainties in the gain were evaluated in accordance with ISO/IEC Guide 93-3: 2008. The 95% confidence interval of the horn antenna gain was reduced from 0.39 to 0.25 dB, when using the phase center location instead of the open end. Then the gains, polarization, and radiation pattern of space-borne antennas were measured: low-, medium-, and high-gain X-band antennas for an ultra small deep space probe employing the polarization pattern method with use of the horn antenna. Comparison between the radiation properties with and without the effect of spacecraft bus was carried out for low-gain antennas. The 95% confidence interval in the antenna gain decreased from 0.60 to 0.39 dB.
The automotive industry is changing rapidly through the evolution of on board and embedded components and systems. Many of these systems rely on the over the air performance of a communication link and the evaluation of these links is a key requirement in understanding both the real world performance, and associated operating limits of a particular system. The operating frequency range of the installed communication systems now extends from the traditional AM bands from 540KHz to almost 6GHz for WiFi. There are several different antenna design options available to cover this range and in many cases, the performance of an antenna when installed on a vehicle differs from a measurement of the same antenna in isolation. There is also a growing use of high frequency RADAR systems operating at frequencies approaching 80GHz that also need to be included in the performance analysis. The behavior of the individual components using conducted methods for example, is an important step but the direct measurement of antenna pattern and data throughput under ideal steady state and also varying spatial and operating conditions is likely to be the most robust method of channel evaluation. There is a steady march toward vehicle autonomy that is pushing the development of increasingly complex and sophisticated sensors, receivers, transmitters and firmware, all installed on an already well populated platform. The interoperability and EMC performance of these embedded systems is an extension to the need for a fundamental understanding of performance. This paper will present some of the available measurement and evaluation options that could be used as part of an integrated test environment which takes advantages of a number of established techniques.
As a result of recent technical and regulatory developments, the millimeter wave frequency band (30GHz – 300 GHz) is being adopted for wide range of applications. Based on array signal processing technologies used for 4G and MIMO, companies are developing small active array antennas operating throughout the millimeter-wave bands. These arrays may include radiating elements and feed structures that are fraction of a millimeter in size and cannot be fed via a coaxial cable. Connection to the antenna is instead performed through a micro-probe more commonly used in the chip industry. MVG-Orbit/FR has developed a compact antenna measurement system which integrates hardware and software necessary to provide antenna gain and radiation patterns of antennas fed with such a micro-probe. To evaluate uncertainties in the measurements of the Antenna Under Test (AUT) gain, directivity, efficiency, pattern, or VSWR, reference antennas are an invaluable tool. The authors have recently driven the development of a micro-probed chip reference antenna. This reference antenna was designed to be mechanically and electrically stable and with reduced sensitive to its mounting fixture and feeding method. Close agreement between measured and simulated characteristics has been achieved. With low losses, the antenna provides good dynamic range and confidence in the measured antenna efficiency and gain. Without chip antenna gain standards, a micro-probed antenna test system requires the use of the insertion loss method for gain calibration. This method requires correction for additional losses such as cables, attenuators, or adaptors that are included in the calibration but not in the subsequent measurement of the AUT. In addition, the micro-probe (which is in the measurement but not in the calibration) should be calibrated and de-embedded from the measurement. Each of these measurements and associated connections and related processing, increases uncertainty and chance of mistakes by the user. It is therefore essential to validate the calibration using a well characterized reference antenna. This paper will outline design requirements and present test results of 60 GHz Chip Reference antennas. Several dozen antennas have been tested. The related uncertainties in the micro-probed antenna measurements will be evaluated with particular emphasis on the gain calibration uncertainty. The paper will also describe the next steps towards developing a chip antenna gain standard, that should reduce gain uncertainties while also significantly simplifying the calibration process.
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.
Current and future beam-steerable phased array antennas require broadband frequency range, multiple-beam operation and fast switching speeds. Near-field antenna testing is an efficient tool to rapidly and securely test antenna array performance, but many of the features of current and next-generation electronically steerable antennas require advances in near-field techniques so as to fully characterize and optimize advanced antenna arrays. For example, system requirements often dictate broadband frequency operation, which presents challenges in terms of probe antenna choices and probe-to-test antenna distances to properly characterize and optimize test antennas with a minimal number of scans and thus satisfy stringent customer requirements in a cost- and time-effective manner. Raytheon is developing near-field antenna test measurement techniques tailored to measure the radiation patterns of multiple beams over wide frequency ranges, to expand the range of test data collected for antenna optimization and customer review. Key test design considerations faced in developing advanced near field characterization techniques will be presented. Custom software to integrate antenna control with the near field measurement system is necessary to provide enhanced capability of characterizing multiple beams. Specialized data processing and analysis tools are needed to process thousands of datasets collected from a single scan, in a timely manner.
This paper summarizes the design, fabrication and characterization of a coplanar waveguide fed modified aperture bowtie antenna operating in the 60 to 90 GHz range. Modifications to the bowtie edges extend the bandwidth up to 40% without increasing radiator area. The antenna was initially designed and measured in the 3-8 GHz frequency band and then frequency scaled to 60-90 GHz. The millimeter wave antenna is implemented on FR408 (er=3.65) and a multilayer laminate. Both substrates can be used in millimeter-wave system design where efficient antennas are needed. Return loss measurements of the antennas are made on a Cascade probe station. The results agree well with simulations in ANSYS HFSS. Until recently, only simulated radiation patterns were available illustrating broadside gain of 5 to 7 dB for these antennas. With the acquisition of a spherical scanner, near-field measurements have been taken of the three antennas from 67 to 110 GHz. The broadside radiation pattern results are compared with simulation. The NSI 700S-360 spherical near-field measurement system used in conjunction with an Agilent network analyzer, GGB Picoprobes and Cascade manipulator allow for on-wafer measurements of the antenna under test.
In the late 1990’s, Maloney et al. began investigating the design of highly pixelated apertures whose physical shape and size are optimized using genetic algorithms (GA) and full-wave computational electromagnetic simulation tools (i.e. FDTD) to best meet the required antenna performance specification; i.e. gain, bandwidth, polarization, pattern, etc. [1-3]. Visual inspection of the optimal designs showed that the metallic pixels formed many connected and disconnected fragments. Hence, they coined the term Fragmented Aperture Antennas for this new class of antennas. A detailed description of the Georgia Tech design approach is disclosed in [4]. Since then, other research groups have been successfully designing fragmented aperture antennas for other applications, see [5-6] for two examples. However, the original fragmented design approach suffers from two major deficiencies. First, the placement of pixels on a generalized, rectilinear grid leads to the problem of diagonal touching. That is, pixels that touch diagonally lead to poor measurement/model agreement. Other research groups are also grappling with this diagonal touching issue [7]. Second, the convergence in the GA stage of the design process is poor for high pixel count apertures (>>100). This paper will present solutions to both of these shortcomings. First, alternate approaches to the discretization of the aperture area that inherently avoid diagonal touching will be presented. Second, an improvement to the usual GA mutation step that improves convergence for large pixel count fragmented aperture designs will be presented. Over the last few years, the authors have been involved with developing the use of the focused beam measurement system to measure antenna properties such as gain and pattern [8]. A series of improved, fragmented aperture antenna designs will be measured with the Compass Tech Focused Beam System and compared with the design predictions to validate the designs. References: [1] J. G. Maloney, M. P. Kesler, P. H. Harms, T. L. Fountain and G. S. Smith, “The fragmented aperture antenna: FDTD analysis and measurement”, Proc. ICAP/JINA Conf. Antennas and Propagation, 2000, pg. 93. [2] J. G. Maloney, M. P. Kesler, L. M. Lust, L. N. Pringle, T. L. Fountain, and P. H. Harms, “Switched Fragmented Aperture Antennas”, in Proc. 2000 IEEE Antennas and Propagations Symposium, Salt Lake City, 2000, pp. 310-313. [3] P. Friederich, L. Pringle, L. Fountain, P. Harms, D. Denison, E. Kuster, S. Blalock, G. Smith, J. Maloney and M. Kesler, “A new class of broadband planar apertures,” Proc. 2001 Antenna Applications Symp, Sep 19, 2001, pp. 561-587. [4] J. G. Maloney, M. P. Kesler, P. H. Harms and G. S. Smith, “Fragmented aperture antennas and broadband antenna ground planes,” U. S. Patent # 6323809, Nov 27, 2001. [5] N. Herscovici, J. Ginn, T. Donisi, B. Tomasic, “A fragmented aperture-coupled microstrip antenna,” Proc. 2008 Antennas and Propagation Symp, July 2008, pp. 1-4. [6] B. Thors, H. Steyskal, H. Holter, “Broad-band fragmented aperture phased array element design using genetic algorithms,” IEEE Trans. Antennas Propagation, Vol. 53.10, 2005, pp. 3280-3287. [7] A. Ellgardt, P. Persson, “Characteristics of a broad-band wide-scan fragmented aperture phased array antenna”, EuCAP 2006, Nov 2006, pp. 1-5. [8] J. Maloney, J. Fraley, M. Habib, J. Schultz, K. C. Maloney, “Focused Beam Measurement of Antenna Gain Patterns”, AMTA, 2012
Coupling a Chip Antenna to an Antenna Measurement System is typically achieved using a co-planar micro-probe. This micro-probe is attached to a probe positioner that is used to maneuver the micro-probe into position and land it on the chip. Through this process, the chip is held by a chuck. Intentional and unintentional radiation from the Chip Antenna will interact with the micro-probe and chuck. From design conception, the antenna designer must take steps to reduce currents on the chip surface to minimize unintended radiation that will interact with both the measurement setup and the surrounding components of the final design. Even with good design practices, residual currents will still remain and radiate from the chip. Combined with intentional radiation from the chip antenna in the upper hemisphere, these radiated fields will impinge on the micro-probe and the probe positioner. Reflections from both the micro-probe and its positioner will reflect and generate interference patterns with the desired signal in the spherical measurement probe. In this paper, we evaluate, to first order, these effects by experimentation on two types of micro-probes (ACP & Infinity). The residual errors are then evaluated using modal filtering tools that further reduce these effects and the results are presented. Finally the dielectric chuck is modeled in simulation to evaluate the effects of the chuck on antenna patterns at 60 GHz and the results are presented.
ABSTRACT Spherical near-field error analysis is extremely useful in allowing engineers to attain high confidence in antenna measurement results. NSI has authored numerous papers on automated error analysis and spherical geometry choice related to near field measurement results. Prior work primarily relied on comparison of processed results from two different spherical geometries: Theta-Phi (0 =?= 180, -180 = f = 180) and Azimuth-Phi (-180 =?= 180, 0 = f = 180). Both datasets place the probe at appropriate points about the antenna to measure two different full spheres of data; however probe-to-antenna orientation differs in the two cases. In particular, geometry relative to chamber walls is different and can be used to provide insight into scattering and its reduction. When a single measurement is made which allows both axes to rotate by 360 degrees both spheres are acquired in the same measurement (redundant). They can then be extracted separately in post-processing. In actual fact, once a redundant measurement is made, there are not just two different full spheres that can be extracted, but a continuum of different (though overlapping) spherical datasets that can be derived from the single measurement. For example, if the spherical sample density in Phi is 5 degrees, one can select 72 different full sphere datasets by shifting the start of the dataset in increments of 5 degrees and extracting the corresponding single-sphere subset. These spherical subsets can then be processed and compared to help evaluate system errors by observing the variation in gain, sidelobe, cross pol, etc. with the different subset selections. This paper will show the usefulness of this technique along with a number of real world examples in spherical near field chambers. Inspection of the results can be instructive in some cases to allow selection of the appropriate spherical subset that gives the best antenna pattern accuracy while avoiding the corrupting influence of certain chamber artifacts like lights, doors, positioner supports, etc. Keywords: Spherical Near-Field, Reflection Suppression, Scattering, MARS. REFERENCES Newell, A.C., "The effect of measurement geometry on alignment errors in spherical near-field measurements", AMTA 21st Annual Meeting & Symposium, Monterey, California, Oct. 1999. G. Hindman, A. Newell, “Spherical Near-Field Self-Comparison Measurements”, Proc. Antenna Measurement Techniques Association (AMTA) Annual Symp., 2004. G. Hindman, A. Newell, “Simplified Spherical Near-Field Accuracy Assessment”, Proc. Antenna Measurement Techniques Association (AMTA) Annual Symp., 2006. G. Hindman & A. Newell, “Mathematical Absorber Reflection Suppression (MARS) for Anechoic Chamber Evaluation and Improvement”, Proc. Antenna Measurement Techniques Association (AMTA) Annual Symp., 2008. Pelland, Ethier, Janse van Rensburg, McNamara, Shafai, Mishra, “Towards Routine Automated Error Assessment in Antenna Spherical Near-Field Measurements”, The Fourth European Conference on Antennas and Propagation (EuCAP 2010) Pelland, Hindman, “Advances in Automated Error Assessment of Spherical Near-Field Antenna Measurements”, The 7th European Conference on Antennas and Propagation (EuCAP 2013)
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.
Methods for determining the uncertainty in antenna measurements have been previously developed and presented. The IEEE recently published a document [1] that formalizes a methodology for uncertainty analysis of near-field antenna measurements. In contrast, approaches to uncertainty analysis for antenna measurements on a compact range are not covered as well in the literature. Unique features of the compact range measurement technique require a comprehensive approach for uncertainty estimation for the compact range environment. The primary difference between the uncertainty analyses developed for near-field antenna measurements and an uncertainty analysis for a compact range antenna measurement lies in the quality of the incident plane wave illuminating the antenna under test from the compact range reflector. The incident plane wave is non-ideal in amplitude, phase and polarization. The impact of compact range error sources on measurement accuracy has been studied [2,3] and error models have been developed [4,5] to investigate the correlation between incident plane wave quality and the resulting measurement uncertainty. We review and discuss the terms that affect gain and sidelobe uncertainty and present a framework for assessing the uncertainty in compact range antenna measurements including effects of the non-ideal properties of the incident plane wave. An example uncertainty analysis is presented. Keywords: Compact Range, Antenna Measurement Uncertainty, Error Analysis References: 1. IEEE Standard 1720-2012 Recommended Practices for Near-Field Antenna Measurements. 2. Bingh,S.B., et al, “Error Sources in Compact Test Range”, Proceedings of the International Conference on Antenna Technologies ICAT 2005. 3. Bennett, J.C., Farhat, K.S., “Wavefront Quality in Antenna Pattern Measurement: the use of residuals.”, IEEE Proceedings Vol. 134, Pt. H, No. 1, February 1987. 4. Boumans, M., “Compact Range Antenna Measurement Error Model”, Antenna Measurement Techniques Association 1996 5. Wayne, D., Fordham, J.A, Mckenna, J., “Effects of a Non-Ideal Plane Wave on Compact Range Measurements”, Antenna Measurement Techniques Association 2014
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.
Antenna measurements at increasing working frequencies carry the difficulty of reliably measuring the signal phase, due to effects of cable bending, thermal drift, etc, and the resulting impedance mismatch which introduces uncertainty in the measurement results. In this paper we investigate the problem of phaseless measurements and phase retrieval for planar near-field measurements, together with the application of probe correction of the retrieved results, to the best of our knowledge the first experimental case of probe correction in phaseless near-field antenna measurements. A phase retrieval method based on an iterative Fourier technique (IFT) is proposed and tested with measurements of a Standard Gain Horn at 60GHz acquired at the planar near-field (PNF) scanner facility at the Technical University of Denmark. The obtained results indicate good agreement with a measured reference pattern within the region of validity when the probe correction is applied after performing the phase retrieval from a pair of uncorrected probe signals. Application of the probe correction before the phase retrieval, on the other hand, shows not satisfactory results. Additional improvements are obtained by introducing spatial filtering at the AUT aperture, thus enhancing performance of the algorithm by reducing phase noise of retrieved fields. Also, a “double-iterated” approach is explored, with additional phase-retrieval iterations after probe correction, with the aim of introducing true electric fields into the IFT.
This paper presents a method for determining the phase center of an antenna based on pattern measurements made over multiple frequencies with a two axis spherical positioning system. Mathematical calculations are used to determine the best-fit sphere for the measured phase data. The origin of the sphere is the phase center of the antenna at the frequency of interest. This method provides fast, flexible multi-frequency measurement of an antenna’s phase center. Results validating the proposed method are presented using both simulated data and measured data. In order to determine the accuracy of this method, a dipole is translated a precise distance within the measurement coordinate system. The difference between phase center measurements made before and after the translation gives an indication of the potential accuracy of the measurements. Also, major contributors to phase center measurement uncertainty are discussed with consideration to reducing the overall phase center measurement uncertainty.