Jeff Fordham, Lars J. Foged, Vicente Rodriguez, Justin Dobbins, Vikass Monebhurrun
The revised version of IEEE Standard 149 is finally available as IEEE Standard 149-2021, approved by the IEEE Standards Association Standards Board (SASB) in September 2021 and effectively published in February 2022. This is the first in-depth revision of the standard since 1979. Rewritten as a recommended practice, the standard is an essential desk reference for anyone involved in antenna measurements. While most instructions on antenna engineering are centered on the designs, types, and numerical methods applied to antennas, the standard focuses on the art of measuring antennas and understanding the best approach for measuring different parameters of antennas. With almost 200 bibliographical references, the standard provides guidance on the methods and approaches to measure realistic implementations of antennas or points the reader to relevant sources of information. No antenna engineer is completely prepared without an understanding of the proper approach to perform antenna measurements and how to estimate the uncertainty of an antenna measurement.
Over the years, the authors, who have been involved in aspects of antenna engineering ranging across areas such as the design of antennas, measurements of complex active antennas, and the design of antenna measurement facilities and systems, have noticed that instruction at the university level is lacking in the antenna measurement area. This is understandable, given the cost associated with constructing a proper laboratory to perform antenna measurements. In addition, common textbooks on antennas dedicate only a minor part to the topic of antenna measurements. Kraus, out of 890 pages, has a 40-page chapter on antenna measurements [1], that is, 4.5% of the book. Balanis dedicates a 44-page chapter of the 925 pages of his book, or 4.7% [2]. Both Kraus and Balanis locate the measurements chapter as the last chapter of their books. Stutzman and Thiele place the chapter that covers measurements before the chapters on numerical methods [3]. Their chapter, which also covers antennas in systems, consists of 31 pages out of the total 643 pages of the book, or 4.8%. The authors regularly observe a lack of understanding on how to perform accurate antenna measurements among a vast majority of antenna engineers graduating from different universities. It is very common to see papers at IEEE Antennas and Propagation Society (AP-S)-sponsored symposia where the presenter essentially compares numerical results to measurements and de facto dismisses the eventual deviations as “manufacturing tolerances” without providing clear information about important measurement details. For example: How was the antenna mounted? How were the cables handled? What was the reflectivity of the range, and how did the stray energy affect the measured pattern features? What was the test distance? How was the gain measured?
IEEE Standard 149-2021 can help technicians and engineers as well as students and Young Professionals understand how to perform low-uncertainty antenna measurements. The members of the IEEE Std 149 Working Group (WG) spent more than eight years revising the previous document—updating the existing content and adding new material. The WG comprised 63 experts on antenna metrology. They dedicated their volunteer time and expertise to produce a document that, while not perfect and potentially not complete, is a repository of knowledge and references related to antenna measurements. In this article, the authors provide a clause-by-clause summary of the contents of the standard, showing the reader why it is a must-have anyone involved with antenna measurements.
In the SASB nomenclature the documents prepared (whether mandatory standards, recommended practices, or guides) follow clauses, as opposed to chapters. In the present section, the authors describe the clauses of the standard and provide information to the reader as to what information the clauses contain and their importance. The changes to the document compared to the original 1979 [4] version have been described in several papers presented at conferences that provided updates on the progress by the WG [5], [6]. In these works, it was mentioned that the main and most significant change to IEEE Standard 149 is the conversion of the standard to a recommended practice. Following discussions within the WG, and to account for the wide range of existing antenna measurement facilities, it was more appropriate to provide recommendations (i.e., the use of “should”) for good measurement practices and describe the variety of available techniques and technologies, rather than to impose mandatory requirements (i.e., the use of “shall”). The document is very useful, both for those designing and evaluating antenna test facilities and for those performing antenna measurements. The revised document retains the IEEE Standard 149 identifier but is now titled IEEE Recommended Practice for Antenna Measurements [7], as shown in Figure 1.
Figure 1. IEEE Standard 149 front cover.
These clauses can be described as housekeeping clauses. They are mandated by the SASB procedures; e.g., they provide information, such as the scope of the standard. One important point is that the standard is centered on passive reciprocal linear devices. However, the techniques described can be used in the testing of additional devices, e.g., active phased arrays, in many cases. Indeed, the standard has an informative annex (Annex D) that describes the over-the-air (OTA) measurement of integrated devices, such as mobile phones. These first clauses also provide definitions for some relevant terms and provide a list of other normative references that are indispensable for the document, such as the IEEE Standard for Definition of Terms for Antennas [8]. Common acronyms are also listed in these clauses. An important point mentioned in Clause 1 is the usage of the time convention. While the standard uses the IEEE-compliant ${e}^{\left({ + {j}{\omega}{t}}\right)}$ time convention, many of the references provided use the physics preference of ${e}^{\left({{-}{i}{\omega}{t}}\right)}$. Guidance is given on how to avoid potential errors from using equations derived using the physics convention instead of the IEEE convention.
Clause 4 deals with the antenna range design, including the selection of the best antenna measurement range for different types of antennas. The WG members considered it important to point out that there is not a single antenna measurement solution that fits all problems. Antennas come in such a variety of designs that not every antenna range is suitable to measure a given antenna. It is desirable to be in the far field of an antenna when measuring its radiation performance. However, a mobile phone-sized antenna having a maximum aperture dimension of about 17 cm, but operating at 22 GHz, requires a test distance in excess of 4.24 m as per the well-known equation for the lower limit distance, R, of the far field given by \[{R}\geq\frac{2{D}^{2}}{\lambda} \tag{1} \] where D is the maximum dimension of the antenna and m is the wavelength.
The clause starts by giving general guidance on what an antenna range should provide, e.g., type of illumination, levels of amplitude taper and ripple, and common errors due to phase variation across the aperture. The clause then provides guidelines for designing outdoor ranges, both elevated and ground reflection ranges. The discussion on ground reflection ranges should be of interest to antenna engineers working in the automobile industry, where some of the antenna systems work at frequencies such that the vehicle must be measured outdoors with the vehicle placed on a metallic turntable. One of the additions to the document compared to the 1979 edition is the discussion on the design of indoor ranges (see Figure 2). Both rectangular far-field anechoic chambers and tapered anechoic chambers are described. It should be noted that ranges for near-to-far-field transformation measurements are not discussed here but in the companion document IEEE Standard 1720-2012 [9], which addresses near-field antenna measurement and is currently under revision. The guidelines for a reflector-based compact range anechoic room are also given in this clause [see Figure 4(a)].
Figure 2. (a) A compact range, (b) a rectangular far-field chamber, and (c) a tapered chamber.
Reverberation chambers are introduced in Clause 4 as an alternative methodology for measuring the efficiency of antennas. An equation is provided for the minimum volume of the chamber to satisfy the criteria of supporting at least 60 resonant modes at the lowest frequency of operation.
Clause 5 discusses all instrumentation in antenna measurements, to include the radio-frequency subsystem as well as the positioning subsystem, which employs electronic control of mechanical positioners to capture spatial radiation patterns. The clause provides guidance and recommendation on the following topics: range antennas, the transmit subsystem, the receive subsystem, the positioning subsystem, and the workstation/software. Topics such as frequency multipliers, the use of remote mixers, dynamic range, amplifiers, and choice of cables are discussed. A discussion of the typical antenna positioning geometries, such as azimuth over elevation, elevation over azimuth, and roll over azimuth, is provided in this clause. The main software functions used on the workstation to control the test are also discussed.
Clause 6 closes the trio of clauses related to the environment and equipment used to measure an antenna under test (AUT). Clause 6 provides recommendations for the evaluation of the range used to perform the measurement. The concept of a quiet zone (QZ) is revisited, and techniques to assess the purity of the QZ are discussed. Methods for measuring the quality of the plane wave incident onto the AUT are provided. An updated discussion of the well-known free space voltage standing-wave ratio method for evaluating the reflectivity in the QZ is also provided. This is probably the first detailed discussion of the method in a standards document since its first description in 1963 [10]. A new addition to the standard is the use of Fourier analysis of QZ performance for chamber validation and diagnostics. In this technique, a plane in the QZ is scanned while sampling the magnitude and phase of the field, a common practice for QZ evaluation. But in this case, a Fourier transform of the data is performed to decompose the measurement into a series of plane waves arriving from different directions. This alternative analysis method can be used to diagnose problems in the range by pinpointing the origin of reflected energy from the lateral surfaces or other features in the range.
While Clauses 4 through 6 comprise a trio related to the tools to perform measurements, Clauses 7 through 11 provide information on measuring specific parameters of an AUT.
This clause provides a series of definitions and guidelines. Among other things, it provides a more precise definition of the far-field condition and the lower limit of the far field depending on the antenna electrical size (see Figure 3). While most people are familiar with (1), it is not recommended that this equation be applied for AUTs that are less than four or five wavelengths in any cross-sectional dimension of the aperture.
Figure 3. The lower limit of the far field depending on the antenna electrical size.
The clause discusses configurations for measuring the amplitude pattern as well as the approach for measuring the phase pattern and the phase center of an AUT.
Clause 8 is probably the most broadly applicable section of the standard, providing information on the measurement of directivity, gain (as defined by the IEEE), and realized gain (as it is often measured) of an AUT. The clause also discusses the antenna factor, a parameter used in electromagnetic compatibility measurements but rarely mentioned in antenna textbooks. Direct methods for measuring the AUT gain, such as the three-antenna method and the extrapolation range method, are covered. These techniques are especially applicable to calibrating gain standards since several measurement methods use gain standards to assess the gain of an AUT. A discussion on potential sources of error on antenna calibrations is provided in the clause.
Clause 9 deals with measurements of the polarization of antennas. After a series of definitions of different parameters related to antenna polarization, the standard provides descriptions and guidance on how to measure and calculate the polarization of antennas. This is one type of measurement where it is important to pay attention to the time convention used in deriving the equations used to obtain the different parameters. Using the wrong time convention will change the sense of the reported polarization.
Clause 10 describes methods for measuring radiation efficiency. In addition to the well-known Wheeler cap technique, the standard introduces the use of reverberation chambers to measure efficiency. Two approaches are provided: one that uses a reference to perform the efficiency measurements and one that incorporates a rotation of three antennas to evaluate the efficiency of each antenna. The pattern integration method is also discussed in the clause.
Clause 11 deals with the measurement of impedance of antennas. To support this clause, there is an informative annex in the standard (Annex F) with additional theory and detail. The clause also describes sources of error and uncertainty. Clause 11 concludes the set of clauses related to measurements of specific antenna parameters.
Clause 12, while not specific to antenna parameters, provides information on specific strategies related to antenna measurements. These strategies include the use of scaled models as well as techniques to generate localized plane waves to measure antennas at shorter distances (including, but not limited to, the compact range reflector). Clause 12 briefly describes the concept of near-to-far-field measurements, which are the topic of [9]. The clause also discusses methods for measuring angle-tracking antennas and gain over temperature for receiving antenna systems that include active electronics. This last topic is a slight deviation from the stated scope of the standard, but the WG agreed that it was important to add the discussion because of the prevalence of active antenna systems.
Clause 13 has been described as the most important addition in the standard [5], [6]. Indeed, a measurement is meaningless without an uncertainty associated with it (and the specification of its units). The clause follows the ISO/IEC Guide to the Expression of Uncertainty in Measurement [11] as directed by the SASB. The standard provides guidance on how to set up an uncertainty analysis. The clause uses the compact range as an example of how to perform an uncertainty analysis. Uncertainty is one of the topics of antenna measurements that are not mentioned in [1], [2], or [3] and yet are of the utmost importance. This makes IEEE Standard 149-2021 a must-have document for all members of the AP-S.
The three final clauses of the standard relate to the daily operation of an antenna range. Clause 14 provides guidance on how to manage a test range, covering topics such as record keeping, periodic calibrations, technician training, and scheduling of testing. Related to the use of the range and its operation, Clause 15 reminds the reader about exposure to electromagnetic fields. While the IEEE document related to this topic is IEEE Standard C95.1-2019 [12], Clause 15 presents two plots that can be easily referenced to check the exposure limits for test personnel. The final clause, Clause 16, reminds the reader about radome effects and the effects of temperature and moisture on antenna measurements and includes considerations for testing antennas that will operate in extreme environments.
As mentioned previously, the standard is an invaluable desk reference for the antenna engineer. It contains a large amount of information that is concentrated in a single, organized document. The included annexes supplement the main document with information to help readers explore further.
An important annex is Annex A, which contains the bibliography. A total of 193 references is provided to direct readers to more depth into some of the topics covered by the standard. For example, [1], [2], and [3] are listed in the bibliography as sources for some of the included topics in the standard.
Annex B discusses the different field regions, that is, the reactive near field, the radiated near field, and the far field. Annex C provides an informative background on the topic of reciprocity. Annex D, as mentioned previously, touches on OTA testing, an area of importance in the mobile phone industry but also now in a wide variety of areas, given the spread of wireless communications.
Annex E provides guidance on boresighting antennas and finding the peak of the pattern, which may not coincide with the mechanical boresight.
The last informative annex, Annex F, provides additional information about impedance measurements as a supplement to Clause 11.
The standard is a repository of knowledge from recognized industry leaders. Armed with it, the antenna engineer can assess different facilities available to perform antenna measurements and ensure that the measurements will have an acceptable level of uncertainty. The authors performed an Internet search of companies that provide antenna pattern measurement services. It was interesting to observe that none of the companies found provide information regarding the reflectivity of the QZ or the test distance. Typically, they mention the frequency range and the largest antenna that can be tested. Most of these facilities have earned an accreditation from one of the laboratory certification agencies, such as the American Association for Laboratory Accreditation (A2LA) or the National Voluntary Laboratory Accreditation Program (NVLAP), signifying compliance with ISO 17025 [13]. Achieving this compliance requires a documented uncertainty analysis. While the companies may be willing to share their uncertainty analyses, an antenna engineer who has read the standard would recognize that many of the uncertainty terms are dependent on the AUT and must be assessed individually.
As an example, consider an antenna engineer who needs to measure the radiation patterns of an antenna with a 20-cm by 20-cm aperture operating at 18 GHz. One available facility claims a frequency range from 300 MHz to 40 GHz for antennas up to 61 cm by 61 cm in a fully anechoic rectangular chamber. Since the antenna is 12m by 12m, the diagonal length is about 17m. Clause 7 indicates that, for this example scenario, the test distance should be at least 9.62 m. The antenna engineer could make a simple inquiry to the provider to determine whether the facility meets this requirement and, regardless of the answer, use the information in Clause 4 to estimate the effects of both amplitude and phase taper at finite measurement distances.
As mentioned previously, antenna pattern measurement service providers do not typically mention the reflectivity level of their ranges. This is a critical term in the uncertainty assessment since the level of reflected energy in the QZ induces an error in the pattern measurement. Clause 6 provides metrics and methods for qualifying an antenna pattern measurement range, and Clause 4 provides guidance for estimating the reflectivity based on the chamber dimensions and absorber treatment. Clause 4 also provides a convenient method to bound the pattern measurement uncertainty due to reflections. To continue our example, let us assume that the range has a QZ reflectivity level of –40 dB. If the AUT has sidelobe levels of –15 dB, then potentially the ratio between the reflected signal observed by the AUT main beam and the –15-dB sidelobe of interest will be –25 dB. Clause 4 shows that the error bound in this scenario is ±0.5 dB. A –25-dB feature in the pattern can potentially result in an error bound of approximately ±1.5 dB. Figure 4, found in Clause 4, shows the error for in-phase and out-of-phase interference between the reflected and direct signals, with our two example scenarios highlighted with green arrows.
Figure 4. In-phase and out-of-phase errors for different reflected-to-direct signal ratios in the antenna range. The solid green lines show the error range for −25 dB and the dashed green lines show the error for −15 dB.
It should be noted that we have only discussed two sources of uncertainty in this article: finite range length and multipath. Clause 13 provides a list of 15 potential error terms to consider along with a specific example of a 15-term uncertainty budget for a compact range. All members of our community, including the engineers and the service providers, now have an updated source of information for assessing uncertainties (Clause 13) and qualifying a range (Clause 6).
IEEE Standard 149-2021 has been published as of 18 February 2022 [7]. This document is a recommended practice with the title IEEE Recommended Practice for Antenna Measurements. This document, while not intended to be a textbook on antenna measurements, fills a gap in the available bibliography for antenna measurements. It provides an overview of antenna measurements that makes this standard a must-have document for any antenna engineer who must oversee the creation of a new antenna design, from concept and model in a software package to final deliverable product. The standard has been significantly revised to reflect modern measurement practices and provide general guidance for improving measurement quality while considering associated uncertainties. The changes make the document an essential reference for anyone involved, not only in antenna measurements, but in antenna engineering in general. Any member of the AP-S should own or have access to a copy of this standard document.
Jeff Fordham (jeff.fordham@ametek.com) is with NSI-MI Technologies, Suwanee, GA 30024 USA. He is a Member of IEEE.
Lars J. Foged (lars.foged@mvg-world.com) is with MVG, Microwave Vision Italy, 00071 Pomezia, Italy. He is a Member of IEEE.
Vicente Rodriguez (vince.rodriguez@ametek.com) is with NSI-MI Technologies, Suwanee, GA 30024 USA. He is a Senior Member of IEEE.
Justin Dobbins (justin.dobbins@rtx.com) is with Raytheon Technologies, Tucson, AZ 85625 USA. He is a Member of IEEE.
Vikass Monebhurrun (vikass.monebhurrun@centralesupelec.fr) is with CentraleSupélec, GeePs, 91192 Paris, France. He currently serves as chair of the IEEE Antennas and Propagation Standards Committee and associate editor of IEEE Transactions on Antennas and Propagation. He is a Senior Member of IEEE.
[1] J. Kraus, Antennas, 2nd ed. Boston, MA, USA: McGraw-Hill, 1988.
[2] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed. New York, NY, USA: Wiley, 1997.
[3] W. Stutzman and G. Thiele, Antenna Theory and Design, 2nd ed. New York, NY, USA: Wiley, 1998.
[4] IEEE Standard Test Procedures for Antennas, IEEE Standard 149-1979, Nov. 1979.
[5] V. Rodriguez, L. Foged, and J. Fordham, “Expected changes and additions to the antenna measurement standard IEEE Std149™,” in Proc. IEEE Conf. Antenna Meas. Appl. (CAMA), 2018, pp. 1–2, doi: 10.1109/CAMA.2018.8530605.
[6] V. Rodriguez, J. Fordham, and L. Foged, “A review of the changes and additions to the antenna measurement standard IEEE Std 149,” in Proc. Antenna Meas. Techn. Assoc. Symp. (AMTA), 2019, pp. 1–3, doi: 10.23919/AMTAP.2019.8906343.
[7] IEEE Recommended Practice for Antenna Measurements, IEEE Standard 149-2021, 2021.
[8] IEEE Standard for Definitions of Terms for Antennas, IEEE Standard 145-2013, 2013.
[9] IEEE Recommended Practice for Near Field Antenna Measurements, IEEE Standard 1720-2012, 2012.
[10] R. E. Hiatt, E. F. Knott, and T. B. A. Senior, “A study of VHF absorber and anechoic rooms,” University of Michigan, Ann Arbor, MI, USA, Report 5391-1-1-F, Feb. 1963.
[11] Evaluation of Measurement Data – Guide to the Expression of Uncertainty in Measurement, Switzerland, Geneva, JCGM Standard 100:2008, 2008.
[12] IEEE Standard for Safety Levels with Respect to Human Exposure to Electric, Magnetic and Electromagnetic Fields, 0 Hz to 300 GHz, IEEE Standard C95.1-2019, 2019.
[13] General Requirements for the Competence of Testing and Calibration Laboratories, ISO/IEC Standard 17025:2017, 2017.
Digital Object Identifier 10.1109/MAP.2023.3282868