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We describe a planar near-field instrument capable of measuring the non-linear response of an electronically steered antenna (ESA) up to the third harmonic while requiring only a single scan with a single probe. The system performs phase-coherent measurements of the aperture near-field at the fundamental frequency, second harmonic, and third harmonic simultaneously, which are then transformed to the far-field. When system losses are appropriately accounted for, these far-fields are accurate representations of the harmonic patterns relative to the fundamental. A broadband dual-polarized probe combined with a specially-configured network analyzer is used to capture all frequencies and both polarizations within a single scan. Using a ultra-broadband probe introduces some limitations to the measurement, but offers a significant increase in measurement speed. In this paper we disclose various architecture and design aspects of the instrument, discuss its advantages and limitations, and compare non-linear PNF measurements with non-linear array simulations and direct far-field measurements.
Compressive Sensing (CS) has been deployed in a variety of fields including wideband spectrum sensing, active user detection and antenna arrays. In massive MIMO arrays, CS has been applied to reduce the number of measurements required to verify the arrays excitation in a production environment. All follow the general approach of creating the sparsity needed for CS by subtracting the measured far-field or near-field of the test array from that of a 'gold standard' array measured under identical conditions. In a previous paper [1] the authors have shown that using a Far-Field Multi-Probe Anechoic Chamber (FF-MPAC) and an optimal sampling strategy CS can offer accurate reconstruction of array excitation with a mean square error (MSE) approaching -40dB using a sampling strategy of just 1.4% of the Nyquist rate. The approach assumed production standard arrays with failure rates up to around 2%. In this paper we extend the concept to using a planar near-field (NF) measurement offering a much more compact test facility that is more suited to the production environment for these antennas. In our initial work the reconstruction of array excitation with a mean square error (MSE) of -30dB was achieved for a 20 x 28 element array antenna at half wavelength spacing using just 1.5% (177 samples) of the samples needed for a conventional NF measurement (12,100 samples) employing back projection to the aperture. Critical to the performance is the realization that the CS samples need to be confined to the central region of the NF measurement plane which for a conventional NF to FF planar antenna pattern measurement would offer a massive truncation error. This paper addresses the optimal sampling strategy needed for this NF approach and presents a statistical performance analysis of the reconstruction accuracy.
First realizations of reconfigurable intelligent surfaces (RIS) are becoming available as research on 6G advances. Consequently, prototypes have to be characterized by means of radiation pattern measurements to confirm the design properties. The main challenges here are the degrees of freedom from independent receiver and transmitter location in combination with surface configurations. We propose a far-field to nearfield measurement setup to conduct first full-sphere CATR measurements of a 5 GHz RIS at RWTH Aachen University. In order to cope with parasitic effects of the near-field probe, applicable post-processing methods including time gating and reconstructed equivalent currents are applied and evaluted.
This paper aims to compare the capabilities and advantages of Plane Wave Generators (PWG) and Compact Antenna Test Ranges (CATR) of similar physical size, operating in the VHF/UHF frequency range. The primary focus of this study is on the benefits of utilizing the PWG at such low frequencies for antenna and device characterization. We demonstrate that the PWG offers a superior approximation to the far-field (FF) plane wave condition in the quiet zone (QZ) compared to similar sized CATR systems. The better performance of the PWG at these frequencies is expected, as this is an unusual frequency range for an optical system such as CATR. Due to the efficient focusing properties of the array, the PWG exhibits significantly reduced side wall illumination and thus resulting reflections within the anechoic chamber. This translates into a substantial improvement in overall measurement uncertainty. The CATR system requires specific edge treatment, such as serrations or rolled edges, which increase the overall system's size and associated cost while reducing the effective area of the reflector. Our findings suggest that at low frequencies such as VHF/UHF, a PWG-based solution can be designed to comparable performance to the CATR system while maintaining a considerably smaller size and lower cost, making it an attractive alternative for low frequency antenna testing at in anechoic environments.
This paper discusses the novel development of a lightweight RF front-end system aimed at enhancing airborne antenna measurements in the far-field. The proposed system leverages advancements in software-defined radio (SDR) technology and high-performance RF front-end systems. While there are instances of SDR applications in UAS measurements, these systems are predominantly designed for anti-UAS and communication purposes, lacking focus on antenna and radar characterization. In contrast, the proposed system is purposefully tailored for RF applications, completely self-contained and airborne, possessing both RX and TX capabilities, and minimizing reliance on ground-based components. The system facilitates transmission and reception in both H- and V-polarization, employing two independent channels. Consequently, it allows for the simultaneous measurement of antenna or radar properties in both copolar (Cp) and cross-polar (Xp) orientations. The comparison between antenna measurements conducted within an anechoic chamber and those carried out utilizing the proposed UAS system demonstrates a substantial level of agreement.
This paper will describe the Huffman Radar Site (HRS), a unique in-situ remote radio frequency calibration and characterization capability located at the Air Force Research Laboratory Sensors Directorate, Wright Patterson Air Force Base (WPAFB), OH. HRS is a part of the OneRY Range complex which consists of Indoor and Outdoor Ranges used to conduct test, evaluation, integration, and demonstration of novel sensing systems and technologies. The Outdoor Range has diverse capabilities at several sites distributed across the local area. Within the Sensors Directorate complex there are three 100 foot antenna towers: the South Tower holds a dish-based S-Band Radar, the East Tower holds a large digital phased array radar, and the West Tower is reconfigurable as needed based on customer requirements. The Huffman Radar site is used to validate the proper functionality of systems on these towers, conduct experiment witness testing, and provide calibration signals for phased-array antennas. The site is primarily used as a Direct Illumination Far Field Range source standing approximately 2 miles away with direct line of sight to the South, East, and West towers. The capability includes full polarimetric transmit from 2.9 to 3.5 GHz and receive from 800 MHz to 6 GHz with future plans to expand the frequency range. This paper will include the design, link budget, hardware implementation, test, and validation of the site. Preliminary far-field antenna pattern data and calibration results for the S-Band Radar system and digital phased-array radar system will be presented. The discussion will include challenges and successes in standing up a multi-function outdoor remote testing capability.
In recent years, many activities have been carried out within the European Association on Antennas and Propagation (EurAAP) and the working group (WG) on measurements in particular. This group constitutes an important framework for collaboration to advance research and development of antenna measurements. The activities are divided in different tasks comprising measurements and comparison of reference antennas, contributions in the revision of IEEE standards on antenna measurements, self-evaluation measurements for facilities and new and emerging technologies for antenna OTA measurements. Special attention is dedicated to the activity on international comparison campaigns and evaluation that span more than 10 years of dedicated work by the WG. This task constitutes a crucial foundation for facilities to document and validate measurement accuracy among participants and provide an important prerequisite for certification of facilities and inputs to standards and research on measurement uncertainties. As regular inter-comparisons are a precious tool for traceability and quality maintenance, the campaigns have become a useful instrument for facilities to obtain and/or maintain an ISO17025 accreditation. International intercomparison campaigns within the WG span the frequency range from UHF-V band using different antennas: a mm-VAST antenna, a set of MIMO PIFA antennas, a SH800 ultra-wideband horn, a BTS1800 base station antenna, a SR40-A offset reflector and a set of chip reference antennas. This paper gives an overview and status on current campaigns within the working group, focusing on the useful criteria for comparing and evaluating large amount of measured antenna data. Updated results on running campaigns and proposed future initiatives will be discussed including an interesting synergy between measurement and simulation modelling tools.
This paper reports the look through losses witnessed for four hygroscopic indoor material groups, namely, wood, paper, brick, and leather employing the VNA-based Swissto12 MCK terahertz transmission waveguide measurements system. This study focuses on materials encountered widely in the interior of indoor environments. The hygroscopic nature of the chosen materials is studied by measuring the look through losses (i.e., penetration losses) for the dry materials followed by the wet ones in the 0.75–1.1 THz frequency range. The moisture or water content may significantly influence the terahertz wave propagation depending on the free and/or bound water percentage. In addition, this acquired knowledge facilitates the characterization as well as localization of these materials precisely and hence, demands thorough investigations. The chosen material samples along with their frequency-dependent material parameters, thicknesses, and roughness are modeled in CST, which gives a further probe into the interesting hygroscopic effects on penetration losses witnessed for the chosen material groups. Utilizing well-known models such as Bruggeman and Landau-Lifshitz-Looyenga, a 1–60 percent moisture content range is employed in the CST simulations. This paper, however, is the first-ever to investigate the characterization of propagation in hygroscopic indoor materials at THz frequencies. Preliminary measurement results exhibit that the look through losses unexpectedly decline for the chosen material groups in the wet state. These unusual effects on look through losses signify that the bound water molecules as compared to free water content manifest less influence on the THz wave attenuation. All details about the measurement setup and material samples along with both measurement as well as simulation parameters are revealed in the full paper to be presented at the upcoming AMTA symposium.
In spherical near-field (SNF) antenna measurements, gain of an antenna under test (AUT) is usually determined by substitution method. In this method, a reference antenna with known gain, typically a standard gain horn (SGH), is measured and processed in the same way as the AUT, that is a full-sphere measurement is done for the SGH and this is followed by the near-to-far-field transformation (NFFT). The AUT gain is then determined comparing the calculated levels of the AUT and SGH far-field signals and using the known SGH gain value. It is always emphasized that in the SNF gain determination by substitution, both antennas must be processed in the same way, and thus measuring the full-sphere near-field data for the SGH is unavoidable. Since typical SGH is not a large antenna, its far-field distancedoes not exceed few meters, which is a usual measurement distance in many SNF setups. The SGH in these cases is measured in the far field and the NFFT does not change the measured SGH pattern shape. It is interesting to find out, in which conditions it is possible to skip the NFFT for the SGH, and thus also its full-spheremeasurement, and use the directly measured SGH data. The SGH measurement can in this case be reduced to a single direction, similar to what is done in the traditional far-field substitution method. In this paper the above question is clarified in detail, paying special attention to probe correction issues as well as to additional measurement uncertainties which may arise due to the explained simplification of measurement procedure.
An origami-based Tightly Coupled Dipole Array (TCDA) is proposed for small satellite applications. The array is formed by a two-layered structure using rigid and flexible substrates to enable accordion-like folding. The proposed TCDA operates across 0.4-2.4 GHz with VSWR < 3 at broadside and across 0.6-2.4 GHz with VSWR < 3 when scanning down to 45 in the E-, D-, and H-plane. An 8 prototype was fabricated using Kapton Polyimide and FR4 and tested to verify the bandwidth and gain of the origami array. The fabricated prototype was demonstrated to be packable, low-profile, and lightweight (only 1.1kg). Notably, when packed, the array has a one-dimensional size reduction of 75%. As will be discussed, the packing compression is made possible by eliminating vertical PCB boards and incorporating the balun feeds within the dipole layer. To our knowledge, this is one of the first foldable, low profile, and low-scanning ultra-wideband arrays in the literature.
An active S-band dual-polarized multifunction phased array radar (MPAR), the Advanced Technology Demonstrator (ATD), has recently been developed for weather sensing and aircraft surveillance. The ATD is an active electronically scanned array (AESA) with 4864 transmit/receive (T/R) modules and was installed in a spherical radome. Simulations and a novel phased array measurement technique have been explored to assess the impact of high reflectivity from a wet radome during rain that can potentially induce voltages exceeding the transmit amplifier breakdown voltage. The measurement technique uses array elements radiating one at a time to illuminate the radome, and uses superposition to quantify the received signal power in a reference antenna on the face of the array. It is shown that when the radome surface is wet and highly reflective, certain electronic steering angles sum to a large reflected signal focused on the array face. This measurement technique can be used prior to high-power phased array radar operation to monitor the magnitude of reflections and help avoid element transmit amplifier failures.
Diagnostic techniques are crucial in antenna development and testing to enhance the Device Under Test (DUT) performances and identify the cause of possible failures in the qualification process. Among different approaches [1]-[8], it has been demonstrated that the equivalent currents method (EQC) [8]-[9], implemented in [10], is one of the most efficient for investigations in various application areas [11]-[13]. Indeed, the generality of the 3D reconstruction surface enclosing the DUT is a key feature, it ensures that this technique is unique and highly suitable for diagnostics, respect to traditional methods based on plane wave expansion. To handle electrically large problems, the EQC method has been initially based on a Fast Multipole Method (FMM) [14]. The recent advent of 5G technologies has led to an increasing need in terms of antenna electrical dimensions. Therefore, a novel technique based on a Nested Skeletonization Scheme (NSS) has been implemented to guarantee a further reduction of memory requirements and computational time. The new capability has been demonstrated in the past for a patch array antenna [15]. In this paper, the diagnostic capabilities of the EQC approach are applied to an early prototype of an electrically large array antenna for 5G antenna measurements applications [16].
This work presents the concept and design of an antenna element, aimed at 5G time division duplex (TDD)-based digital massive MIMO applications, operating in millimeter waves (mm-waves). The proposed radiating structure is based on printed slot antennas, fed by a substrate integrated coaxial line (SICL) for significantly reducing mutual coupling among the array elements. Furthermore, the slot has its own cavity for creating a broadsidedirection beam, without increasing mutual coupling. Numerical results demonstrate 1.31 GHz bandwidth at 26 GHz, 6.4 dBi gain and beamwidth of 70° and 80° in the main orthogonal planes. A two-element array is reported as a proof-of-concept and the mutual coupling between its elements has been kept lower than 32 dB from 25 to 27 GHz, illustrating its potential for scalability to high-order massive antenna TDD arrays.
A compact ultra-wideband (UWB) antenna operating from 300 MHz to 6 GHz was developed for operating with a Lunar Heat Flow Radiometer (LHR) system is presented. The antenna was required to fit within 36 cm x 36 cm x 10 cm volume with an emphasis of small antenna height so that it can be mounted under rovers. This paper presents an innovated design which combine a dielectric-loaded TEM horn mode from 2 GHz to 6 GHz and bowtie dipole mode 300 MHz to 2 GHz. The simulation results show a minimum realized gain of 2.2 dBi at 300 MHz and the gain monotonically increases to approximately 15 dBi at 6 GHz and maintains approximately constant gain and patterns from 2 to 6 GHz
The Plane Wave Generator (PWG) concept has recently been presented for millimeter wave applications [1-2]. The PWG has attracted interest, also because of its unique application in direct testing of 5G/6G enabled devices while in use by life people or mounted on suitable phantoms. This test feature is important to evaluate the shadowing effect by the user and the effectiveness of distributed array system on devices to overcome the shadowing. In this paper, we investigate the feasibility and achievable measurement accuracy in such scenarios. Using the measured performance of the PWG reported in [1-2], the measurement scenario is emulated accurately and compared to the reference case.
Mm-wave 5G base-stations are capable of emitting unprecedented EIRPs, necessary to achieve its high data rates. This capability introduces the opportunity to also use this wireless resource to wirelessly power IoT devices. However, the passive recipients of such power densities would need large enough apertures to harvest appropriate power levels to operate, which would naturally limit their angular coverage. In this work, we present an unconventional solution to this problem through the implementation of a passive beamforming network—the Rotman lens—in the receiving mode, as an intermediate element between antenna arrays and rectifiers to enable the surprising combination of high gain and wide angular coverage. The fullyprinted, flexible Rotman lens, operating in the mm-wave regime, is equipped with eight antenna ports and six beam ports, selected based on a scalability study. Tested in both planar and bent configurations, the Rotman lens demonstrates a robust, ultrabroadband behavior, with minimum variations in its gain and angular coverage over more than 20 GHz of bandwidth. These structures promise to power the next generation of passive IoT devices at distances exceeding 100 m using 5G base-stations, with the transmission of the full 75 dBm EIRP allowable by the FCC in the 5G/mm-wave bands, thereby enabling the emergence of ultra-low-cost mmIDs for ubiquitous sensing for smart-city and smart-infrastructure applications.
Mm-wave 5G base-stations are capable of emitting unprecedented EIRPs, necessary to achieve its high data rates. This capability introduces the opportunity to also use this wireless resource to wirelessly power IoT devices. However, the passive recipients of such power densities would need large enough apertures to harvest appropriate power levels to operate, which would naturally limit their angular coverage. In this work, we present an unconventional solution to this problem through the implementation of a passive beamforming network—the Rotman lens—in the receiving mode, as an intermediate element between antenna arrays and rectifiers to enable the surprising combination of high gain and wide angular coverage. The fullyprinted, flexible Rotman lens, operating in the mm-wave regime, is equipped with eight antenna ports and six beam ports, selected based on a scalability study. Tested in both planar and bent configurations, the Rotman lens demonstrates a robust, ultrabroadband behavior, with minimum variations in its gain and angular coverage over more than 20 GHz of bandwidth. These structures promise to power the next generation of passive IoT devices at distances exceeding 100 m using 5G base-stations, with the transmission of the full 75 dBm EIRP allowable by the FCC in the 5G/mm-wave bands, thereby enabling the emergence of ultra-low-cost mmIDs for ubiquitous sensing for smart-city and smart-infrastructure applications.
Flared notch (“Vivaldi”) arrays have been a subject of great interest since the mid 1990s for use in broadband phased-array systems. These arrays are popular in large part due to their ultra-wide bandwidths, which can span multiple octaves, exceeding the bandwidths of the individual flared notch elements themselves. This effect is achieved via strong inter-element coupling, a departure from the conventional wisdom of minimizing mutual coupling between elements in a phased array. The benefits of this design choice have been widely reported on in the literature - however, this dependence on element coupling also places serious constraints on array performance, especially with regards to scan angle, active impedance, and array efficiency, which often go unreported. In addition, reliance on inter-element coupling necessitates an array that can be safely approximated as “infinitely” planar. If an array does not strictly meet this condition, significant VSWR issues can result, especially for elements near the edges of the array. This paper discusses the common pitfalls inherent in practical flared-notch array design that are often overlooked in the literature. To aid in this analysis, a network-centric approach to array modeling is demonstrated that allows for an examination of both element- and array-level performance metrics in a way that minimizes computation time and resources. Special attention is paid to parameters such as active impedance as a function of scan angle, which, though vital to array performance, are often mischaracterized by “infinite array” approximations commonly used by engineers in the design phase. The effects of mutual coupling on different array performance metrics, both beneficial and detrimental, are examined in detail so that an informed decision can be made on the suitability of the flared-notch topology for a given application.
As modern antenna array systems for MIMO and 5G applications are deployed, there is increased demand for measurement techniques for timely calibration, at both research and commercial sites.[1] The desired measurement method must allow for the de-embedding of information about the closed digital signal chain and element alignment, and must be performed in the near-field. Current means of measuring large arrays cover a variety of methods. Single-element gain and pattern calibration must cover the parameter space of element weightings and is extremely time-consuming, to the point where the measurement may take longer than the duration over which the array response is stable.[4] Two other popular methods are the transmission of orthogonal codes and the use of holography to reconstruct a full-array pattern. The first of these methods again requires extremely long measurement time. For an array of N elements and weightings per element W_n, the matrix of orthogonal codes must be of an order greater than NW_n.[4][3]. This number varies with the form of W_n depending on whether the array is analog or digital, but in both cases for every desired beam configuration, an order-N encoding matrix must be used. The second method relies on illuminating subsets of elements within an array and reconstructing the full pattern.[2] Each illuminated subset, however, neglects some amount of coupling information inherent to the complete system, making this an imperfect method. In this work we explore the development of a sparse set of measurements for array calibration, relying on coherent multi-channel data acquisition of wideband signals at 75 GHz, and the hardware characterization and post-processing necessary to perform channel de-embedding at an elemental level for a 4x1 system. By characterizing the complete RF chain of our array and the differential skew and phase response of our measurement hardware, we identify crucial quantities for measuring closed commercial systems. Additionally, by combining these responses with precise elemental location information, we consider means of de-embedding elemental response and coupling effects that may be compared to conventional single-element calibration information and full-pattern array measurements. [1] C. Fulton, M. Yeary, D. Thompson, L. Lake, and A. Mitchell. Digital phased arrays Challenges and opportunities. Proceedings of the IEEE, 104(3):487–503, 2016. [2] E. N. Grossman, A. Luukanen, and A. J. Miller. Holographic microantenna array metrology. Proceedings of SPIE, Passive Millimeter-Wave Imaging Technology VIII, 5789(44), 2005. [3] E. Lier and M. Zemlyansky. Phased array calibration and characterization based on orthogonal coding Theory and experimental validation. 2010 IEEE International Symposium on Phased Array Systems and Technology (ARRAY), pages 271–278, 2010. [4] S. D. Silverstein. Application of orthogonal codes to the calibration of active phased array antennas for communication satellites. IEEE Transactions on Signal Processing, 45(1):206–218, 1997.
There are some antennas where rapid validation is required, maintaining a reduced measurement space and sufficient accuracy in the calculation of some antenna parameters as gain. In particular, for cellular base station antennas in production phase the measurement time is a limitation, and a rapid check of the radiation performance becomes very useful. Also, active phased arrays require a high measurement time for characterizing all the possible measurement conditions, and special antenna measurement systems are required for their characterization. This paper presents a single or dual cut near field antenna test procedure for the measurement of the gain of antennas, especially for separable array antennas. The test set-up is based on an azimuth positioner and a near to far field transformation software based on the expansion of the measurements in cylindrical modes. The paper shows results for gain measurements: first near to far field transformation is performed using the cylindrical modes expansion assuming a zero-height cylinder. This allows the use of a FFT in the calculation of the far field pattern including probe correction. In the case of gain, a near to far field transformation factor is calculated for theta = 0 degrees, using the properties of separable arrays. This factor is used in the gain calculation by comparison technique. Depending on the antenna shape one or two main cuts are required for the calculation of the antenna gain: for linear arrays it is enough to use the vertical cut (larger dimension of the antenna), for planar array antenna 2 cuts are necessary, unless the array was squared assuming equal performance in both planes. Also, this method can be extrapolated to other kind of antennas: the paper will check the capabilities and limitations of the proposed method. The paper is structured in this way: section 1 presents the measurement system. Section 2 presents the algorithms for near to far field transformation and gain calculation. Section 3 presents the validation of the algorithm. Section 4 presents the results of the measurement of different antennas (horns, base station arrays, reflectors) to analyze the limitations of the algorithm. Section 5 includes the conclusions.