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This paper extends the authors prior studies to develop a more flexible definition for the shape of a blended rolled edge compact antenna test range (CATR). This is accomplished by utilising a more sophisticated definition for the junction contour. This definition ensures the reflector surface is smooth and provides additional parameters that can be used to optimise the performance of the CATR enabling wall illumination and quiet-zone performance to be managed and balanced. As with the authors prior work, a novel, parallel, physical optics based, genetic optimisation is performed that, over subsequent generations, breeds optimal designs for each test case selecting the preferred design from many thousands of potential mutated candidates. Results are presented and discussed for several CATR designs that illustrate the concept and achievable performance highlighting the utility of a hybrid serrated blended rolled edge CATR reflector.
The application of a series of interconnected single-axis motion controllers distributed throughout an antenna measurement range has some important advantages when compared to multi-axis controllers that must reside in a central control room. The technologies used for distributed real-time control based on the Ethernet for Control Automation Technology (EtherCAT) standard are discussed, as are advantages and tradeoffs to be considered. The greatly reduced requirement for signaling between the control room and the remote axes reduces cost in system design and manufacturing. In practice, the shorter electrical signal runs can enhance electromagnetic interference (EMI) performance, noise immunity and safety.
At the Boeing 9-77 Range, we have used various dielectric strings and fishing ropes for target support. An advantage of a string-system is that when not at broadside to the incident wave, the strings would give rise to the least interference to the object being measured. A good example is a 60-ft long rod lifted from ground by the upper turntable (UTT) to the quiet-zone center, rotated horizontally and being measured. [1-2] For the long rod with abruptly terminated ends, there is often a ringing, called the Gibbs phenomenon, which modulates the responses at both ends. Yet, for a 40-ft long vertical metal cylinder supported by a rope through its center, it was curious that the ringing did not show up. By reviewing the metal-dielectric interference, we now realized that the dielectric rope must have contributed an opposite effect such that the ringing ceased. These results are described and discussed. [3-4]
The aim of this research is to understand modeling techniques tailored specifically for low electrical conductivity materials, such as e-threads (σ~104 to 106 S/m) and other conductive polymers (σ < 104 S/m), for a wide range of antenna design applications (e.g., implantable antennas, flexible wearable antennas, and more). Commercial datasheets for such materials primarily report DC conductivity data. However, it has long been reported that conductivity of these materials exhibits frequency dependence, with notable increase in losses at higher frequencies, attributed to phenomena like surface roughness and skin effect. Our study involves a systematic exploration of diverse materials suitable for modeling low-conductivity scenarios, leveraging the capabilities of CST Microwave Studio. This involves the usage of various numerical solvers for analysis, with a goal to optimize antenna design in free-space as well as in proximity to the human body. Our analytical framework encompasses not only the evaluation of Radio-Frequency (RF) parameters such as return loss, gain, and antenna efficiency, but also extends to encompass system-level performance metrics, such as computation time and memory requirements. Overall, the proposed approach enables the identification of the most suitable modeling approach for antennas fabricated via low-conductivity materials, empowering near real-world simulation results.
Radomes are structures or enclosures designed to protect antenna and associated electronics from the surrounding environment and elements such as rain, snow, UV light, and strong wind while at the same time not impacting the performance of the antenna. In some cases, radome designs include frequency selective surfaces (FSSs) embedded within the inner, outer, or intermediate interfaces of the radome. When properly designed, the FSS embedded radome structure can enhance the performance of an antenna system by filtering out unwanted frequencies. The design of Radomes, especially those containing multiple layers and curved frequency selective surface (FSS) elements, are extremely complex, with the modeling and simulation of these systems taking days and even weeks to complete. In this paper, we present advanced computational tools for fast and accurate simulation of the FSS embedded radomes using characterized surfaces. A detailed study on different FSS elements for their frequency response of the reflection and transmission coefficient behavior is also presented. Simulations are performed to study the effects of insertion losses, boresight error and effect on the antenna side lobes. Computational resource comparisons for simulations of actual structure of the radome versus those simulations using characterized surfaces are presented.
As the demand for efficient and accurate characterization of mmWave antennas grows, compact antenna test ranges (CATRs) have become preferred alternatives to traditional far-field ranges due to their smaller size requirements. CATRs transform spherical waves into planar waves at short distances using a parabolic reflector. The quality of the CATR’s quiet zone depends on minimizing edge diffractions caused by fields reflecting off the reflector’s rim. Techniques like serrated and blended rolled edges are used to reduce these diffractions. While blended edges perform better, serrated edges are more commonly used due to their ease of manufacturing and lower cost. To enhance the convenience, affordability, and performance of CATRs, this work introduces a 3D-printed blended edge reflector for a Ka-band system. Manufactured on a desktop 3D printer, this high-performing reflector shows promising results. Additionally, a surface roughness analysis of CATR reflectors quantifies the impact of surface roughness on the purity of plane waves in the quiet zone across various frequencies. Measurement results from the additive manufactured reflector align with TICRA GRASP simulations. This work aims to improve efficiency and accuracy in mmWave and sub-terahertz frequency measurements, which require high precision in antenna characterization.
Free space material measurements illuminate a material or component with wave propagating through space. Algorithms for inverting intrinsic properties or thickness from free space measurements usually assume an ideal plane wave. This is an approximation because a typical incident beam is finite in extent and comes from a nearby aperture. In reality, the beam consists of a distribution of plane waves around the propagation direction. Typically, the illumination spot is minimized to measure different areas of a material and characterize homogeneity, or because the component itself is limited in size. A smaller spot leads to a wider distribution of plane-waves, which causes an effect called space loss, where the illuminating beam spreads as it travels. An ideal plane wave does not have space loss, so the plane-wave assumption results in systematic error when space loss is present. This paper derives a correction for the space loss phenomenon and applies it to thickness inversions used in microwave spot probe measurements. The correction is demonstrated on commercial microwave probes and quantified with a series of computational electromagnetic simulations. These calculations are discussed in terms of microwave mapping of radomes to measure performance and establish their compliance with design specifications.
A free-space material measurement for a small dielectric plate using a truncated Gaussian beam whose beam width is smaller than the material under test (MUT) is described. Measurement results of two glass plates of different thicknesses at W-band (75-110 GHz) show its validity and the minimum beam width of the truncated Gaussian beam for the reliable material property measurement of a small planar MUT.
Additive Manufacturing (AM), or 3D printing, has gathered increased interest for radio frequency (RF) applications in recent years due to its ability to easily fabricate complex shapes at a low cost. One such application of interest is additive manufacturing of dielectric materials. Through varying the percent fill volume of a print, the bulk material properties can be changed. One method of altering the percent fill volume is through the use of lattice structures, or repeating geometric patterns, through a volume. The lattice is appropriately designed to result in a sample with a specific percent fill volume. This percent fill volume defines the ratio of material to air which in turn defines the bulk dielectric constant of the sample. This paper demonstrates the use of AM and lattice structures to alter the bulk material properties. The latticed material designs are validated through simulated and measured samples. These results are compared to common dielectric mixing approximations to gauge the accuracy of these approximations for latticed AM materials. Included is a parametric study of commonly used lattice structures at different percent fill volumes which is analyzed for variations on bulk dielectric constant due to geometry variations. Theoretically, the bulk dielectric constant should not change so long as the lattice unit cells are sufficiently smaller than the wavelength of the frequency of operation. Additionally, since many 3D printing materials are not well characterized, or do not provide, the necessary material properties critical for RF design, like dielectric constant and losses through the material, a free space lens system is utilized to initially characterize and compare some common 3D printing materials.
This paper describes a new materials measurement method that includes a sensor embedded within a ground-plane to continuously measure complex permittivity of an adjacent material. The sensor works with a 1-port vector network analyzer (VNA) to collect amplitude and phase of the sensor reflection signal, which is then converted to intrinsic dielectric properties or sheet impedance. The complexity of the fields near this sensor makes a conventional analytical method to relate reflection data to dielectric permittivity impractical. Instead, this sensor uses a computational electromagnetic (CEM) inversion method based on finite difference time domain (FDTD) simulations to derive real and imaginary dielectric properties from the amplitude and phase of the measured reflection. This paper describes the sensor design and inversion method. Additionally the sensor is demonstrated on several material types including i) sheet materials that may be manufactured in an in-line process and ii) concrete, which is a material whose properties change as it cures.
This paper describes a new materials measurement method that includes a sensor embedded within a ground-plane to continuously measure complex permittivity of an adjacent material. The sensor works with a 1-port vector network analyzer (VNA) to collect amplitude and phase of the sensor reflection signal, which is then converted to intrinsic dielectric properties or sheet impedance. The complexity of the fields near this sensor makes a conventional analytical method to relate reflection data to dielectric permittivity impractical. Instead, this sensor uses a computational electromagnetic (CEM) inversion method based on finite difference time domain (FDTD) simulations to derive real and imaginary dielectric properties from the amplitude and phase of the measured reflection. This paper describes the sensor design and inversion method. Additionally the sensor is demonstrated on several material types including i) sheet materials that may be manufactured in an in-line process and ii) concrete, which is a material whose properties change as it cures.
This paper extends the time-domain gated response isolation scheme for full polarimetric calibration with a modified Thru-Reflect-Match procedure for network analyzer selfcalibration where precise knowledge of the metrology standards is not required. Cross-polarization contributions from the measurement setup are neglected to simplify the procedure. A simulated cascade analysis is included to demonstrate the relative scattering parameter error of the sample under test when the measurement setup cross-polarization level is neglected. The featured calibration analysis leverages a 4x4 scattering parameter matrix notation to capture the polarimetric scattering at each cascaded stage and develops a 16-term error correction factor model to account for cross-polarization scattering contributions from the measurement sample. Finally, a wire-grid polarizer is used as a modified Match standard where a series of interrogations at multiples orientations, in combination with Thru and Reflect measurements, enables cross-polarized scattering channels to be characterized. This polarimetric self-calibration approach uses physically realizable metrology standards and accounts for all error terms for precision focus beam system measurements.
A prototype of a previously presented design of a microwave oil/water ratio measurement system for geological applications was built and tested in a commercial flow loop. The prototype used a vector network analyzer to measure the reflection (S11) and transmission (S21) coefficients of the fluids under test. The fluids tested had different compositions of dearomatized oil, low salinity water and methane. This paper presents results from the flow loop evaluation of the present oilbrine- ratio (OBR) meter prototype. Transmission and reflection spectrums were obtained for different oil, water, and methane combinations. Transmission data shows good sensitivity and accuracy for most water volume fractions (WVF) below 70%. Transmission in WVF above 70% could be inverted by using a different frequency band and results could also be combined with S11 data to improve accuracy. In addition, the effect of gas volume fraction (GVF) above 50% is seen as a significant increase in transmission, for high WVF, relative to the de-gassed case. In the case of WVF less than 45%, the effect of gas can be measured by a shift in the cut-off frequency. Thus, the measuring system could also be used for gas kick detection.
The reflectivity of foam absorber materials is governed by the correct loading and mixture of carbon and other supplicants such as fire retardants. In order to assess the reflectivity of the absorbers various measurement setups are applied, each having different advantages and disadvantages in terms of frequency coverage and RF performance. The measurement setups are used both in the quality control (QC) as well as for product development. Especially for the product development case, it is important to understand limits of these setups as the lower the reflectivity gets, the more difficult it becomes to detect minute differences between different variants of the absorbers. For reflectivity measurements of microwave absorbers, the available dynamic range and calibration-quality of the setup plays a vital role in this respect. By determining the uncertainty of the measurement setups, a clear assessment can be made to the quality of the measurement and the product to insure consistent QC, as well as plan for the product development.
In this work, a tunable frequency-selective surface is designed with a center frequency that can be varied with an applied DC voltage. An equivalent circuit representation of the FSS is derived from a finite-difference time-domain (FDTD) simulation of the passive FSS, allowing tuning circuits for the FSS to be designed in common circuit simulation tools such as SPICE. A comparison between the spectral response obtained from FDTD and equivalent circuit modeling (ECM) in SPICE shows that under certain conditions, ECM provides good agreement with full-wave analysis, is less computationally intensive, and provides physical insight. The ECM technique enables rapid design and analysis of various trade-offs, such as those between resonant frequency tunability and bandwidth. The ECM-designed circuit is then validated with full-wave analysis of the designed structure with active components using an FDTDSPICE hybrid co-simulator. Finally, applicability of the chosen active FSS topology as a metasurface for free-space dielectric material characterization is discussed.
This paper describes two reflection methods to measure highly conductive coatings at VHF frequencies: 1) a resonant method based on eddy-current sensing at HF and VHF frequencies, and 2) a wideband method at VHF and UHF frequencies, based on a shorted transmission line combined with and computational electromagnetic (CEM) simulations to invert surface impedance. In both cases, the methods are able to determine surface impedances with sensitivities of a small fraction of an ohm. Both methods have strengths and weaknesses with respect to ease of calibration, sensitivity, frequency range, and use on non-flat surfaces. This paper describes both approaches and presents measurements on a variety of conducting materials and coatings. The resulting properties are also compared with DC conductivity measurements collected with a four-point probe system. The predicted accuracy for both methods is presented based on simulated data and empirical measurements.
Progressing towards highly automated vehicles, radar systems have been developed into reliable assistance systems of environmental perception for a wide spectrum of mobile applications in air, on sea and rails, and especially on roads. This success as well as further improvements necessitate the precise characterization of radar objects with radar cross-section (RCS) measurements throughout the manifold parameter space, including frequency, aspect angles or even illumination and observation angles in bi-static radar constellations. According to the IEEE RCS standard 1502-2020, parasitic effects from ground reflections have to be taken into account in the test setup in almost all RCS measurement systems. In ground-plane ranges, multipath signal propagation is even considered intentionally. In this paper, the influence of ground reflections on bi-static radar measurements has been investigated both analytically and experimentally. A geometry-based analytical model was applied to calculate interference and the resulting small-scale fading of the electric field strength at the receiver. The geometry consists of independently arranged transmitter, receiver, and scatterer. The model includes antenna crosstalk, ground reflections for different relative heights of transmitter and receiver, and the scattered signal contributions. As a result, six paths are considered in terms of their time delays and phases, resulting from the classical two-way propagation model in a point-to-point link plus the four-way radar propagation model including ground reflections. The model yields interference gains between 0 and 4, where the maximum value is uniquely obtained at equal distances between transmitter and scatterer, and between scatterer and receiver, respectively. The variations of the bi-static angle lead to further fluctuations, which confirm to expectation and will be described in the full paper. The analytical model was validated with bi-static radar measurements in a semi-anechoic chamber with a metallic ground floor. As a canonical radar target, a metal sphere was measured in the frequency range between 1GHz and 10GHz at different heights and distances. The measurement results confirm the analytical model and provide the basis for further extensions of practical relevance, e.g., the reflectivity parameters of the ground (e.g., dry and wet road surfaces). Funded by: Federal Ministry of Education and Research (BMBF), grant number 16ME0164K
The near-to-far-field transformation (NTFFT) technique adopting the plane-rectangular (PR) scanning is the most simple one from the analytical and computational points of view and can be suitably employed when characterizing antennas which exhibit pencil beam radiation patterns. In recent years, NTFFTs using the nonconventional planar wide-mesh scanning (PWMS) have been developed. They allow a remarkable measurement time saving with respect to that adopting the classical PR scanning, since their raster grid is characterized by meshes which become larger and larger as their distance from the scanning plane center increases. These NTFFTs have been obtained by applying the non-redundant sampling representations of the electromagnetic fields to the voltage detected by the scanning probe and adopting suitable AUT modellings for volumetric and quasi-planar AUTs. The evaluation of the AUT far field is then got by applying the PR NTFFT, whose input data are accurately recovered through optimal sampling interpolation expansions from the collected PWMS samples. In both the conventional and non-conventional scannings, the sampling points are reached through an x-y scanner. However, the finite resolution of the probe positioners and/or their imprecise control can prevent to exactly collect the near-field samples at the prescribed sampling points and imperfections in the mechanical rails driving the motion of the probe can cause a deviation from the considered measurement plane. Accordingly, 3-D positioning errors, which can be revealed via laser interferometric techniques, affect the acquisition. The aim of this work is to develop an effective NTFFT with PWMS from 3-D probe positioning error affected near-field measurements. To this end, the so named k-correction (Joy and Wilson, AMTA Proceedings 1982) will be used to compensate the positioning error related to the deviation from the considered measurement plane. Then, an iterative procedure (D’Agostino et al., International Journal of Electronics and Communications 2020) will be applied to retrieve the near-field samples at the points specified by the non-redundant sampling representation from those obtained at the previous step and affected by 2-D positioning errors. Numerical tests will show the capability of the procedure to fully compensate the 3-D positioning errors affecting the acquisition of the PWMS samples.
Among the near-field - far-field (NF-FF) transformations, that with spherical scan is the most appealing due to its feature to allow the whole radiation pattern reconstruction of the antenna under test (AUT). To considerably save measurement time, spherical NF-FF transformations for AUTs with one or two predominant dimensions, requiring a minimum number of NF data, have been developed by using the non-redundant sampling representations of the electromagnetic fields and adopting suitable AUT modellings. Another effective possibility to save the measurement time is to make faster the scan by collecting the NF data through continuous and synchronized movements of the probe and AUT. To this end, non-redundant NF-FF transformations with spherical spiral scan have been recently proposed by exploiting the unified theory of spiral scannings for volumetric and non-volumetric AUTs. However, the characterization of heavy and large AUTs (such as, e.g., a vehicle) in a NF spherical facility from measurements collected over the full scanning sphere can become infeasible. To ensure a mechanically stable support and guarantee a high repeatability of the measurements, a more viable way is to place the AUT on a turning metallic ground plane and characterize the considered AUT from the NF data acquired over a hemisphere. In recent works, instead to set to zero the NF data required by the classical NF-FF transformation, which would be acquired over the lower hemisphere, it has been proposed to synthesize them by properly applying the image theory, thus avoiding that the truncation error affects the FF reconstructions. This work aims to propose an efficient spherical spiral NF-FF transformation for volumetric AUTs, using a minimum number of spiral data, which, due to the presence of an infinite perfectly conducting ground plane, are collected over a proper spiral wrapping the upper hemisphere. Once the voltage NF data which would be acquired over the spiral wrapping the lower hemisphere will be properly synthesized, then an efficient 2-D optimal sampling interpolation scheme will allow the recovering of the NF data required by the classical spherical NF-FF transformation. Numerical tests will show the accuracy of the developed non-redundant spherical spiral NF-FF transformation.
A free-space material measurement using the scattering parameters of a planar MUT (material under test) placed between Tx and Rx antennas is suitable for non-destructively testing the MUT without physical contact and precise machining in a high-frequency range. Improving the measurement uncertainty of the material parameter of an MUT extracted from a free-space material measurement requires accurate and precise measurement of the scattering parameters of the MUT, which is highly dependent on the characteristics of impedance standard (or reflect standard) and method used in calibrating the material measurement system, including a VNA (vector network analyzer). A free-space material measurement system usually needs at least three independent reflect standards (e.g., short, open, and load for coaxial case) for the calibration at both sides of an MUT in free space. Unfortunately, it is not easy to implement the reflect standards in free space. Recently, a planar offset short has been proposed as a free-space calculable reflect standard. The magnitude of its reflection coefficient is unity, and the phase is the linear function of the offset of the short and the signal frequency. This paper reviews the recently developed one-/two-port calibration methods using a planar offset short for a free-space material measurement in a millimeter-wave frequency range. An adapter characterization scheme, which is widely utilized to measure the scattering parameters of a non-insertable device (e.g., SMA to 3.5 mm adapter) by using a two-tier one-port calibration, may be applied to a free-space one-port calibration. On the other hand, an unknown thru calibration method, widely used to measure the scattering parameters of non-insertable devices whose connectors could not mate together (e.g., a coaxial to waveguide adapter), may be applied to calibrate a free-space two-port measurement system. These works use three planar offset shorts as free-space reflect standard, which gives the phase difference of 120° between the reflection coefficients of the planar shorts at the center frequency of the operating frequency band of a waveguide. A review of the two calibration methods and measurement results in the W-band (75-110 GHz) will be presented.