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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.
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.
Modern Aerospace, Naval and Defense platforms are overwhelmed with radiofrequency (RF) signals competing for spectrum. RF co-site interference has become a major problem due to the RF interference from jamming, radio stations nearby, or even from civilian communications such as mobile phones. It can be a problem on military platforms like surface warships, land vehicles, and aircraft where many different RF transmit and receive antennas must share a relatively small space. This can be a communications nightmare in which separate RF systems inadvertently step on each other's signals, causing an RF communications fratricide problem that can also be compounded by intentional or accidental RF jamming. It has become more important than ever to address these issues that arise due to RF co-site interference. In this paper, we present advanced simulation tools for antenna placement, antenna coupling and cosite interference on electrically large naval and air platforms using Altair Feko and Wrap softwares. S-parameter coupling matrix of various antennas (both in-band and out-of-band) are computed using either full-wave solutions such as MoM, MLFMM or using asymptotic methods such as PO, RL-GO and UTD. Alternatively Coupling Loss Matrix, defined as the power ratio between powers at the terminals of the transmitting and receiving antennas can be computed using equivalent sources. S-Parameter matrix or Coupling Loss values are then used to study the parameters of co-site/collocation interference such as inter-modulation products, adjacent channel interference and harmonic interference. Furthermore, we will also discuss the options to mitigate collocation interference by adding appropriate filters.
Robot-based measurement systems offer a high number of degrees of freedom for the configuration of the targeted antenna measurement. Especially in comparison to the conventional planar, cylindrical or spherical measurement chambers, complex measurement sequences can be exploited. In addition, the actual measurement path is not decisive for an antenna measurement since the data is acquired only at discrete sampling positions. Spline-based measurement trajectories provide a way to obtain the measurement data at the intended positions without specifying a fixed path. Instead, it is up to the robot controller itself to calculate and optimize the path between the various spline support points, for example regarding the greatest path speed. However, further optimization potential can be accessed by reducing the number of path-defining spline support points and instead recording the measurement data on the resulting path. This sampling data is thus not on a regular or uniform sampling grid, but on the path optimized by the robot controller, both in terms of position and orientation. Accordingly, a pointwise probe correction is essential so that the actual position and orientation of the probe can be considered in the calculation of the spherical mode coefficients. In order to evaluate the radiation pattern transformed into the far field of the antenna under test as well as the required measurement time, the spline-based measurements are compared to a conventional spherical measurement as usually performed with roll-over-azimuth positioners. The results show that spline-based motion sequences enable faster antenna measurements, since the actual trajectory is no longer predetermined, but can be optimized by the robot controller itself for the underlying measurement setup.
Antennas on airborne platforms are subject to harsh environmental conditions such as rain and hail, as well as a wide range of pressure and temperature conditions. In this work, we propose a low-cost and easy to fabricate combined pressure and temperature test-bench for emulating the antenna performance while airborne. The specific antenna under test (AUT) is a millimeter wave 3d printed helix antenna enclosed in a specifically designed radome and fed with a 2.4mm connector. The entire system is subjected to airspeeds in excess of 200 m/s and operational altitude from the sea level through several kilometers. In the intended application, there are two possible pressure conditions that are considered, specifically open and closed. The open scenario assumes that the interior of the antenna is exposed to the ambient pressure level. In the closed case, the antenna has internal pressure that acts on the radome from inside at the higher altitudes in addition to the outside wind load due to the airspeed. Also, at higher altitudes, the temperature can drop to < -30˚C whereas at low altitudes it can be as high as 50˚C or more. Therefore, the structure needs to maintain its high-quality performance over the 80˚C temperature gradient. Note that both pressure and temperature can affect the antenna RF performance due to the drift of tolerances. To ensure proper operation, it is necessary to test the antenna when it is subjected to the above-discussed conditions, and for that purpose, the cost-effective combined pressure and temperature test bench is engineered for environmental tests of airborne antennas. Test-beds are mainly made of commercial off-the-shelf parts and in-house-made frames with all components integrated into one assembly. The system is developed for antennas having diameters and lengths of 125mm and 200mm, respectively, and occupies a relatively small volume. Experimental results that include live monitoring of VSWR during the variation in temperature from -20˚C to 50˚C and pressure from vacuum of half atmosphere to 20 PSI will be presented. This work is funded by the Office of Naval Research (ONR) grant # N00014-21-1-2641
Currently to determine the basic parameters of shales, namely Maturity (which indicates production potential), geochemical analysis need to be performed. These analyses may take days to weeks, depending on laboratory availability. Moreover, by the time the samples get to the laboratory, they could be damaged or poorly preserved, thus creating a significant source of uncertainty in the measurements. Pyrolysis is a method that introduces a sample of rock, of known mass, into a sealed oven that is programmed to heat the sample according to a prescribed rate of increasing temperatures that terminates at 650°C. During the initial heating, upon reaching a threshold in temperature somewhere around 300-350°C, a significant amount of hydrogen is recorded which is called the “S1” peak. With further increases in temperature beyond 350°C, another threshhold is reached at 550-600°C, where yet more hydrogen is evolved from the rock and the peak recorded at that stage is called “S2”. A microwave heating and testing for geological applications was tested with different shale samples. The system consists of a dual-mode microwave cavity, where heating and measuring is performed simultaneously with two different microwave sources. The cavity has a diameter of 104.92 mm and a height of 85 mm. A small shale sample of 9.8 mm diameter by 15 mm height is introduced in a quartz vial with an inner diameter of 9.8 mm and 120 mm. Depending on the electrical losses, the sample could be heated up to 1200°C. Initial complex permittivity measurements show that the imaginary part exhibits relaxation processes at specific temperatures. These temperatures coincide with the expected temperatures of the S1 and S2 peaks of the pyrolysis method, from which we can compute the vitrinite reflectance of the sample, which is an indicator of Maturity. Thus, allowing for quick estimates of maturity, which allows for real time decisions on the development of unconventional resources.
Hybrid radome-antenna designs can enable novel applications and unique benefits that would be difficult to achieve with standalone radomes and antennas. Examples of such designs are provided which use simple antennas and novel radomes to reduce antenna size and weight, to generate and steer antenna beams without use of complex phased arrays and beam forming networks, and to enable precise direction finding with only two antenna elements.
Free space material measurements at VHF and UHF bands require antennas that are necessarily large and heavy to accommodate the long wavelengths in these bands. Large antennas make measurement less practical and more expensive. This paper presents a new flat lens antenna technology, which enables significant reductions in size and weight compared to conventional wide bandwidth horn antennas. These new antennas utilize artificial dielectric loading combined with lossy materials to give directivities similar to much larger and heavier horns. This paper also presents the direct application of these antennas for free space dielectric material characterization. Example measurements of dielectric specimens are shown with a pair of 200 MHz to 4 GHz antennas.
A microwave water-cut meter for production fluids applications was designed and a basic test performed. The meter uses a vector network analyzer to measure the reflection (S11) and transmission (S21) spectra of the material under test (MUT), such as production fluids, oil spills, rock cores or soil. The initial design concept consisted of a pair of waveguides whose ends face each other and are placed on the inner surface of the pipe/core holder. The waveguides have a diameter similar to the main pipe and are filled with specific low loss materials with dielectric constant similar to that of the fluid in the pipe. Based on the initial design, a refined water-cut meter design was optimized, via numerical simulations, built and tested. To maximize bandwidth, the improved design adds ridges to the original cylindrical waveguide and optimizes the feed details to maintain an impedance match to the feed connectors. Results show that the ridges in the waveguide significantly improve transmission compared to just the waveguide alone. Initial experimental results show the measuring system is sensitive to the water content of production fluids.
Electrical properties of materials are requisite to analyze and design electromagnetic (EM) devices and systems. Free-space material measurement method, where the measurand is the free-space scattering parameters of an MUT (material under test) located at the middle of transmit (Tx)/receive (Rx) antennas, is suitable for non-destructively testing the MUT without prior machining and physical contact in high frequency ranges. This paper proposes a free-space two-tier one-port calibration method using three planar offset shorts with the respective offset of , ⁄, ⁄ for the measurement of the full scattering parameters of a reciprocal planar MUT from two successive oneport calibrations. Measurement results of a glass plate of 4.775 mm thickness are shown in W-band (75-110 GHz).
We present a new method for measuring thin, polymer sheets using a slotted rectangular coaxial transmission line (RCoax). This method allows a sheet of material to be inserted into the R-Coax slot, greatly simplifying the measurement procedure over traditional waveguide methods. The permittivity inversion is performed with the aid of computational simulations of the RCoax conducted across a range of expected dielectric properties. In particular, the slotted R-Coax device was optimized to enhance signal strength but has no simple analytical solutions for inversion. This new measurement technique is demonstrated on several thicknesses of commercial polyethylene terephthalate (PET) films, with a maximum thickness of 10 mils (0.254 mm). Due to the coaxial geometry, this technique does not have an intrinsic lower frequency cutoff and has an upper frequency cutoff near 3 GHz from over-modeing within the transmission line, though this frequency range could be extended by shrinking the fixture. However, the signal strength and calibration stability limit the useful range of permittivity measurement to 0.5-3 GHz for 10 mil thick specimens (and a range of ~1 GHz-3 GHz for 0.5 mil thick specimens). Repeatability for the real part of the permittivity ranged between 2-5% and loss tangents of ~0.006 were measured. Thus, this paper demonstrates the R-Coax measurement technique as a potential QA tool for microwave frequency electrical properties of thin polymer films.
In this paper, a design of a linear-to-circular polarization converter based on a 3D printed anisotropic metamaterial substrate (AMS) is presented. The AMS is a stack of thin sheets of acrylonitrile butadiene styrene (ABS) material, with air gaps in between, placed over a ground plane. This produces a metamaterial structure composed of periodic anisotropic unit cells, enabling the conversion of a linearly polarised (LP) incident wave to a circular polarised (CP) reflected wave. Results demonstrate that the proposed 3D printed AMS provides good angular stability. Using the AMS as a substrate, a CP antenna application is proposed operating within the L1 GPS reducing the complexity of designing/feeding and fabricating of the primary antenna.
This paper presents a wide incident angle metasurface unit cell element applied to a reconfigurable intelligent surface (RIS) for beamforming and beam-steering applications in the 26 GHz frequency band from the fifth generation of mobile communications (5G) frequency range 2 (FR2). Each metasurface unit cell is based on a printed frequency selective surface (FSS) loaded with a varactor diode. The FSS-based structure is based on a circular loop at the top and a slot-based ground plane at the bottom resulting in a 0.25x0.25λ0 total area. The complete unit cell element encompasses four conducting layers, in which the first two ones form the FSS. RF chokes are printed at the middle layer to isolate the DC circuit, and the bias lines are routed at the fourth layer. The unit cell has been conceived using the full-wave electromagnetic solver ANSYS HFSS. Its numerical results demonstrate a reflection phase shift up to 180º and reflection magnitude higher than 0.4 at the 26 GHz frequency band for incident wave angle from 0 to 50º. The proposed reconfigurable intelligent surface might be applied to future wireless communication systems, planar antenna reflectors, and vortex beam generation.
In this work, a simulation-based study is presented exploring the effect of surface roughness of 3D printed plastics on the accuracy of material parameter extraction. A homogenous sample material is placed inside the W-band waveguide and the S-Parameters are simulated. Two different methods for estimating the dielectric properties of the sample using the simulated S-Parameters are presented (i) NRW (Nicolson-Ross-Weir) technique-based estimation method, and (ii) Feko optimization-based estimation method. An error analysis study is presented to understand the percentage of error due to the surface roughness of the 3D printed plastics. For N7 grade surface roughness, NRW predicts 14% error in material parameters due to surface roughness, whereas Feko optimization method predicts 10% error compared to estimation without any surface roughness. Process outlined in the paper can be used to estimate effect of surface roughness of waveguides on material property measurements at mm wavebands such as W-band.
Resistive materials are often employed in antenna or absorber design for radio frequency (RF) applications. Causal material models are needed when modeling wideband RF systems using time-domain numerical models (e.g. FDTD). To this end, the frequency-dependence, from 10’s of MHz to 10’s of GHz, of spatially patterned and un-patterned resistive-cards (R-cards) were measured using free space and specialty materials measurements fixtures. Specifically, the complex sheet-impedance of two R-card specimens were measured at VHF frequencies using either an 8.5-inch slotted rectangular-coax (R-coax) or a recently developed resistive material mapping probe (RMMP). At GHz frequencies measurements were conducted using a standard 2’ focused beam lens system. The multi-band complex-impedance data were fit using a set of causal sheet material models. Typically, the fit errors are in the 1-3% range for causal models of measured data over two-plus decades of bandwidth.
Over the years many methods have been developed and used for measuring permittivity and permeability of materials. The most widely used methods are: 1) free-space techniques; 2) cavity perturbation techniques; and 3) transmission line of waveguide methods. Each technique has its own advantages and limitations. The free-space methods are employed when the material is available in a big sheet form. These measurements are less accurate because of unwanted reflections from surrounding objects, difficulty in launching a plane wave in a limited space, and unwanted diffraction from the edges of the sample. The resonant cavity measurement or cavity perturbation techniques are more accurate. Recently "epsilon-near-zero (ENZ) metamaterials have received much attention for several interesting phenomena like super-coupling, transparency and cloaking devices and pattern reshaping at microwave and optical frequencies. The rapid growth and excitement of ENZ materials was due to their ability to achieve very long wavelength in zero permittivity material, allowing propagation in a static-like manner. This paper presents the evaluation of complex dielectric permittivity and magnetic permeability of materials using planar ENZ tunnel structure with substrate integrated waveguide technology. The changes in resonance frequency and quality factor are related to the dielectric permittivity and magnetic permeability properties of the sample through Cavity Perturbation Technique. ENZ tunnel structure has very high sensitivity, which yields more accurate results when compared to other techniques, such as perturbation of conventional cavities. Design, optimization, and simulation of the ENZ tunnel structure at microwave frequencies is presented. Simulations are performed on various dielectric and magnetic samples using the cavity perturbation technique of the ENZ tunnel structure and validated with measured data.
Electrical material parameters such as permittivity and permeability are a prerequisite to analysis and to design of EM devices and systems.For the measurements of the EM material parameters, coaxial/waveguide methods and cavity method are used in low frequency range whereas free-space method is suitable in high frequency range. In free-space method, one of the non-destructive methods without prior machining of a MUT (Material Under Test), TRL (Thru-Reflect-Line) calibration method is used when the free-space measurement system has a linear slide to precisely adjust the separation distance between transmit (Tx) and receive (Rx) antennas, and GRL (Gated-Reflect-Line) calibration method in the case that the separation distance between the two antennas is fixed. As one of the well-known calibration methods, SOLR (Short-Open-Load-Reciprocal) calibration method assumes seven unknowns in the free-space material measurement configuration, i.e., the port-#1-side directivity, match, and tracking (E_DF, E_SF, E_RF) between the VNA test port #1 and the MUT, the port-#2-side directivity, match, and tracking (E_DR, E_SR, E_RR) between the VNA test port #2 and the MUT, and the quotient ?/…. This calibration, first performs two one-port calibrations at the port-#1-side and port-#2-side to determine (E_DF, E_SF, E_RF) and (E_DR, E_SR, E_RR) using three reflection standards, and then performs one transmission measurement using a reciprocal two-port standard (in this case, thru) for determining ?/…. Calibration of a free-space material measurement system by using SOLR method requires three free-space reflection standards. Recently, a planar offset short is proposed as a free-space reflection standard because its reflection property has the magnitude of unity and the phase proportional to the offset of the offset short. This paper proposes the SOLR method using three planar offset shorts with the respective offset of (0, ?/6, 2?/6) for calibrating a free-space measurement system. Effects due to the thickness of the MUT are compensated by a de-embedding process. The thinner the thickness is, the better results this method can get. The proposed method does not require a linear slide to precisely adjust the separation distance between Tx and Rx antennas of the measurement system. Theoretical details and measurement results will be presented at the symposium.