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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.

Anna Stumme, Alexander Golding, Mark Dorsey, Scott Rudolph, October 2023

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.

Christopher Howard, Kenneth Allen, Bill Hunt, October 2023

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.

Andreas Schwind, Isabella Varga, Willi Hofmann, Matthias Hein, October 2022

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

Florindo Bevilacqua, Francesco D'Agostino, Flaminio Ferrara, Claudio Gennarelli, Rocco Guerriero, Massimo Migliozzi, October 2022

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.

Francesco D'Agostino, Flaminio Ferrara, Claudio Gennarelli, Rocco Guerriero, Massimo Migliozzi, October 2022

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.

Ljubodrag Boskovic, Mohamed Elmansouri, Dejan Filipovic, October 2022

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

Jose Alvarez, David Jacobi, Poorna Srinivasan, October 2022

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.

Ronald Lavin, Michael LoRe, Andy Lee, Michael Carr, October 2021

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.

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