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The sampling of the field first-order spatial derivative, in addition to the field itself, enables an increase of the sampling step to twice that of the standard sampling criterion – and thus facilitates a reduction of the measurement time. Here, we investigate so-called derivative probes and their usage for spherical near-field antenna measurements.
Larger low-observable targets are being mounted onto RCS pylons. In many cases not only Azimuth rotation of the target, but a degree of movement in elevation is desired. This requires in many cases a large number of positioning cables to run from the base of the pylon to the tip where the rotator is placed. At the same time the low-observable qualities of the target call for pylon ogives with higher ratios to minimize the background RCS of the pylon that supports the target. The higher ratios call for very thin structures that cannot handle the weight of the rotator or have not enough space for the control and power cable to be fed to the rotator. A way of solving this problem is to have a variable ratio pylon, where the ogive at the tip is different from the ogive on the main body of the pylon. To analyze these pylons a higher-order basis-function method of moments (HOBFMoM) approach has been used in the past [1]. To conform the quadrilateral flat patches to the round geometry of the pylon, patches smaller than 0.3λ were used. While this was still an advantage over the typical 0.1to 0.05λ patches it placed limits on the highest frequencies that could be analyzed give the available computational resources. In this paper the authors present an approach to the meshing of the structure that allows for computing the monostatic RCS at frequencies in the x-band for a 2.4 m tall pylon. In addition, the effects of the non- physical absorber terminations are further analyzed.
We present a bench top demonstration of dropped-channel polarimetric compressive sensing to recover range profiles of a simple scene. Four antennas (H and V transmit and H and V receive) are connected to an arbitrary waveform generator and digital oscilloscope with programmable attenuators and phase shifters inline to control crosstalk. Range profiles of the scene are generated for three measured channels; the fourth is reconstructed from information imbedded in crosstalk using basis pursuit denoising. Reconstructed range profiles are shown to agree with measurements of all channels obtained without crosstalk contamination. Thus, the bench top setup demonstrates the potential use of dropped-channel polarimetric compressive sensing to reduce data storage and transmission burden while preserving full pol information.
We report a new class of wearable loop sensors for monitoring human kinematics (particularly, joint flexion angles) while overcoming limitations in the state-of-the-art. Previous studies have demonstrated the feasibility of these loop sensors using tethered connections to a network analyzer. In this work, we take a major step forward to demonstrate untethered operation for the sensor. To this end, transmitter and receiver boards are designed and integrated into the loops. The transmitter board sends a Radio-Frequency (RF) power of 5.68 dBm at 34 MHz upon a 50 Ω load, while the receiver board detects the power level and transmits the data to a nearby personal computer (PC) via Bluetooth. Flexion tests are conducted upon a tissue-emulating phantom to validate the setup. To quantify performance, we calculate the root mean square error (RMSE) between the estimated angle from our sensor and the gold-standard angle from a marker-based motion capture camera system, as well as Pearson’s correlation coefficient (ρ). The proposed sensor shows outstanding performance with an average RMSE of 0.670° and an average ρ of 0.99966. Overall, our sensor outperforms state-of-the-art wearable kinematic technologies by being highly accurate, seamless, lightweight, unobtrusive to natural motion, and reliable over time.
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.
In modern integrated radar systems conventional antenna measurements are often impractical due to the lack of access to the antenna feed points. For frequency modulated continuous wave radars, the two-way radiation pattern can be characterized with a reflector while utilizing the integrated transmit and receive module. However, some post-processing steps are required for this measurement method to obtain the frequency-resolved radiation characteristic. This paper takes a closer look at the fast Fourier transform (FFT) and inverse FFT with the associated window functions and the necessary range gating including zero-padding based on simulations. A sufficiently wide range gating is necessary to reconstruct the frequency resolution of the antennas correctly. Yet a trade-off between the required wide range and the filtering of mutual coupling and reflections from the environment has to be made in the case of real measurements. Moreover, depending on whether a frequency in the center or at the edge of the chirp is to be reconstructed, different window functions provide the most accurate result.
The standard Near-Field antenna characterization allows to reconstruct the Far-Field pattern over the whole visible domain, even if, in many cases, the partial characterization of the Far-Field pattern just along some cuts can be sufficient, and becomes preferred if realized in shorter measurement time with respect to the standard case. A method for Partial Characterization has been proposed. The approach provides a general framework and defines the optimal distribution of the near-field samples required to reconstruct the Far-Field pattern along the cut of interest. The main features of the method are presented, and the performance is verified, experimentally, for two test cases.
It is well-known that any structure in the proximity of a radiating antenna will affect its radiation pattern. This is one of the reasons that a vehicle-mounted antenna tends to be tested while mounted onto the actual vehicle. There is a current discussion regarding how the vehicle should be tested. Traditionally, metallic turntables are used, with the tested vehicle resting on this conductive half-space. Several new spherical near field (SNF) ranges elevate the vehicle over a floor treated with RF absorber to obtain a quasi-free-space pattern. Discussions regarding which method is better are on-going. One of the arguments in favor of the free-space SNF range approach is that, using computational methods, the equivalent radiating currents that radiate the measured near fields, be it over a spherical surface or a non-canonical surface, can be computed. The equivalent radiating currents computed on a triangular element mesh are then imported onto a quadrilateral element mesh on a higher order basis function method of moments (HOBF-MoM) package. Once imported into the HOBF-MoM these currents can be used as excitations to obtain the far field. Within the HOBF-MoMit it is possible to place these equivalent currents in the presence of a metallic (PEC) using symmetry. A new development that allows for the use of an arbitrary Green’s function hence it is possible to get the far field from the computed equivalent currents in the presence of a dielectric half-space. Thus, the theoretical radiation pattern of the vehicle mounted antenna can be computed when the vehicle is on concrete, dirt, or even salt water. In this paper the authors present the latest work performed using this approach to place free space measured antennas over a PEC or dielectric half-space. Results show the potential of this approach. The higher order basis functions allow for the modeling of large structures with reduced number of unknowns. Thus, the antenna under test can be then placed in proximity to not only various half-space materials, but also to towers, buildings, or spacecraft.
Measuring radar cross-section (RCS) in high-noise environments remains a challenge. This paper presents an advanced signal processing framework that uses statistical dimensionality reduction to effectively separate the signal of interest from environmental noise. The proposed technique consists of two main steps. First, background subtraction and a gating technique are used to preprocess the measured data, separating and extracting the target’s reflectivity distribution from unwanted room contributions. Then, principal component analysis (PCA) is employed to analyze the target’s scattering image and localize its main scattering centers. To validate the proposed algorithm, a perfectly electrically conductive (PEC) scaled UAV model is manufactured and tested. The analysis of the experimental results demonstrates that the suggested technique effectively removes background and clutter, providing reliable RCS measurements in noisy environments.
In this paper, we present an overview and comparison of various experimental techniques to identify the room scattering contribution to the overall measurement uncertainty in spherical near-field systems. Our primary objective is to determine the upper bound of the uncertainty due to room scattering in the automotive multi-probe system at the Pulsaart by AGC facility. This facility is designed for comprehensive vehicle testing across a broad frequency range from 64 MHz to 6 GHz. At the lower end of this frequency range, room scattering significantly impacts the overall antenna measurement uncertainty budget, making it crucial to quantify the upper bounds of this error. The considered experimental techniques to determine the room scattering contributions include measuring the same Antenna Under Test (AUT) at multiple translated positions within the chamber, employing advanced post-processing techniques to eliminate room scattering and identify its effects, and combining both approaches. This activity aims to define the room scattering contribution, particularly at lower frequencies, to the range’s overall antenna measurement uncertainty budget.
A previously presented prototype of a microwave oil/water ratio measurement system for geological applications has provided positive and applicable results in a commercial flow loop test. The prototype is a two-port circular waveguide traversed by a pipe. Both waveguides face each other on opposite sides of the pipe and are filled with dielectric material, which is molded to match the inside walls of the pipe. For maximizing transmission, avoiding higher order modes, the microwave waveguides in the system are designed with a diameter slightly smaller to that of the pipe. To expand the applications of the current system, a more thorough understanding of the waveguide propagation on a transversal pipe system and its limitations is needed. However, the complexity of the geometry makes the problem analytically intractable. Three dimensional simulations for transversal pipes geometries were performed in COMSOL 6.2 from 1 GHz to 4 GHz. Initial numerical results show waveguide mode propagation in the pipe at lower frequencies and pipe mode propagation at frequencies passed the optimum transmission frequency where the horizontally propagating waveguide/pipe mode wavelength are slightly smaller than the diameter of the waveguide/pipe intersection. Higher order modes can be seen in both waveguide and pipe at frequencies above 2.5 GHz.
We discuss an approach to represent the electromagnetic field radiated/scattered by oblong objects. The field representation exploits Vector Prolate Spheroidal Wave Functions (VPSWFs) which are computed by a stable and accurate numerical scheme and take into account the a priori information about the geometry of the radiator/scatterer. The presented numerical results highlight the satisfactory accuracy and the convenience of the approach against the classical representation by spherical harmonics.
This paper presents a numerical investigation into the cylindrical mode filtering method and the application of the Compressed Sensing (CS) for far-field single-cut antenna pattern measurements. For measuring a single cut antenna pattern in a far- or quasi-far-field distance, cylindrical mode filtering using an intentional offset effectively removes multipath reflections from the test environment. The CS algorithm enhances this method by enabling sampling on an irregularly spaced grid. This investigation uses numerically simulated data to examine the cylindrical mode filtering method, addressing questions about the mechanism of modal separation and sparsification facilitated by the coordinate translation of the pattern to the rotation center. It also discusses potential limitations of the method, including aspects not previously covered in the literature. We then assess the efficacy of modal recovery via CS compared to FFT and pseudo-inverse methods for both non-sparse and sparse modal data. For non- sparse data, the L1 minimization used by CS can accurately compute the antenna modes but does not offer advantages in terms of reducing the number of samples needed. When the modal data is sparse, the CS algorithm not only allows for irregular sampling but also reduces the number of samples below the Nyquist rate. Additionally, the study evaluates the algorithm’s robustness against added noise and compares its performance with traditional dense data acquisition schemes. The findings provide greater insights into the cylindrical mode filtering method and validate the effectiveness of the CS algorithm.
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.
The extrapolation method is widely used for antenna absolute far field gain calibration. The technique involves measuring responses between precisely aligned antenna pairs across varying distances. Previous studies have suggested that how one measures the separation distance—whether from aperture face to face or from phase center to phase center—doesn't influence the resulting far-field gain. However, our present study demonstrates that this assumption is incorrect. The choice of reference points for measuring separation distance can indeed impact the computed far-field gains. Our investigation shows that using the distance from the phase centers provides the most accurate far-field gain. Through numerical experiments and measurement data, we illustrate the discrepancies in the far-field gains caused by different distance definitions. Since the phase center of the antenna under test is usually unknown in practice, finding the phase center separation distances to apply to the extrapolation calculation isn't straightforward. To address this, we introduce a novel searching algorithm that varies an offset distance during polynomial fitting. This generates various convergence curves with different trends and rates, allowing for the accurate determination of phase center separation distances. The proposed algorithm not only enhances the accuracy of the antenna gain extrapolation method but also provides the phase center information of the antenna under test, all without requiring additional measurements.
This paper presents a tracking and localization system for passive RFID tags. The localization and tracking system comprise a rotatable RFID reader and sixteen fixed passive tags spread around the room. By strategically positioning passive tags, we demonstrate the possibility of tracking and localizing any passive RFID tag that enters the system. The localization algorithm represents each tag as an x and y coordinate, with the reader representing the origin. The algorithm runs every 0.5 seconds to update all tag locations. The algorithm uses the distance that the passive tag is from the reader and the angle from the positive x-axis the tag lies on to locate the passive RFID tag. The algorithm finds distance using the RSSI (Received Signal Strength Indicator) value and the tag's angle by taking the active tags' average position in that region. This system is helpful for localization and tracking applications. In environments like warehouses or large outdoor areas, where it's crucial to track items or individuals across a vast space.
This paper presents an analysis of the truncation errors of co-pol and cross-pol data by comparing a far-field pattern obtained from simulation, with different patterns obtained from the near-field to far-field transformation for different scan area sizes. It is shown how these errors are reduced when the scan area is larger, the reason being that more significant fields are being captured by the probe; however, the improvement comes at the expense of longer measurement time. From this problem, a new method is proposed where the system makes sure to measure all the significant fields and avoid the insignificant ones, reducing the measurement time and increasing the accuracy.
The plane-polar approach for near-field antenna measurements has attracted a great deal of interest in the open literature during the past four decades [1, 2, 3, 4, 5, 6, 7]. The measurement system is formed from the intersection of a linear translation stage and a rotation stage with the combination of the axes enabling the scanning probe to trace out a radial vector in two-dimensions facilitating the acquisition of samples across the surface of a planar disk, typically being tabulated on a set of concentric rings. In its classical form, the probe moves in a fixed radial direction and the AUT rotates axially. However, with the ever more prevalent utilization of industrial multi-axis robots and uninhabited air vehicles (UAV), i.e. drones, being harnessed for the task of mechanical probe positioning, such systems offer the possibility of acquisitions being taken across non-planar surfaces. In this paper an accelerated, rigorous, near-field to far-field transform for data that was sampled using a polar acquisition scheme that is based on a Fourier-Bessel expansion [4] is developed and presented that can be employed in the above circumstances. This highly efficient, robust, transform enables near-field data acquired on planar, and non-planar, surfaces to be transformed to the far-field providing the acquisition surface is rotationally symmetric about some fixed point in the x,y-plane with z being purely a function of the radial displacement. The utility of the non-planar acquisition interval stemming from the ability to minimize truncation effects without needing to increase the measurement size. The transform efficiency stems from the utilization of the fast Fourier transform (FFT) algorithm with the rigor and robustness deriving from the avoidance of recourse to approximation, e.g. piecewise polynomial interpolation cf. [7]. Numerical results are presented and used to verify the accuracy and efficiency of the novel transformation, as well as to confirm convergence of the requisite Bessel series expansion and sampling theorem.