AMTA Paper Archive
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Intercomparisons of Site VSWR Measurement Methods using Mode Filtering, Time Domain and Spatial Sampling Techniques
The validity and viability of using frequency domain mode filtering to qualify an EMC chamber above 1 GHz has been demonstrated in a previous study . The novel approach overcomes the difficulties with under sampling encountered in the traditional spatial sampling method adopted by CISPR 16-1-4, and it also has the distinct advantage over the time domain method adopted by ANSI C63.25.1 in that broadband and low ring-down antennas are not required. In this study, we further examine one of the assumptions made in the previous study to translate the quasi-far-field pattern to the rotation center. The approximate method is compared to a more rigorous method by using a quasi-far-field to far-field transformation first before applying the phase translation and subsequent mode filtering. In this paper, we further validate the method by conducting an intercomparison study based on measurements conducted in a 3 m anechoic chamber to show the correlations of the mode filtering method to the CISPR and the time domain (TD) site VSWR methods. We demonstrate how the proposed method improves the test repeatability, lowers measurement uncertainties, and increases measurement efficiencies.
Errors and Prerequisites of the Short-Time Measurement and Transformation of Continuously Modulated Fields
Near-field far-field transformations (NFFFTs) are usually performed for time-harmonic fields. In cases where insitu antenna measurements are required and the antenna under test (AUT) is not accessible for specifically tailored test signals, the need for handling time-modulated fields arises. The shorttime measurement (STM) approach offers a way to deal with continuously modulated fields while a time-harmonic NFFFT can be employed. We present results of numerical simulations to demonstrate and characterize the STM approach for the case of a cylindrical measurement geometry as found in UAV-based antenna measurements. We further derive guidelines from the simulation results that describe the applicability of the STM for different measurement situations.
Analysis of Time and Direction of Arrival (TADOA) Data using Basis Pursuit in the AFRL One-RY Antenna Measurement Range
Time and Direction of Arrival (TADOA) analysis of field probe data has been an accepted method for characterizing stray signals in an antenna measurement range for many years (, ). Recent uncertainty investigations at the OneRY range have shown a need for increased resolution to isolate and characterize energy in TADOA images so that resources can be carefully applied to reduce the uncertainty from these stray signals. This is accomplished by modeling the TADAO image as the solution to a Basis Pursuit (BP) l1 minimization problem. This paper outlines the model development and shows concrete examples from OneRY field probe data where BP allows for the identification of stray energy which was previously difficult to find. We also show how the BP optimization context can be using to remove contamination from the data through the inclusion of additional basis functions (). I.J. Gupta, E.K. Walton, W.D. Burnside, “Time and Direction of Arrival Estimation of Stray Signals in a RCS/Antenna Range,” Proc. of 18th Annual Meeting of the Antenna Measurement Techniques Association (AMTA '96), Seattle WA, September 30-October 3, 1996, pp. 411-416. I.J. Gupta, T.D. Moore, “Time Domain Processing of Range Probe Data for Stray Signal Analysis,” Proc. of 21st Annual Meeting of the Antenna Measurement Techniques Association (AMTA '99), Monterey Bay CA, October 4-8, 1999, pp. 213-218. B.E. Fischer, I.J. LaHaie, M.H. Hawks, T. Conn, “On the use of Basis Pursuit and a Forward Operator Dictionary to Separate Specific Background Types from Target RCS Data,” Proc. of 36th Annual Meeting of the Antenna Measurement Techniques Association (AMTA '14), Tucson AZ, October 12-17, 2014, pp. 85-90.
A High Precision Group Delay Measurement Method for Circular Polarized High Gain Antennas
In this contribution we demonstrate a method to measure the absolute Group Delay (GD) of a high gain dual feed offset reflector antenna for circular polarized signals in Ku- and S-band by which we reach a measurement accuracy better than 10 picoseconds. At first we discuss the definition and different possible measurement methods of GD. We specifically show that the utilization of the antennas phase centre does not lead to the demanded measurement accuracy. Instead we propose a measurement method that uses an electrically small Reference Antenna (RA). We use the measurement of the GD of the RA as a reference for the GD of the Antenna Under Test (AUT). Therefore the exact positions of the reference planes of the corresponding wave guide ports have to be ensured. For this we made use of a theodolite. These measurements must be performed in a Compensated Compact Range to meet the strict requirements of plane waves. Here the CCR of the Lab for Satellite Communication, Munich University of Applied Sciences was used. The GD of the (electrically small) RA is determined by measuring the GD of two identical RAs separated by an exact known free space distance and by referencing these measurements to the measured GD of the same arrangement, where the free space is bypassed by a long high precision rectangular wave guide with well-known dimensions. We demonstrate that by using a soft gating method the accuracy of the measurement results can be tremendously improved. Measurement results parametrized by the width of the gate window in the time domain are discussed. We further discuss the accuracy of the measurement results quantitatively and we especially show, that the influence of an antenna misalignment is negligible, as long the alignment error is smaller than the one dB power beam width. The measurement campaign was commissioned by the European Space Agency (ESA) to meet the requirements of the project Atomic Clock Ensemble in Space (ACES). By ACES a microwave link is used to compare the times given by different atomic clocks in space and on earth, so three ACES ground terminals were tested.
Time Gating Based on Sparse Time Domain Signal Reconstruction from Limited Frequency Domain Information
Time gating is one of the most widespread techniques to suppress the effect of unwanted echoes on antenna measurements. It just requires the measurement of the antenna under test (AUT) for a carefully chosen bandwidth and frequency step size and the isolation of the direct AUT signal contribution from the echo contribution in time domain is quite intuitive. Although a frequency sweep is usually fast compared to the axes movement, it might become the speed limiting factor for large measurement bandwidths. Thus, time gating techniques that need a minimum bandwidth are beneficial. Therefore, a gating method is presented that reconstructs a time domain signal with high resolution from a minimum measurement bandwidth based on the assumption that the time domain signal is sparse, i.e. it mainly consists of samples with low amplitude and only few samples with high amplitude which are related to the peaks of the direct and the echo signal. The effectiveness of the proposed method is compared with the well known fast Fourier transform (FFT) and matrix pencil method (MPM) based techniques using echoic near-field antenna measurement data.
Implementation and Testing of Engineered Anisotropic Dielectric Materials
Several instances in antenna design are known where an anisotropic material is useful ; however, finding a naturally occurring anisotropic material with the required dielectric tensor is often an impossibility. Therefore, artificially engineered anisotropic dielectric materials must be designed, tested, and implemented. In a previous paper by the authors , the design and initial measurement of an anisotropic material in Cartesian coordinates was presented along with predictions of how the material could be used to extend the bandwidth of a simple antenna structure. In this paper we shall present the final implementation of the anisotropic material (with a tensor implemented in cylindrical coordinates) along with data on the material properties, the resulting antenna bandwidth, and radiation pattern. Design considerations for implementation of this approach shall be discussed along with practical limitations. Data shall also be presented on an unexpected result showing that that a reduced volume of anisotropic material produces favorable results. Measured data shall be compared with values predicted using finite difference time domain (FDTD) software and applications of this new broadband antenna for range operations will be discussed. . D. Tonn, S. Safford, M. Lanagan, E. Furman, S. Perini, “DESIGN AND TESTING OF LAYERED ANISOTROPIC DIELECTRIC MATERIALS”, AMTA 2015 Proceedings, Long Beach CA, October 2015.
A study of the Low-frequency Coaxial Reflectometer measurement procedure for evaluation of RF absorbers’ reflectivity
This paper presents a study on the low-frequency coaxial reflectometer measurement procedure. A time domain gating algorithm is developed by ETS-Lindgren and the results are validated after comparing to the Keysight 8753-time domain algorithm. The in-house time gating algorithm is then applied to the simulated reflectivity results of absorbers in reflectometer to the simulation results of the same absorbers with plane wave excitation using finite element method numerical computation. Based on the simulation results, the operable upper frequency limit and the minimum length of the straight coaxial section for the reflectometer are suggested. The errors introduced during measurement due to higher order modes are studied and the permissible limit for the errors is analyzed. The different higher order modes and their effects on field distribution are studied. The impact of the non-uniform field distribution on the absorber reflectivity measurement is also discussed.
A New Method for VHF/UHF Characterization of Anisotropic Dielectric Materials
Recent interest in anisotropic metamaterials and devices made from these materials has increased the need for advanced RF material characterization. Moreover, the quest for measurement of inhomogeneous and anisotropic materials at VHF and UHF frequencies has long been one of the primary stretch goals of the RF materials measurement community. To date, the only viable method for these types of materials has been either fully filled or partially filled VHF waveguides, which are large, expensive, and slow. This paper introduces a new fixture design that greatly simplifies the process of obtaining intrinsic properties for inhomogeneous and anisotropic dielectric materials. The fixture combines low frequency capacitance and high frequency coaxial airline concepts to measure cube shaped specimens, and is termed an “RF Capacitor”. Furthermore, a significant limitation of past measurement methods is their reliance on approximate analytical models to invert material properties. These analytical models restrict the available geometries and frequency ranges that a measurement fixture can have. The present method avoids this limitation by implementing a new inversion technique based on a full-wave, finite difference time domain (FDTD) solver to exactly model the measurement geometry. In addition, this FDTD solver is applied in a novel way to enable inversion of frequency-dependent dielectric properties within seconds. This paper presents the fixture design and calibration for this new measurement method, along with example measurements of isotropic and anisotropic dielectric materials. In particular, 3” cube specimens are measured and the bulk dielectric properties in the three principal planes are determined by measuring the same specimen in three different orientations within the measurement fixture. Finally, calculations are presented to show the relative accuracy of this method against a number of probable uncertainty sources, for some characteristic materials.
Scattered Fields from a Panel
There is a lot of interest in measuring the scattered fields from a panel. The panel could be a frequency selective surface (FSS), could consist of lossy dielectric material, resistive material, etc. For these measurements, the panel is mounted in a large ground plane (perfectly conducting) that mimics an infinite ground plane and the back scattered/bistatic scattered fields are measured. These measured fields contain the scattering from the panel under test as well as the diffracted fields from the junction between the panel and the ground plane, and it is quite difficult to discern the two field components. Alternatively, one can measure the scattered fields over a frequency band in the near zone using a fixed transmitting antenna while the receiving antenna is displaced to scan a planar surface or a linear scan. Note that the measurements are similar to one-way probing. The total measured scattered fields can be processed to isolate the scattering from the panel of interest. In this paper, we will present various signal processing techniques that can be applied to the measured scattered field data. These techniques include high resolution down range processing (tie domain), time domain near field focusing, etc. We will also show that it is straight forward to obtain the reflection and transmission coefficient of the panel from the near field measured data.
Simulation Experiments with Ultra-Wideband Antennas and Arrays in the Time Domain
The performance of a typical narrowband antenna array is reduced by mutual coupling between radiating elements. The degree to which this inter-element coupling occurs may be correlated with the resonant characteristics and tendency for late time ringing of an individual element. The parabolic reflector impulse radiating antenna (IRA) is an ultra-wideband (UWB) antenna which by virtue of its design and aim to radiate a very short time-domain signal demonstrates significantly decreased late time ringing. Given this quality, the suitability and performance of the reflector IRA in an array configuration is examined. Modeling and simulation of the reflector IRA is accomplished using commercially available software and single antenna results are compared to measured data. Full-wave simulation of arrayed reflector IRAs in varying physical configurations and excitation modes is performed The relative levels of coupling and degradation of radiation pattern and signal quality are discussed.
An Novel Near-Field Imaging Method Using FrFT Technology
We present a microwave near-field imaging method using Fractional Fourier Transform (FrFT). Since the FrFT of finite chirp signal including a Fresnel integral, the scattering field of target in Fresnel zone can be described in the FrFT form. It is a valuable expression of scattering, because it unified the expression of scattering field in three different range along with distance, Rayleigh zone, Fresnel zone, and Fraunhofer zone. Then, one can get the target image from the Fresnel scattering field using the inverse FrFT (IFrFT) technology. The fast algorithm of FrFT or IFrFT has the same algorithm complexity as Fast Fourier Transform (FFT), which made a fast near-field imaging method compare to conventional method like Back Projection (BP), Time Domain Correlation (TDC), etc. Two near-field imaging results, one simply target and one complex target, will be presented to prove the method.
A Dual-Linearly-Polarized Horn with Low Sidelobes for the upper VHF range: The Quasi-Open-Boundary Quad-Ridged Antenna
RCS Measurements at the upper half of the VHF range of the spectrum have become increasingly important. This type of measurement is usually performed in an outdoor RCS range. The present paper shows a design for an antenna that can be used to illuminate a reflector or as the illuminating structure in a RCS measurement. The antenna is fairly compact given the wavelength and exhibits a low VSWR and a good time domain performance for use with pulses. The new antenna has low side-lobes that otherwise could illuminate adjacent structures to the outdoor range and reduce the dynamic range of the measurements, this is an improvement over the Open-Boundary Quad-Ridged Horns Introduced over the past 9 years. The new Feed is a Quasi-Open-Boundary Horn, in which RF absorber material is used to create the Open Boundary behavior, but an enclosed structure is used to block the potential side-lobe radiation.
Range Domain Filtering: Application to Zero Doppler and High Doppler Removal
Two filtering applications using the Fourier Transform of frequency diverse (chirp) data into the range/time domain are demonstrated. First, a Zero Doppler Clutter (ZDC) estimate acquired by averaging in the frequency domain is optimized by pre-filtering in the range domain. By suppressing information based on location and/or return value prior to inverse transforming back to the frequency domain, non-zero Doppler scatterer information can be omitted from the ZDC estimate prior to averaging. Second, by considering the phase difference between range bins of adjacent samples, high-doppler artifacts can be identified and removed in the range domain. Additional rules based on range domain information are used to further refine the filtering. The test case for high-doppler removal is an ISAR scenario with flying insects moving much faster than the rotating speculars of the target scene.
An Artificial Lossy Dielectric Material Standard for RF Free Space Measurements
A new material validation and verification standard is designed to imitate the behavior of a lossy dielectric absorber. This standard is constructed from well-characterized, low-loss materials in a manner that ensures manufacturing repeatability. The performance of this standard is verified with S-parameter and permittivity measurements in a free space focused beam system and with finite difference time domain simulations. A sensitivity analysis, based on a series of simulations, is presented to quantify the uncertainty in the measured S-parameters due to dimensional and alignment variations from the ideal design values.
A Standard for Characterizing Antenna Performance in the Time Domain
We derive here a simple function describing antenna performance in the time domain. This single function describes antenna performance in both transmission and reception, and in both the time and frequency domains. The resulting equations are as simple as one could hope for. The function isolates the performance of the antenna from the impedances of the source, load, and feed lines. From this function one can simply derive such conventional frequency domain quantities as gain, realized gain, and antenna factor. It is hoped that this function will be adopted as an IEEE standard for time domain antenna performance.
Evaluating the Time Domain Performance of Spiral Antennas Using Near Field Measurements
Ultra wideband (UWB) systems use short pulses in order to achieve high data rate wireless communications and/or radar resolution. Thus, UWB antennas should be designed carefully, both in time and frequency domains, with the system performance in mind. Time domain characterization of an antenna can be performed first by measuring the frequency domain transfer function of a direct link consisting of two identical antennas. Then, the time domain response is obtained by post processing the frequency domain data using the Inverse Fast Fourier Transform (IFFT). This paper discusses frequency and time domain performance of four-arm equiangular and Archimedean spiral antennas operating in mode 2. The frequency domain transfer function is synthesized using complex far field information measured in a spherical near-field chamber from 2GHz to 12GHz. The synthesized approach is validated using simulation and direct link measurements. The quality of radiated pulses is evaluated in terms of fidelity factor over a full field of view, a task not trivial for the direct link measurements.
Reflectivity Evaluation in NF antenna Measurement Facilities Using Gated Time - Domain Technique
A widely used time-gating technique can be effectively implemented in near-field (NF) antenna measurements to significantly improve the measurement accuracy. In particular, it can be implemented to reduce or remove the effects of the following measurement errors : -multiple environmental reflections and leakage in outdoor or indoor NF ranges -edge diffraction effects on measurement accuracy of low gain antennas on a ground plane  In addition, reflectivity in the range can be precisely localized, separated and quantified by using the time – gating procedure with only one addition (a subtraction operation) added to the standard near-field to far-field (NF – FF) transformation algorithms. In this paper a step by step procedure is described which includes acquisition of near-field data, transformation of the raw near-field data from the frequency to the time domain, definition of the correct time gate, transformation of the gated time domain data back to the frequency domain, and the transformation of the time gated near-field data to the far-field. The time gated results, as already shown in , provides for more accurate far-field patterns. In this paper it is shown how the 3D reflectivity/multiple reflections in the measurement chamber or outdoor range can be determined by subtracting the time gated results from the un-gated data. This technique is illustrated through use of several measurement examples. It is demonstrated that the time gated method has a clear physical explanation, and, in contrast with other techniques [4,5] is less consuming (does not require mechanical AUT precise offset installation, additional measurement and processing time) and allows for a better localization and quantization of the sources of unwanted radiation. Therefore, this technique is a straightforward one and is much easier to implement. The main disadvantage cited by critics regarding use of the time gating technique is the narrow frequency bandwidth used in many NF measurements. However, it is shown, and illustrated by the examples, that the technique can be effectively implemented in NF systems with a standard probe bandwidth of 1.5:1 and an AUT having a bandwidth as low as 5% to 10%.
Efficient Analysis of Multi-Layer Periodic Structures Using FDTD
Many periodic structures are often built up of multiple layers to improve the electromagnetic performance such as the frequency bandwidth. Two approaches can be employed to analyze multi-layer structures: one is to formulate and analyze a multi-layer structure in its entirety; the other is to compute the generalized scattering matrix (GSM) for each layer and then obtain the total GSM of the structure by simple matrix calculations. The second approach is more flexible and efficient to practical problems where several layers may be cascaded in arbitrary sequences. This paper describes an efficient procedure to analyze multi-layer periodic structure using the finite difference time domain (FDTD) method. Based on the constant horizontal wavenumber approach, the procedure first computes the GSM of each periodic layer. The scattering parameters of the entire multi-layer structure are then calculated using the cascading formulas. The validity of this algorithm is verified through several numerical examples including frequency selective surfaces (FSS) with different periodicities and under different incident angles. The numerical results of the developed approach show good agreement with the results obtained from the direct FDTD simulation of the entire structure, while the new procedure saves the computational time and storage memory.
A new absorber Layout for a spherical near field scanner
A well designed absorber configuration is a key factor for precise antenna measurements. Unfortunately, even a scanner covered with pyramidal absorbers can cause reflections that could degrade the measurement accuracy. A novel scanner absorber configuration using bent absorbers is presented in this paper. Another problem is that in most cases it is necessary to remove the absorbing material at the scanner to change the antenna under test. The absorbers covering the scanner suffer abrasion caused by the frequent manual movement. For this reason it was also the intention to find a faster and easier solution which also preserves the absorbing material. The new and the old absorber layout were benchmarked using a number of spherical nearfield measurements as well as time domain reflection measurements with a broadband probe antenna. A comparison of the results is also shown in this paper.
Generation of a pseudo time domain holography from frequency swept measurements
This paper presents the methodology to generate a pseudo time domain holography from frequency swept measurements. This is an approximation to the time domain holography (TDH) invented by the authors [1,2], which opens a new possibility for antenna diagnostics using conventional instrumentation and in the absence of time domain measurements. Practical examples using two spaceborne antennas are provided and discussed.
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