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This paper introduces a new spherical near-field sampling grid, which we shall refer to as the equally-spaced sampling grid, where the sample points are spread over the surface of the measurement sphere using an almost-uniform triangular tessellation of the unit sphere. By spacing the points evenly over the surface of the sphere, the grid uses 50% the number of sample points as the classical grid. In addition, it can be shown that the equally-spaced grid requires the use of fewer points than required by the commonly cited “Nyquist criteria” at certain values of ka. Since the spherical modes required for the SNF-to-FF transform cannot be obtained from the equally-spaced grid through traditional FFT methods, the solution is demonstrated by other means. The details behind a brute-force matrix inversion method and a brief discussion on the condition number of the equally-spaced sampling grid will be included. Some comparisons will be shown between results obtained using the brute-force matrix inversion method and results obtained using the more computationally-efficient conjugate gradient method (as proposed by Wittmann, Alpert, and Francis).
Electrically small antennas present tremendous design challenges. Plagued with a small radiation resistance and high quality factor (thus narrow bandwidth), these types of antennas are difficult to accurately measure. For use in HF communication applications, the problems associated with the entire development cycle become even more pronounced. This paper focuses on the development of two such electrically small HF antennas for a vehicular platform, specifically the Amphibious Assault Vehicle (AAV). The primary design objective is to develop antennas that operate over the entire near-vertical incidence (NVIS) band (2 – 10 MHz) with a minimum of 3kHz bandwidth. Additional design objectives are low profile, broadside directive pattern, and high power handling capability. The inverted L antenna and the half loop antenna were selected as probable candidates for this application. At 2 MHz, the antenna – vehicle system fits within the envelope ka < 0.2, where k is the free space wave number and a is the radius of a sphere completely enclosing the radiator. The full scale antenna design and performance were evaluated using method of moments and finite element method codes FEKO and HFSS respectfully. It is observed that the presence of the real ground plane poses a serious challenge for well established modeling techniques and considerable care must be exercised to obtain credible design data. For measurement validation and characterization of the antenna/vehicle interaction, a set of scaled antenna and vehicle prototypes were developed. Rapid prototyping and 3D printing were employed to build a scaled model (1:50 scale) of the complete antenna – vehicle system. The step-by-step process from the computational model to the measurement validation is discussed along with the description of the adopted fabrication techniques. In the concluding section of the paper, the measured results from the scaled model are presented alongside the simulated results. The good agreement between these results paves the way towards the successful use of such scaled model testing for more complicated antenna designs in the future.
Historically, the inverse synthetic aperture radar (ISAR) reflectivity assumption has been used in the implementation of Image-Based Near Field-to-Far Field Transformations (IB-NFFFT) to estimate monostatic far field radar cross-sections (RCS) from monostatic near field radar measurements. The ISAR assumption states that all target scattering occurs at the location of the incident field excitations, i.e., the target is composed entirely of non-interacting localized scatters. Certain non-localized scattering phenomenon cannot be effectively handled by the IB-NFFFT approach with the ISAR assumption. Here we have used the adaptive Gaussian representation, which is a joint time-frequency decomposition technique, to coherently decompose near field measured data into two subsets of scattering features: one subset of localized scatterers and the other of non-localized scatterers. The localized scattering features are processed through the IB-NFFFT as typical, which includes compensating for the R4 fall-off present in the near field measured data. The non-localized scattering features, more appropriately scaled, are then coherently added back in to the post-NFFFT localized scattering phase history. Although this does not properly transform the non-localized scattering features into the far field, it does avoid the over-estimation error associated with improperly compensating distributed non-localized scattering features by a R4 power fall off based strictly on downrange position.
If a planar near-field (PNF) scanner is large and there is insufficient temperature regulation in the chamber to keep ordinary thermal expansion/contraction from causing unacceptable position errors, then consideration must be given to compensation techniques that can adjust for the changes. Thermal expansion/contraction will affect almost everything in the chamber including the floor, the scanner structure, the encoder or position tapes, the AUT support, and the mount for any extra instrument(s) used to measure and correct for position error. Since the temperature will generally cycle several times during a lengthy acquisition, error-correction solutions must account for the dynamic nature of the temperature effects. This paper describes a new automated tracking-laser compensation subsystem that has been designed and developed for very large horizontal PNF systems. The subsystem is active during the acquisition to account for both static and dynamic errors and compensates for those errors in all three dimensions. The compensation involves both open-loop corrections for repeatable errors with high spatial frequency and closed-loop corrections for dynamic errors with low spatial frequency. To close the loop, laser data are measured at a user-defined interval between scans and each scan that follows the laser measurements is fully compensated. The laser measurements are fully automated with no user interaction required during the acquisition. The challenges, goals, and assumptions for this development are listed, high-level implementation considerations are provided, and resulting measured data are presented.
Inter-comparisons and validations of antenna measurement ranges are useful tools allowing the detection of various problems in the measurement procedures, thus leading to improvements of the measurement accuracy and facilitating better understanding of the measurement techniques. The maximum value from a validation campaign is achieved when a dedicated Validation Standard (VAST) antenna specifically designed for this purpose is available. The widespread use of the known VAST-12 (12 GHz) antenna, developed by the Technical University of Denmark (DTU) in 1987 and operated by the DTU-ESA Facility since 1992, demonstrates the long-term value of the dedicated VAST antennas [1]. The driving requirements of VAST antennas are their mechanical stability with respect to any orientation of the antenna in the gravity field and thermal stability over a given operational temperature range. The mechanical design shall ensure extremely stable electrical characteristics with variations typically an order of magnitude smaller than the measurement uncertainty. At the same time, it must withstand high g-loads under frequent transportations and it shall also support convenient handling of the VAST antenna (practical electrical and mechanical interfaces, low mass, attachment points for lifting, etc.). Today, there is a well identified need for increased operational frequencies to get access to large bandwidth for broadband communication. Upcoming satellite communication services utilize up/down link at K/Ka-bands, while the use of Q/V bands is contemplated for the feeder links in the coming years. In response to this need, a millimeter-wave VAST (mm-VAST) antenna is currently under development in a collaboration between the DTU and TICRA under contract from the European Space Agency. In this paper, the electrical and mechanical requirements of the DTU-ESA mm-VAST antenna are discussed and presented. Potential antenna types fulfilling the electrical requirements are briefly reviewed and the baseline design is described. The emphasis is given to definition of the requirements for the mechanical and thermal stability of the antenna, which satisfy the stringent stability requirement for the mm-VAST electrical characteristics. [1] S. Pivnenko et al., “Comparison of Antenna Measurement Facilities with the DTU-ESA 12 GHz Validation Standard Antenna within the EU Antenna Centre of Excellence”, IEEE Trans. Antennas Propagat., 2009, vol. 57, no. 7. pp. 1863-1878.
In this work, we propose a new method for measuring the gain of a circularly polarized antenna at millimeter wave (mm-wave) and terahertz (THz) frequencies. Traditional methods of CP-gain measurement require measurement of linearly polarized gain in two orthogonal planes. However, for mm-Wave frequencies, this method is not practical since rotation of antenna under test can cause errors in measured phase. To avoid this, we present a novel phase-less method. For validation, circularly polarized slot array antenna under test is measured at 106 GHz. The antenna design and fabrications process is also presented.
One design of a smart plasma antenna is to surround a plasma or metal antenna by a plasma blanket in which the plasma density can be varied. In regions where the plasma frequency is much less than the antenna frequency, the antenna radiation passes through as if a window exists in the plasma blanket. In regions where the plasma frequency is high the plasma behaves like a perfect reflector with a reactive skin depth. Hence by opening and closing a sequence of these plasma windows this design can be computerized to electronically steer or direct the antenna beam into any and all directions. The plasma windowing design is one approach to the smart plasma antenna design. The beamwidth can vary from an omnidirectional radiation pattern with all the plasma windows open or a very directional radiation pattern when only one plasma window is open. The advantages of the plasma blanket windowing design are: 1. Beam steering of one omnidirection antenna with the plasma physics of plasma windowing. 2. A reconfiguable directivity. 3. The beamwidth can vary from an omnidirectional radiation pattern with all the plasma windows open to a directional radiation pattern with less than all the plasma windows open.
Satellite plasma antennas benefit from the lower thermal noise at the frequencies they operate. Ground-based satellite antennas point at space where the thermal noise is about 5K. A low thermal noise, high data rate satellite plasma antenna system is possible with low noise plasma feeds and a low noise receiver. Satellite plasma antennas can operate in the reflective or refractive mode. Satellite plasma antennas can be flat or conformal and effectively parabolic. Electromagnetic waves reflecting off of a bank of plasma tubes get phase shifted as a function of the plasma density in the tube. This becomes an effective phase array except that the phase shifts are determined by the plasma density. If the plasma density in the tubes is computer controlled, the reflected beam can be steered or focused even when the bank of tubes is flat or conformal. In the refractive mode, the refraction of electromagnetic waves depends upon the density of the plasma. In the refractive mode, steering and focusing can be computer controlled even when the bank of tubes is flat. For two-dimensional steering and/or focusing, two banks of plasma tubes are needed. Receivers can be put behind satellite plasma antennas operating in the refractive mode.
Plasma frequency selective surfaces (FSS) use plasma instead of metal for the FSS elements. In conventional metal FSS, each layer has to be modeled using numerical methods and the layers are stacked in such a way to create the desired filtering for antenna radomes. Genetic algorithms are sometimes used to determine the stacking needed for the desired filtering. This can be complicated, numerically expensive process. The plasma frequency selective surfaces can be tuned to a desired filtering by varying any or all of the density, size, shape, and spacing of the plasma elements for reconfigurable filtering in antenna radomes. As the density of the plasma is increased, the plasma skin depth becomes smaller until the elements behave as metallic elements and we create filtering similar to FSS with metallic elements. Up until the metallic mode for the plasma, our theory and experiments showed that the plasma FSS had a continuous change in filtering. A model for a plasma FSS is developed by modeling the plasma elements as half wavelength and full wavelength dipole elements in a periodic array on a dielectric substrate. The theoretical model with numerical predictions predicted results in good agreement with our experiments on the plasma FSS.
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.
In array signal processing; i.e. direction of arrival estimation, digital beam forming/ nulling one needs to know the antenna array manifold (relative gain and phase of individual antenna elements) as well as the relative response of RF front end of the various channels. Here the RF front end is defined as LNA, filters, various down convertors and A/Ds. Since RF front end response is highly dependent on the physical environment, in general, nearly real time calibration of the RF front end is carried out. To accomplish this, a pilot signal is injected in various channels or a strong signal is received from a known direction. The received pilot is isolated, and the isolated signal is processed for relative calibration of the various front ends. One simple processing technique is to divide the signal received at a given frequency by the channel of interest with the signal received by the reference channel at that frequency. This technique works fine when the pilot signal has high SNR (20 dB or so). For low SNR, the front end calibration will have large variance and also bias, which is undesired. One can also cross correlate the signal received by the two channels at the frequency of interest, and normalize the cross correlation with the autocorrelation of the signal received by the reference channel. We will show that for low SNR of the pilot signal, this approach can lead to bias in the estimated relative magnitude. We will also present a novel approach that leads to unbiased estimate with small variance. The approach is based on signal space idea. At the given frequency, we generate a covariance matrix that contains the correlation between the signals received by various channels. Next, the principle eigenvector (corresponding to the largest eigenvalue) of the covariance matrix is calculated. The eigenvector is adjusted such that its element corresponding to the reference channel is unity. Then the other elements of the eigenvector yield the relative response of the front end of various channels. We have applied the suggested approach to real world data with very good results. Some examples are discussed in the paper.
Abstract – This paper introduces a method for calibrating the gain and the frequency dependent phase center locations of Log Periodic Dipole Arrays (LPDAs). The method builds upon the three antenna method, but is conducted over a PEC ground plane in an Open Area Test Site (OATS). Similar to the traditional three antenna method, three pairings of transmission measurements are taken. In each measurement, one antenna is set at a fixed height above the ground plane, while the other antenna is scanned in height over 1 to 4 m heights. Magnitude and phase responses between the two antennas are taken at multiple heights. Measured results are fit to a theoretical model using a complex fit algorithm. From this process, the gain and frequency dependent phase center locations of each antenna can be solved. Measurement data show that it is effective in reducing systematic uncertainties associated with assuming fixed phase center locations. In addition, unlike other calibration methods over a conducting ground plane, no assumptions are made about the antenna patterns. This method provides an accurate, versatile and fast method for calibrating LPDAs from as low as 100 MHz.
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.
Antenna, Radome, and RCS testing systems rely on high-fidelity positioner systems to provide high-precision positioning of articles for RF testing. Historically, the industry has relied on linear PID control techniques in torque, velocity, and position control loops on individual axes to drive the positioners. Recently, advancements have been made in the use of advanced control hardware including multiple-DOF laser and optical feedback devices, brushless DC motors, VFD AC motors, and multi-drive torque-biased actuation. Advanced control techniques including single-axis error correction, multi-axis global error compensation, and multi-axis coordinated motion have been implemented to improve positioner accuracy. Here, a survey is conducted of control technologies in other industries such as machine tools and industrial robotics. An assessment is conducted on the viability of other advanced techniques to provide insight into the potential future control and capabilities of positioning systems in the RF testing industry. Candidate advanced techniques include gain scheduling and sliding-mode control which could provide improved accuracy over a wider range of conditions including varying loads and operating points caused by differing movement speeds or large variations in static loading. Dynamic input-shaping and feed-forward techniques could help suppress dynamic vibrations and improve dynamic tracking behavior for improved continuous-measurement scanning accuracy. Adaptive and non-linear control techniques might improve disturbance and error rejection for improved accuracy while managing dynamic-behavior drift allowing for adaptation to long-term positioner changes without re-tuning.
A Gaussian electromagnetic radiation is always attractive for many scientific research and some practical applications. The importance of the Gaussian microwave beam, especially for high power microwave region of operation, is that its maximum energy density is concentrated on its axis. A two-spiral corrugated Bragg reflector, at the inner surface of a circular waveguide, is a novel way to provide such a radiation. The Bragg reflector has been designed and optimized using the fully electromagnetic HFSS tool. Such a reflector converts the operating TM01-mode of the Cerenkov devices to the forward TE11-mode with a Gaussian microwave beam at the output. The use of the Bragg reflector is not only to reflect the injected TM01-mode but also to convert it to a clean TE11-mode pattern. A cold test structure is fabricated to test the theoretical predictions of the microwave transmission versus frequency and the dispersion characteristics. The dispersion relation is found from the discrete measured resonant frequencies and wave numbers of a cavity containing eight periods of the slow-wave structure. Generally, a slow wave structure has N periods will exhibit N+1 resonant frequencies when shorted at planes of mirror symmetry. The main purpose of this study is to experimentally determine the dispersion relation of the structure. Test results using a vector network analyzer showed a good agreement with the simulations for the excitation of the TM01-mode at 10 GHz.
In military applications, where low sidelobes and high precision in beam pointing are vital, a phased array antenna beamformer requires to be calibrated regarding the cabling that connects the beamformer to the antenna and mutual coupling between antenna elements. To avoid problems associated with mismatched phase transmission lines between the beamformer and the antenna and include the coupling effects, beamforming network characterization must be done with the antenna integrated to the beamformer. In this paper, a method to characterize the beamformer of a slotted waveguide array antenna in the antenna measurement range is introduced. The antenna is a travelling wave slotted waveguide array scanning in the elevation plane. The elevation pattern of the antenna is a shaped beam realized by a phase-only beamformer. The calibration channel includes serial cross-guide couplers fed by a single waveguide line. The channel is integrated to the waveguide lines of the antenna. In the first phase of the characterization, the far field pattern of each antenna element is obtained from the near field measurements at the “zero” states of the phase shifters. In the second stage, all states of the phase shifters are measured automatically using the calibration channel described above. The results of calibration channel measurements are used to determine the changes in phase and magnitude for different states of phase shifters. The phase and magnitude of the peak point of the far field pattern is referenced to the zero state measurement of the calibration channel. Phase only pattern synthesis is carried out using the results of both zero-state near field and calibration channel measurements and the required phase shifter states are determined accordingly. Measured patterns show good agreement with the theoretical patterns obtained in the synthesis phase.
This paper describes the mechanical design, control instrumentation and software for a precision model positioning system developed for use in the Experimental Test Range (ETR) electromagnetic test facility at NASA Langley Research Center. ADC has a contract to design, build, and install major components for an updated indoor antenna characterization and scattering measurement range at NASA Langley Research Center. State-of-the-art electromagnetic systems are driving a demand to increase the precision and repeatability of electromagnetic test ranges. Sophisticated motion control systems can help meet these demands by providing electromagnetic test engineers with a level of positioning fidelity and testing speed not possible with previous generation technology. The positioning system designed for the Experimental Test Range at NASA Langley Reseach Center consists of a rail positioning system and four rail positioning carriages: an antenna measurement positioner, scattering and RCS measurement pylon, an azimuth rotator to support foam columns, and an electric personnel lift for test article access. A switching station allows for rail positioning carriages to be quickly moved on and off of the rail system. Within the test chamber there is also a string reel positioning system capable of positioning test articles within a 40’ x 40’ x 25’ volume. Total length of the rail system is 112’ with laser position encoding for the final section of the rail system. Linear guide rails are used to support the carriages and each carriage is position with a rack and pinion drive. Rails mount to steel weldments that are supported with 8” diameter feet. Capacity of the rail system is 7,300 lbs. A switching station allows for positioning components to be moved off of and onto the rail system independently and a place to dock positioning components when they are not in use. A curved linear guide rail supports the switching station so that the platform can be rotated manually. Hardened tapered pins are used to align the switching station with mating rail segments. The scattering and radar cross section (RCS) measurement pylon is a 4:1 ratio ogive shape and has a 3,000 lb load capacity. A pitch rotator tip or spline driven azimuth tip can be mounted to the pylon. The spline drive shaft can be removed to allow for the pitch tip to be mounted to the end of the pylon. Total height of the pylon is 18’ from the floor to the pitch positioner mounting plate. Keywords: RCS, Scattering, Pylon, Positioner, Antenna Design, Rotator, Instrumentation, electromagnetic, Radio Frequency, Radar
Antenna measurement facilities face their physical limits with the growing size of today’s large and narrow packed antenna farms of telecom satellites but also of large unfurlable reflector antennas for low frequency telecom applications. The special operational constraints that come along when measuring such large future antennas demand for new measurement approaches, especially if the availability or realization of present measurement systems with large anechoic chambers is not an option. This paper presents a new system called PAMS (Portable Antenna Measurement System). The most characteristic part of PAMS is that the RF instrumentation is installed inside a gondola that is positioned by an overhead crane. The gondola is equipped with one or several probes to scan the near-fields of the antenna under test. With a modified crane control the gondola can be placed anywhere within the working space of the crane, which is considered as being giant in comparison to measurement volumes of existing large antenna test facilities. The whole system supports but is not limited to common classical near-field scanning techniques. Thanks to new near-field to far-field transformations the system can deal with arbitrary free form scanning surfaces and probe orientations allowing measurements that have been constrained by the classical near-field theory so far. The paper will explain the PAMS concept on system level and briefly on sub-system level. As proof of concept, study results of critical technologies are discussed. The paper will conclude with the status about on-going development activities.
Constitutive parameter characterization of a biaxial anisotropic material using a rectangular waveguide requires three separate samples; each one a different orientation of the parent biaxial anisotropic sample. The Waveguide Rectangular to Waveguide Square (WRWS) characterization method is an alternative, more efficient method, to the rectangular waveguide method because the WRWS method requires only one cube sample of biaxial anisotropic material to perform complete parameter extraction. This cube sample fits uniformly without gaps in the waveguide sample holder and can be indexed to accommodate all orientations required for characterization. The WRWS waveguide transitions insure that only single (TE10) modes are present and thus leads to closed form solutions for the material properties - an advantage over other existing techniques requiring higher-order modal analysis and subsequent numerical root search for extraction. Each WRWS transition mounts to the sample holder and the waveguide test ports of a Vector Network Analyzer and is calibrated using a TRL technique. A biaxial anisotropic test sample was designed based upon crystallographic symmetry, mixing theory and verified in rectangular waveguide measurements. WRWS test data is collected and constitutive parameters are extracted from each orientation of the biaxial anisotropic cube. This method of extracting biaxial anisotropic constitutive parameters using the WRWS system is evaluated in both experiment, simulation and validates the WRWS method. Theory, experimental and simulated results are presented to show that a cubic sample and WRWS measurement system can be efficiently and effectively used to measure biaxial anisotropic materials.
In recent years a number of analyses and simulations have been published that estimate the effect of using a probe with higher order azimuthal modes with standard probe corrected spherical transformation software. In the event the probe has higher order modes, errors will be present within the calculated antenna under test (AUT) spherical mode coefficients and the resulting asymptotic far-field parameters [1, 2, 3, 4] where the simulations were harnessed to examine these errors in detail. Within those studies, a computational electromagnetic simulation (CEM) was developed to calculate the output response for an arbitrary AUT/probe combination where the probe is placed at arbitrary locations on the measurement sphere ultimately allowing complete near-field acquisitions to be simulated. The planar transmission equation was used to calculate the probe response using the plane wave spectra for actual AUTs and probes derived from either planar or spherical measurements. The planar transmission formula was utilized as, unlike the spherical analogue, there is no limitation on the characteristics of the AUT or probe thereby enabling a powerful, entirely general, model to be constructed. This paper further extends this model to enable other measurement configurations and errors to be considered including probe positioning errors which can result in ideal first order probes exhibiting higher order azimuthal mode structures. The model will also be used to determine the accuracy of the Chu and Semplak near-zone gain correction [5] that is used in the calibration of pyramidal horns. The results of these additional simulations are presented and discussed. Keywords: near-field, antenna measurements, near-field probe, spherical alignment, spherical mode analysis. REFERENCES A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, Antenna Measurement Techniques Association (AMTA) 34th Annual Meeting & Symposium, Bellevue, Washington October 21-26, 2012. A.C. Newell, S.F. Gregson, “Higher Order Mode Probes in Spherical Near-Field Measurements”, 7th European Conference on Antennas and Propagation (EuCAP 2013) 8-12 April 2013. A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction”, Antenna Measurement Techniques Association (AMTA) 35th Annual Meeting & Symposium, Columbus, Ohio, October 6-11, 2013. A.C. Newell, S.F. Gregson, “Estimating the Effect of Higher Order Azimuthal Modes in Spherical Near-Field Probe Correction”, The 8th European Conference on Antennas and Propagation (EuCAP 2014) 6-11 April 2014. T.S. Chu, R.A. Semplak, “Gain of Electromagnetic Horns,’’ Bell Syst. Tech. Journal, pp. 527-537, March 1965