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Recent advances in Computational electromagnetic (CEM) simulations made them possible to be a cost-effective solution for designing and characterizing antenna measurement facilities. Using both full wave and asymptotic techniques, it is possible to characterize the performance of large measurement facilities such as anechoic chambers and compact antenna test ranges (CATR) with large parabolic reflectors. In this paper, we present the use of full wave techniques such as Finite Element Method (FEM) and Multi level Fast Multipole Method (MLFMM) as well as asymptotic techniques such as Physical Optics (PO) and Ray-Launching Geometrical Optics (RL-GO) for quiet zone characterizations as well as emulation of antenna measurements in both anechoic chambers as well as CATRs. Computational resources required for different techniques have been compared.
Backscattering responses of range-extended objects include those attributed to traveling-wave effects, typically caused by the termination of the object. Diagnosing the nature of scattering mechanisms contributing to the composite response is essential to modifying the object's radar signature. ISAR images reveal essential information on the location and nature of scattering features by decomposing the frequency/angle data into basis functions corresponding to independent point scatterers. When applied to responses of objects exhibiting other basis functions, such as those for traveling-wave scattering, ISAR images reveal unexpected results that can obscure proper interpretation of the scattering mechanisms. Because traveling-wave lobes are restricted to limited ranges of grazing angles and are frequency dependent, however, localizing their effects from high-resolution images can be elusive. Specifically, the traveling-wave responses are not readily distinguished from direct or diffracted responses. The paper deals with backscattering data collected on a slender cylindrical rod of 183cm length and 0.635cm diameter for aspect angles 0-180 degrees over a frequency range of 2-10GHz with polarization parallel to the plane of incidence, intended to emphasize effects of traveling waves at the rod's grazing angles. In spite of its relatively simiple geometry, the linear rod object presents complicated responses owing to the combined effects of traveling waves and multiple diffractions. Although ISAR images properly locate point scatterers, an understanding of the imaging process provides clues on the expected location of image elements corresponding to more complicated scattering features. The angle and frequency dependence of each scattering mechanism is illustrated in the paper by frequency responses, range responses, and ISAR images. The total scattering resonse of the rod for grazing incidence is characterized by at least 5 distinct scattering mechanisms, each interacting with the others as a function of viewing angle. Because images of traveling- and diffracted-wave components overlap for some grazing angles, their relative responses preclude separation. The results provide an example of the complex nature of scattering from a simple shape subject to traveling-wave effects.
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, artificial anisotropic dielectric materials must be designed, tested, and implemented. In this paper we shall present a layered artificial anisotropic dielectric material with a biaxial permittivity tensor. This material is designed to be used in conjunction with an antenna in order to improve antenna bandwidth. The design motivation behind this material shall be discussed, along with its implementation, the measurement of its permittivity tensor, and testing characterization with a prototype antenna. Results from CST Microwave Studio® simulations and the mixing rules from dielectric material science will be compared with the measured data. Test fixture design and instrumentation will also be presented. Predictions on various types of artificial anisotropic dielectrics suitable for future applications will also be discussed.
The boresight roll scan is a simple yet powerful tool for antenna range characterization and diagnostics. In this type of measurement a linearly-polarized antenna with high axial ratio (such as a standard gain horn) is rotated about its mechanical boresight axis while magnitude and phase data are collected. Post-processing of these data provides a wealth of information about the source polarization characteristics, and can also be used to diagnose common problems such as receiver compression, mechanical misalignment, drift, and flexing cables. This paper describes the theory and implementation of the boresight roll scan, and provides examples of how different types of range errors manifest in the processed data.
We describe millimeter-wave near-field measurements made with the new National Institute of Standards and Technology (NIST) robotic scanning system. This cost-effective system is designed for high-frequency performance, is capable of scanning in multiple configurations, and is able to track measurement geometry at every point in a scan. We have measured a WR-5 standard gain horn at 183 GHz using the spherical near-field method. We compare these results to a theoretical model and to a direct far-field measurement.
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
There are different applications where the radiation level of the electromagnetic waves are needed to be controlled or reduced. One way to achieve a functional passive control of the radiation level is by the use of electromagnetic-wave absorbers. The absorption efficiency is not only gained by the electromagnetic characteristics of the base material but also by the geometrical shape of the absorber specifically for wideband absorbers. One of the main applications of wideband absorbers is in anechoic chambers. In anechoic chambers, the walls of the chamber are lined with different absorbing panels each of which have different geometrical shapes. Two major groups of wideband absorbers are the wedge and pyramidal absorbers. When an infinite wall is lined completely with one type of these absorbers, the resulting electromagnetic problem will be a 1-dimensional (for the case of wedge absorber) or a 2-dimensional (in the case of pyramidal absorber) periodic-boundary-condition problem. A semi-analytical method based on a multi-conductor-transmission-line model has been previously introduced to solve the 1-dimesional problem (wedge absorbers) [1]. A modified method has been developed and will be introduced for the 2-dimensional problem (pyramidal absorbers). Some examples comparing the simulation results with the measurements ones will show the efficiency of the proposed method for the pyramidal absorbers. [1] A. Enayati, and A. Fallahi,“ Full-wave modeling of wedge absorbers”, ATMS 2014, Chennai, India.
There is an increasing interest in the use of adaptive antenna arrays for applications such as interference suppression, beamforming, and direction of arrival estimation. However, optimal use of an antenna array requires precise knowledge of the relative gain and phase responses of all elements in the array for all directions of interest (the antenna manifold). One generally obtains the antenna manifold through carefully controlled indoor measurements in an antenna test range. While these measurements are very precise and may include a range of frequencies, this approach has drawbacks in that it is time consuming and costly to perform for a large number of antenna arrays. One may also measure an antenna array's manifold outdoors using signals of opportunity. For Global Navigation Satellite Signal (GNSS) antennas, the satellite signals are the signals of opportunity. These measurements are quick and inexpensive if one has access to the digitized samples of the antenna outputs. In this paper, we discuss and compare our measurements of a multiple element antenna array using the indoor and outdoor approach. We show that there is a very good agreement between the two approaches and that outdoor measurements may be a viable substitute for indoor measurements in many applications.
Electrically small antennas (ESAs) are neither efficient nor good radiators because their radiation quality factor is considerably high. It is therefore critical to add appropriate matching networks (MNs) to the antenna to enhance its realized gain and therefore its performance over a large frequency range. For receiver applications, the important factor for the electrically small antenna is the signal-to-noise ratio (SNR). In this paper, the design of stable non-Foster circuits (NFCs) to improve the performance of the ESA in terms of the realized gain and the SNR has been achieved. Measurements of the signal and noise of the electrically small antenna with and without non-Foster circuits has been performed in an outdoor environment. Key steps of the measurements will be shown including the post-processing of the data, which is an important step to reduce the effect of undesired signals. Based on the measured data, it is shown that the Non-Foster circuits improve both antenna gain and SNR by more than 15dB over a wide frequency range, namely, 100MHz to 700MHz, with respect to the case without a NFC. To the best knowledge of the authors, this improvement is maintained over the widest frequency band among all the published work. Excellent agreement between simulation and measurement results in terms of gain and signal improvements is obtained and will be highlighted in our presentation.
Applications using millimeter-wave antennas have taken off in recent years. Examples include wireless HDTV, automotive radar, imaging and space communications. NSI has delivered dozens of antenna measurement systems operating at mm-wave frequencies. These systems are capable of measuring a wide variety of antenna types, including antennas with waveguide inputs, coaxial inputs and wafer antennas that require a probing station. The NSI systems are all based on standard mm-wave modules from vendors such as OML, Rohde & Schwarz and Virginia Diodes. This paper will present considerations for implementation of these systems, including providing the correct RF and LO power levels, the impact of harmonics, and interoperability with coaxial solutions. It will also investigate mechanical aspects such as application of waveguide rotary joints, size and weight reduction, and scanner geometries for spherical near-field and far-field measurements. The paper will also compare the performance of the various mm-wave solutions. Radiation patterns acquired using some of these near-field test systems will be shared, along with some of the challenges encountered when performing mm-wave measurements in the near-field.
This paper will investigate practical aspects of measuring antennas operating with pulsed RF signals of very narrow width, down to 100 ns. Modern network analyzers provide very high IF bandwidth, theoretically allowing measurement of very narrow pulses. But in practice there are several factors that limit the minimum pulse width that can be measured accurately. These include lowpass filtering in the IF path, limited receiver dynamic range, and trigger synchronization issues. On large near-field scanners, the RF path length change with probe motion becomes significant when the pulses are short. If the RF system uses a pulsed reference signal, a delay line may be required to align the test and reference pulses arriving at the receiver. Even with a CW reference, the varying propagation delay in the test channel places a limit on the minimum pulse width. The paper will present a detailed investigation of these issues along with measurement examples. It will compare measured performance of a standalone VNA, a VNA with remote mixers, and the NSI Panther receiver.
Radome enclose antennas to protect them from environmental influences. Radomes are ideally electrically transparent, but in reality, radomes introduce transmission loss, pattern distortion, beam deflection, etc. Radome diagnostics are acquired in the design process, the delivery control, and in performance verification of repaired and newly developed radome. A measured near or far-field may indicate deviations, e.g., increased side-lobe levels or boresight errors, but the origin of the flaws are not revealed. In this presentation, source reconstruction from measured data is used for radome diagnostics. Source reconstruction is a useful tool in applications such as non-destructive diagnostics of antennas and radomes. The radome diagnostics is performed by visualizing the equivalent currents on the surface of the radome. Defects caused by metallic and dielectric patches are imaged from far-field data. The measured far-field is related to the equivalent surface current on the radome surface by using a surface integral representation together with the extinction theorem. The problem is solved by a body of revolution method of moment (MoM) code utilizing a singular value decomposition (SVD) for regularization. Phase shifts, an effective insertion phase delay (IPD), caused by patches of dielectric tape attached to the radome surface, are localized. Imaging results from three different far-field measurement series at 10 GHz are presented. Specifically, patches of various edge sizes (0.5?2.0 wavelengths), and with the smallest thickness corresponding to a phase shift of a couple of degrees are imaged. The IPD of one layer dielectric tape, 0.15 mm, is detected. The dielectric patches model deviations in the electrical thickness of the radome wall. The results from the measurements can be utilized to produce a trimming mask, which is a map of the surface with instructions how the surface should be altered to obtain the desired properties for the radome. Diagnosis of the IPD on the radome surface is also significant in the delivery control to guarantee manufacturing tolerances of radomes.
RCS measurements are often comprised of a combination of the coherent summation of many things in addition to the desired target. Those other things contribute to error in RCS measurements and include noise, clutter and background, which can be further characterized according to specific types. An approach has been developed that is capable of capturing and separating certain types of noise, clutter and background based on specific forward models to include RFI, target support (e.g., pylon), and many others, such that engineers can clearly see the separated components and selectively choose to include, exclude, or edit as the case may be. This approach affords far more flexibility than classic image edit reconstruct (IER), and offers more editing accuracy than Fourier-based approaches including entire phase history based approaches. This paper describes the basic approach and shows examples with measured data.
The Space Communications and Navigation (SCaN) Testbed was developed to investigate the applicability of software defined radios (SDR) to NASA space missions, study the operation of SDRs and their waveform applications in an operational space environment, and reduce cost and risk for future space missions using SDRs. The SCaN Testbed, developed at NASA’s Glenn Research Center, is currently installed on the International Space Station and has line-of-sight connection to NASA’s Space Network and compatible ground stations. To characterize the operation of the SDRs and their waveforms, a new and unique capability was developed at GRC, which uses purposeful antenna off-pointing to provide a wide range of power levels to the input of the SDR. With this capability, a radio can be more fully tested and characterized on-orbit. This paper describes the new antenna off-pointing capability and methodology, and how it was applied to characterize the on-orbit performance of an S-Band radio in the SCaN Testbed. It provides details of the antenna pointing system control algorithm, gimbal articulation limitations, medium gain antenna pattern profile, and phase limitations associated with the medium gain antenna. Finally, the paper presents test results and lessons learned.
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.