Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
For narrow beam antennas or track antennas some parameters like main beam or null position and 3 dB beamwidth need to be determined with an accuracy of less than a mill or mrad. With Near Field measurements, the Far Field is normally calculated by FFT-processing. This does, however, not provide the required accuracy. Nevertheless, the measured Near Field data contains information about any Far Field point. An iterative approach is presented to determine the Far Field antenna characteristics with high accuracy.
A new strategy reducing the effect of the environmental noise in the evaluation of the radiated far field by means of a near-field far-field transformation technique is presented. A plane-polar scanning system is considered although the approach holds for general scanning geometries. Numerical and experimental results confirm the effectiveness of proposed the technique.
A new strategy reducing the effect of the environmental noise in the evaluation of the radiated far field by means of a near-field far-field transformation technique is presented. A plane-polar scanning system is considered although the approach holds for general scanning geometries. Numerical and experimental results confirm the effectiveness of proposed the technique.
In this paper we present an improved theoretical modelling for the free space technique for measuring the complex permittivity of materials at microwave frequencies. The theory was developed for a transmission set-up with two identical pyramidal horn antennas. By performing a spectral decomposition of the aperture fields, the new model takes the effect of the non plane wave character into account when the sample is not placed in the far field of the transmitting antenna. With the use of the new theoretical model it becomes possible to place the sample much closer to the antennas without infringing the theoretical assumptions since no plane wave incidence is needed. In this way the transversal dimensions of the sample can be reduced significantly. The validity of the new theoretical model was verified by measurements on many dielectric (Plexiglas, polystyrene,…) and lossy materials. A comparison was made with the values obtained when the usual plane wave theory is used.
This paper presents several enhancement tools that were developed to improve the Georgia Tech Spherical Far-Field/ Near-Field Antenna Measurement Range. Measurement amplitude and phase drift was quantified by sampling an antenna measurement signal over long time intervals while leaving the AUT rotation positioners fixed. A return-to-point drift correction tool was implemented to correct for the long-term drift component for spherical surface measurements. Temperature sensitive components of the receiver were moved from an area with severe temperature variations to a temperature stable area to reduce the phase variation. A software tool was developed to display a histogram of the variation in repeated spherical scan measurements. Histogram vales show that drift correction improves the repeatability of an antenna pattern measurement. The shapes of the histograms have been helpful in identifying random and deterministic variations.
The imaging of radar targets is typically accom plished by measuring the radar cross section (RCS) of the target as a function of frequency and az imuth angle. We measure a third dimension of the RCS by tilting the target and collecting data for conical cuts of the RCS pattern. This third dimension of data provides the ability to estimate the three-dimensional location of scattering centers on the target. Three algorithms are developed in order to process the three-dimensional RCS data.
New windows which allow the user to select the level of sidelobe suppression near the DFT resolution limit are reported. By a parametric study, we identify the truncated Lorentzian and Gaussian functions as better choices compared with the popular Hann windows.
RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.
RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.
The UCLA hi-polar planar near-field scanner has a novel implemen tation which results in a polar sampling grid. The scanner was used to perform measuremen t comparisons using three sampl in g arrangements: polar, thinned-polar, and linear-spiral sampling. The data acquired using each was processed to the far-field for both simulated and measured near-field data. Excellent agreement was observed.
Compact ranges have found wide application for antenna measurements, RCS measurements, and, most recently, for spacecraft payload measurements. Each of these ap plications requires certain special features of the range optics, positioning systems, electronics, and software. The system design of a compact range measurement sys tem for making all these types of measurements presents a number of challenges. This paper will discuss the system aspects of the design of a multi-purpose compact range facility. Items of inter est include the RF electronics design, the positioning sys tem design, the optimization of the reflector and feeds and the specialized software design.
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
The ESAT-TELEMIC division at Katholieke Universiteit Leuven (KUL) has three antenna ranges: an indoor Far-Field range, an indoor planar Near-Field range and an outdoor Far-Field range. The positioning equipment is of a variety of manufacturers. The division launched an effort to modernize the range complex and add automatic measurement capabilities, while still retaining control of all three ranges from one control console and using one positioner controller, one angle readout and a single receiver to save costs. The system upgrade included some electrical refurbish ment of the positioning equipment and the replacement of all the old control and data recording equipment with Orbit Positioner Controller/Programmer, Power Control Unit and combined Near-Field and Far-Field software. Control of all three sites is achieved using a special Orbit Junction Box. With the new configuration all three ranges can be operated in fully automatic mode, one range at a time. The software package controls both Near-Field and Far Field measurements using compatible data formats and human interfaces.
ERIM is currently investigating several near-field to far-field transfonnations (NFFFfs) for predicting the far-field RCS of targets from monostatic near-field measurements. Each of the techniques uses approximate tions and/or supporting information to overcome the need for the bistatic near-field data which is required to rigorously transfonn a target's scattered field from the near zone to the far zone. Our focus has been on spheri cal near-field scanning, since this type of collection geometry is most compatible with existing RCS ranges. One particular NFFFT is based on the reflectivity approximation commonly used in ISAR imaging to model the target scattering. This image-based NFFFT is the most computationally efficient technique under con sideration, because, despite its theoretical underpinnings, it does not explicitly require image fonnation as part of its implementation. This paper presents an efficient discrete implementation of the image-based NFFFT, along with numerically-simulated examples of its perfonnance. The advantages and limitations of the technique will be discussed. A simplified version which applies to high aspect ratio (length-to-height) targets and requires only a single great circle (waterline) data in the near field is also summarized.
The RCS of extended objects measured in the near field is subject to errors induced by the spherical nature of the incident and scattered wavefields. A number of techniques have been applied to estimate far-field responses from results of monostatic near-field measurements. While the results indicate successful transformations for linear scatterers, the lack of a sound theoretical basis brings into question the appli cability to general objects. The paper explores the theoretical basis of the far-field transformation of RCS data and the consequence of the limited data obtained from monostatic measurements. The limitations of approaches reported to date [1-4] are explored from conceptual and physical con siderations with the goal of establishing reasonable expectations for practical methods. Examples using simulated and measured near-field data are presented to illustrate successes and failures of the algorithms in transforming results to far-field RCS.
This paper examines the possibility of increasing the speed of Near-Field measurement of an Antenna, by reducing the number of measurement points and by determining the degree of truncation permissible while maintaining a prescribed degree of precision of the reconstructed far-field. The Near-Field of a planar radiating array is analysed in depth. A formulation and a procedure to correct the spectral domain of the field are established. It is shown that correction in the spectral domain can improve the accuracy of the Far-Field while using the same amount of Near-Field data. The technique has a good potential to be applied to Near Field data of large radiating Antennas leading to new information about the accuracy and speed of measurement achievable.
We investigated two methods of probe-position error correction to determine how well the corrected results compare to the uncorrupted far field: the k-correction method and the Taylor-series method. For this investigation, we measured a 1.2 m dish at 4 GHz and a 1.2m by 0.9m phased array at 2.2 GHz. Measurements were made first without position errors and then with deliberate z-position errors. We perfonned probe position error correction using both methods and compared the results to the error-free far field. For errors up to A/4, the fifth-order implementation of the Taylor series correction was slightly better than the k-correction. For errors of ')..J2, the k-correction was better than the Taylor-series correction.
A method to transform Fresnel field data to far-field data with probe correction, based on a non linear least squares algorithm, is presented. The functional to be considered is the expression of the Fresnel field radiated by an array of isotropic sources located on the antenna aperture, and the complex excitations are the coefficients that minimize the rms error between the measured data and the functional values. The intermediate step of determining the complex excitations can be used as a diagnostic tool. Probe pattern correction has been included in the method, improving the performances of antenna measurement systems placed in small size anechoic chambers.
Far-field patterns obtained from planar near-field measurements of a prototype of the AMSU-B radiometer antenna by phase retrieval at 94 GHz are presented in this paper. Comparison with results from a compact range facility show good agreement within the main beam A modified algorithm takes into account any misalignments of the two intensity data sets so that the RMS near-field error metric comparing retrieved and measured values converges to < -30 dB. Phase retrieval is revealing itself as a useful technique to be applied to electrically large antennas at frequencies extending into the millimetre and sub millimetre bands.
Daimler-Benz Aerospace AG (Dasa) in Munich, Germany designed, developed, build and tested most of the INTELSAT VIII antennas. RF test requirements and results are presented for the Hemi/Zone antennas. These tests cover the Beam Forming Networks (BFN), the feed array in the cylindrical near field facility at ambient temperature and in a temperature range from -61 to +85 deg centigrade, and finally the complete antenna sub system, without and with satellite mock-up, in the large Compensated Compact Range. Dasa and TICRA software was used to calculate the far field results from the measured BFN coefficients and from the feed array results measured in the near field facility. Also alignment aspects are considered.