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A special purpose 80 element linear phased array antenna was aligned using an iterative phase cycling method. First, the array was aligned to yield maximum main-beam power in the reactive near-field zone and then in the far-field zone. A record of the phase-shifters settings achieved for each zone was kept for use as look-up table during operation. In situ electronic main-beam steering was performed to compare sidelobe performance for the two cases. This report describes the measured results obtained using the phased cycling alignment procedure and compares the measured one-way radiation pattern for the two distance conditions.
Large arrays require large separations between the transmit antenna and the antenna under test (AUT) to measure pattern parameters in the far field. For the subject AUT, a range of 6 miles with a spurious signal level of -58 dB was necessary to obtain the required accuracy. Measurements have been performed on a significantly shorter range without serious degradation. The antenna was focused for the angle of electronic scan and the resulting pattern measured. The theoretical far field patterns were compared with the calculated focused patterns for the short range. The maximum sidelobe error of 1/2 dB occurred at 60 degrees scan. There was no noticeable degradation in beamwidth, gain, or foresight at any scan angle. A 6-mile range would have produced a 2-dB sidelobe error. The measured range reflection level was -50 dB. The transmit dish with sidelobes of 22 dB was replaced with an array that had 40 dB sidelobes. This change reduced the reflections to below the required -58 dB. The antenna was focused using a range calibration technique and the measurements substantiated the theory.
Under many circumstances it is necessary to experimentally estimate the radar cross section of targets in a cluttered environment. A significant reduction in the clutter can be obtained when cross range filtering can be done. In this experimental RC measurement concept, scattering measurements are performed using a moving radar antenna. Thus scattering as a function of target plus clutter versus aspect angle in the near field can be measured. Next, a back projection algorithm can be used to estimate the scattering as a function of position in the neighborhood of the target. The known range to which the signal is to be focussed is used to project back to the target area. An estimate of the RCS at points along a line in the plane of the target is computed. The clutter responses can then be removed from the data, and the remaining target-only values projected forward again (possibly to the far field) to estimate the RCS of the target alone.
The hardware and software developments undertaken to upgrade two far-field measurement facilities - a 12-m anechoic chamber and a 35-m outside range - are described. A method (termed quasi-far-field, QFF) for deriving antenna far-field patterns from a single plane scan at a distance less than the traditional distance of 2D2/? is described. The QFF technique involves pattern sample and subsequent pattern transform and reconstruction, from the easement distance to the far-field distance. A discussion of the limitations inherent in the QFF transform, including range length, is given. Experimental results for measurements made on circular-aperture antennas with both symmetric and asymmetric illumination, and on antennas with elliptical apertures, are described.
For transforming a Fresnel region pattern to a far-field pattern, we present here two methods, the "discrete beam sampling" method (DBSM) and the "displaced beam" method (DBM), which allow an accurate characterization for both linear as well as circular antenna apertures. Both methods assume a simple Fourier transform relationship between the aperture field distribution and the far-field of the antenna. The Fresnel region field is then essentially perturbed by an aperture quadratic phase error assumed to exist because of the finite distance at which the field pattern is characterized. Numerical simulation and its results are presented to show the accuracy of the reconstructed far-field data. Finally, an error analysis is performed to show the sensitivity of the above two methods.
The nonuniform sampling technique utilizes measured (or simulated) amplitude and phase far-field data at nonuniformly sampled data points and constructs the pattern from these limited number of measured data. The technique relies on the fact that the antenna far-field pattern is proportional to the Fourier transform of a function which is related to the induced current on the antenna. The application of nonuniform sampling technique becomes important in the situation for which it will be difficult (or impossible) to measure the far field at regular intervals. In this paper, the application of the nonuniform sampling technique is demonstrated for antenna pattern measurements. The foundation of the technique is first reviewed and the required mathematical steps for the implementation of the technique is summarized. Both one dimensional and two dimensional cases are reviewed with attention given to the applicability of closed form expressions for the determination of the sampling coefficients. Numerical results are presented and comparison to measurements are shown. In particular, the application of this technique to a recently proposed space-station based antenna experiment is presented.
The nonuniform sampling technique utilizes measured (or simulated) amplitude and phase far-field data at nonuniformly sampled data points and constructs the pattern from these limited number of measured data. The technique relies on the fact that the antenna far-field pattern is proportional to the Fourier transform of a function which is related to the induced current on the antenna. The application of nonuniform sampling technique becomes important in the situation for which it will be difficult (or impossible) to measure the far field at regular intervals. In this paper, the application of the nonuniform sampling technique is demonstrated for antenna pattern measurements. The foundation of the technique is first reviewed and the required mathematical steps for the implementation of the technique is summarized. Both one dimensional and two dimensional cases are reviewed with attention given to the applicability of closed form expressions for the determination of the sampling coefficients. Numerical results are presented and comparison to measurements are shown. In particular, the application of this technique to a recently proposed space-station based antenna experiment is presented.
In recent years there has been an increasing demand for antenna calibrations at millimeter wave frequencies. Because of this the National Institute of Standards and Technology (NIST) has been developing measurement capabilities at millimeter wave frequencies. The development of gain and polarization measurement capabilities have been previously reported. This paper reports on the development of the capability to measure an antenna pattern which has been achieved during the last year. Measurement accuracies of better than 4 dB have been achieved for sidelobes which are 40 dB below the mainbeam peak. NIST is now providing a new measurement service for antenna patterns in the 30-50 GHz frequency range.
A flexible near-field antenna test-facility is presented. This system gathers all that is necessary to design, to debug and to validate the high performance antennas which are made by ESD. ARAMIS has been operational since January 1988. Its applications are: - Near-field measurements (for diagrams): * planar, * cylindrical. - High speed field mapping (for default analysis): * planar radiating surface, * cylindrical radiating surface. - Generation of element excitation (active phased array testing): * planar antennas, * cylindrical antennas. - Direct far-field measurements (probes, small antennas), - Circuit measurement (S parameter). The facility features a specially designed scanner. Thanks to its six degrees of freedom, this positionner allows the differents types of measurements to be made. The instrumentation is based upon the HP 8510 B network analyzer. A single computer performs the measurements, transforms the data and presents the graphics (linear diagrams, color maps, three-dimensional colored projections). In order to grant a high scan speed, the system uses the FAST CW mode of the HP 8510 B. An external trigger is provided during the motion process of the probe. A rate of 500 measurements/sec. has been proved. This on-the-fly process is clearly depicted. Experimental results are presented which include: - Low sidelobe (-38 dB) antenna diagrams. - Default analysis through: * Amplitude mapping (leakage short-circuit in a microstrip antenna). * Phase mapping (out-of band comparison between two radiating element technologies). * Measurement of excitation laws. * 3-D transformation. - Simultaneous on-the-fly acquisition of up to three antenna outputs.
Implementing an antenna test range has traditionally been viewed as a major and costly undertaking, requiring significant long term facility planning, computer hardware interfacing, and software development. This paper describes a complete low cost, yet high accuracy portable near-field measurement system that was privately built for less than $2,000 and interfaced to a PC compatible computer. The design and operation of this system, including the scanner, microwave hardware, and computer system will be described. This system has since been extended into a commercial product capable of providing rapid and accurate measurements of small to medium size feeds and antennas within a small office or lab space at significantly lower cost than standard antenna test techniques. The system has demonstrated an equivalent sidelobe noise level of less than -50 dB, includes a probe corrected far-field transform and holographic back projections, and can output pattern cuts, contour plots, 3D plots, and grey scale images of antenna performance.
A study of RF testing methods was conducted for the Radarsat SAR antenna. The implementation tolerances of a planar and a cylindrical near-field facility were computed, by simulation of the effects of different types of measurement errors on the reconstructed far field. The results are presented and the two types of near-field facility are compared.
We describe a portable system for performing microwave holography of reflector antennas. This technique derives the complex (amplitude and phase) aperture current distribution from the measured complex far field of an antenna. The amplitude of the current distribution displays directly the effects of feed and support leg shadowing, and illumination taper. The phase of the current distribution is used to optimize feed and/or sub-reflector location, and to generate a table of recommended panel adjustments.
This paper reports on a project carried out at Georgia Tech to reduce forward scattering from the top edge of far-field range diffraction fences over a wide frequency band. It is shown that the addition of serrations with length greater than ten wavelengths and a flower petal shape reduce the stray radiation in the quiet zone by as much as 10 dB. Several variations on the basic shape are investigated and computed results are shown.
A new spherical near field test facility is under development by Canadian Astronautics Limited at the David Florida Laboratory in Ottawa. It provides for a wide range of antenna measurements, including far-field, far-field from near field, and near-in and very near-in field reconstruction. Many user-friendly, user-interactive, and graphics features are incorporated. This paper outlines some of the underlying concepts for the facility.
In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.
The radar cross section of large targets has previously been measured on large outdoor far field ranges. Due to environmental and security limitations of outdoor ranges, low cost indoor compact ranges are preferred. To optimize compact range performance and to minimize size, careful attention must be paid to the design of feeds which are required for the proper illumination of the reflector. This paper describes a new polarization diversified parasitic multimode/corrugated (PMC) feed for a compact range reflector. The performance attributes of the PMC feed are presented. The PMC feed provides several advantages over other known commercially available compact range feeds.
A far-field antenna range has been assembled on the roof of the Electrical Engineering building at Virginia Tech. Antenna radiation patterns and polarization patterns can be measured. The system consists of two Scientific-Atlanta azimuth positioners, a Scientific-Atlanta 1711 receiver, a Scientific-Atlanta 1832A amplitude display unit, a DC motor controller, a synchro-to-digital converter, an IBM PC, and signal sources. The DC motor controller has been interfaced to the PC along with the synchro-to-digital converter, forming a closed loop positioning control system that can be used with either of the azimuth positioners. One of the positioners is used for the antenna under test while the other positioner controls the polarization of the transmit antenna. The receiver and amplitude display provide a 60 dB dynamic range for antenna measurements. The PC has been programmed in TURBO Pascal to control the antenna positioner, record antenna patterns, store pattern data on disk, and provide antenna pattern plots. This modular approach provides permanent storage on PC disk of all measurements as well as allowing many plot combinations including linear or logarithmic form and rectangular or polar format.
For over a decade the National Bureau of Standards (NBS) has used the planar near-field method to accurately determine the gain, polarization and patterns of antennas either transmitting or receiving cw signals. Some of these calibrated antennas have also been measured at other facilities to determine and/or verify the accuracies obtainable with their ranges. The facilities involved have included near-field ranges, far-field ranges, and compact ranges. Recently, NBS has calibrated an antenna to be used to evaluate both a near-field range and a compact range. These ranges are to be used to measure an electronically-steerable antenna which transmits only pulsed-cw signals. The antenna calibrated by NBS was chosen to be similar in physical size and frequency of operation to the array and was also calibrated with the antenna transmitting pulsed-cw. This calibration included determining the effects of using different power levels at the mixer, the accuracy of the receiver in making the amplitude and phase measurements, and the effective dynamic range of the receiver. Comparisons were made with calibration results obtained for the antenna transmitting cw and for the antenna receiving cw. The parameters compared include gain, sidelobe and cross polarization levels. The measurements are described and some results are presented.
A roof-top antenna range has been installed at the Bellcore facility in Red Bank, New Jersey. This facility is used as a far field range to measure highly directive antennas at millimeter wave frequencies. Theoretical and experimental studies were performed to characterize the range environment and identify reflections. Two computer programs were used to analyze the strength and location of interfering signals at both UHF and millimeter wave frequencies. These programs use Geometrical Optics and the Geometrical Theory of Diffraction to predict the location and strength of diffracted and reflected energy from the surrounding structures. Both singly and doubly diffracted interferences were considered. A bi-static radar, with an 850 MHz carrier, bi-phase modulated by a 40 Mbit/s pseudonoise code, was used to measure the impulse response of the environment. The antenna range measurements are compared with the analysis done at 850 MHz and calculated results are printed for the behavior of the range in the millimeter wave regime.
A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.