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A technique is presented for the measurement of antenna patterns, in which a long, thin wire is moved past the antenna aperture while the changes in reflection coefficient at the antenna feed are recorded. By suitable processing of these data, the antenna pattern can be calculated.
This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
An image processing method which uses the bistatic scattered fields of a target obtained in a compact range is presented in this paper. The transmitting and receiving antennas can be either two compact ranges or one compact range and a horn antenna. The compact range reflector can be either focussed or defocussed so that a near field situation can be simulated. The bistatic scattered fields are collected as a function of frequency and the angle of rotation of the target. Then they are processed coherently to determine the cross-range and down-range scattering centers of the target. Experimental results are presented to validate this image processing technique.
Performance verification of an adaptive array requires direct, real-time sampling of the antenna pattern. For a space-qualified array, measurements on a far-field range are impractical. A compact range offers a protected environment, but lacks a sufficiently wide field of view. Conventional near-field measurements can provide antenna patterns only indirectly. This paper shows how far-field antenna patterns can be obtained in a relatively small anechoic chamber by focusing a phased array in the near-field. The focusing technique is based on matching the nulls of far-field and near-field antenna patterns, and is applicable to conformal or nonuniform phased arrays containing active radiating elements with independent amplitude and phase control. The focusing technique was experimentally verified using a 32-element, linear, L-band array. Conventionally measured far-field and near-field patterns were compared with focused near-field patterns. Very good agreement in sidelobe levels and beamwidths was achieved.
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.
This paper considers hardware and software issues associated with accurate RCS, antenna, and near field antenna measurements. In particular we examine methods for making accurate measurements at high speed using existing network analysis equipment, such as the HP8510B. Techniques which allow for fundamental mixing are examined from the viewpoint of enhanced dynamic range and speed. Harmonic mixing techniques are also discussed and limitations related to IF bandwidth and harmonic locking are presented. The realtime requirements of software systems for these applications are presented and operating system considerations are analyzed. Interface attributes are examined with a view toward use with multi-tasking operating systems and the real-time requirements of high speed measurement systems.
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.
A 400 element phased array antenna has been constructed at the GEC-Marconi Research Centre. Each radiating element is fed from its own phase shifter. The radiation patterns of this array have been measured using a recently constructed Cylindrical Near Field Test Facility. The radiation pattern is obtained on a two dimensional grid and contains both amplitude and phase information. It is therefore possible to transform these data back to the array aperture to obtain the array excitation amplitudes and phases. The spatial resolution obtained in the aperture is a function of the angular coverage of the radiation pattern used. The effect of deliberately introduced phase errors on the calculated aperture data is shown.
The Synthetic Aperture Radar Antenna for the European Remote Sensing Satellite ERS-1 is a 10 by 1 metre deployable slotted waveguide array operating at 5.3 GHz. Electrical performance of the complete antenna is derived at the end of the environmental test programme from near field measurements on a planar NF scanner. In order to obtain very early information on electrical integrity of the flight model antenna, suitable for pre- and post-environmental comparison, a fast electrical functional test was implemented in the total test sequence. It basically consists of a 2D slot probing of a well distributed number of slots in combination with complex input impedance measurements. The paper describes the method and presents results of different test steps. The data of pre-/post-environmental measurements are compared.
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.
The National Institute of Standards and Technology (NIST) has developed a multi-axis controller and software data acquisition system that has improved probe position accuracies in near-field scanning. This extends the usefulness of the NIST planar near-field scanner to higher frequencies. This system integrates programmable power supplies into an existing planar measurement system with new software that controls the power supplies and the data acquisition. It provides the higher positioning accuracy required for millimeter wave measurements at a reasonable cost. This system uses the NIST planar near-field scanner's existing DC motors, computer and laser. The programmable power supplies are connected to the motors, with a separate power supply for each motor'a armature and a common power supply for each of the motor's field windings. This allows for concurrent movement in each axis and eliminates delays in switching between axes. Directional control, motor protection, and special software features are implemented by logic control.
This paper describes techniques for coherently suppressing multipath and other error sources in planar near-field measurements. Of special interest is a simple, yet effective technique of suppressing axial multipath and mutual coupling between the nearfield probe and an antenna. This is of particular value in the testing of low sidelobe antennas. Traditionally, self comparison tests with different separations between the probe and the antenna under test are used to identify the magnitude of multipath errors. What is not generally realized is that these tests can be used to produce a coherent estimate of the induced error, which can often be suppressed. A series of tests was performed with a small X-band phased array antenna, resulting in a reduction of the sidelobe noise background from a 25 dB level to better than 50 dB.
Obtaining the tangential electric fields on a planar near field surface has many important applications. The need for this information may not, however, justify the expense of a planar near field test range. A technique has been developed to obtain planar near field data from spherical near field measurements. Spherical near field measurements can be performed on practically any range that employs a roll over azimuth positioner and a phase/amplitude receiver.
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.
The National Bureau of Standards (NBS) has been measuring antennas and dealing with the problems of leakage for the past twenty years. This paper will discuss the various methods of detecting leakage, typical sources of leakage, how to correct leakage problems, and the effects that leakage can have on calibration.
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.
We have reviewed the sampling-interval requirement associated with the algorithmic problem of extrapolating near-field radiation measurements to the far zone and concluded that the far-zone sampling rule (d?=?/D) works as well in the Fresnel portion of the near zone. In addition, we find that the angular window, W, over which the Fresnel-zone field must be measured is approximately W - 2D/R radians in width, where D is the nominal diameter of the antenna and R the range at which the near-field data are taken. This guideline is valid when one uses an integral extrapolation scheme, as opposed to a modal one, since the paraxial approximation gives some assurance that field contributions from points outside the sampling window will contribute negligibly to the far-zone amplitude. We have also looked at the sampling requirements associated with extrapolating near-field RCS measurements to the far zone and concluded that windowing techniques can reduce the magnitude of the bistatic scanning task dramatically.
It is common practice in antenna measurements to use a moderately directive source (or 'probe') antenna, to minimise the effect of reflections. The illumination of the test antenna then exhibits a degree of non-uniformity. A correction scheme has previously been proposed for spherical near-field measurements. This paper describes a new probe-correction algorithm that can be used in conjunction with spherical near-field or 'conventional' measurements. It is operable with a minimum amount of measured data (for either the test antenna or the probe). It may also be used for probe-correcting calibration measurements using a gain horn.