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Recently, several deployable, ground-to-ground col lection systems have been developed for the assessment of aircraft RCS on the flight-line. The majority of these systems require bulky rail or scanning hardware in order to collect diagnostic imaging data. The measurement technique described in this paper, while not a "cure-all", does eliminate the need for bulky hardware by allowing the collection system to move freely around the target while collecting radar backscattering data. In addition, a nearfield-to-farfield transformation (NFFFT) algorithm is incorporated in the process to allow the collection of scattering data collected in the near field to be processed and evaluated in the far field. The techniques described in this paper are a part of a data conditioning process which improves the data quality and utility for subsequent analysis by an automated diagnostic system described elsewhere in this proceedings [1]. The techniques are described and demonstrated on numerically simulated and experimentally measured data.
A time domain near-field far-field transformation technique based on a non redundant plane-polar sampling representation of the field is presented. The method allows to obtain the far-field with a minimum number of samples and/or a reduction of the scanning area. Various computational schemes are presented.
A new holographic technique for array diagnosis is discussed. Numerical and experimental results are shown in case of linear arrays and compared with the performance of the “classical” holographic approach based on FFT. Furthermore, the working progress on a sub-optimal algorithm specifically developped for diagnosis of large arrays is presented.
We describe a technique which employs amplitude-only measurements of an unknown antenna combined with a synthetic reference wave to produce a hologram of a near-field antenna distribution. The hologram, which may be recorded by amplitude-only receiving equipment, is digitally processed using an enhanced theory which allows complete removal of the spurious images normally encountered with optical hologram reconstruction. The recovered near-field data are then processed using standard algorithms to calculate antenna far-fields. We present the theoretical formulation and results of measurements obtained on an 1.2 m reflector antenna.
We describe a technique which employs amplitude-only measurements of an unknown antenna combined with a synthetic reference wave to produce a hologram of a near-field antenna distribution. The hologram, which may be recorded by amplitude-only receiving equipment, is digitally processed using an enhanced theory which allows complete removal of the spurious images normally encountered with optical hologram reconstruction. The recovered near-field data are then processed using standard algorithms to calculate antenna far-fields. We present the theoretical formulation and results of measurements obtained on an 1.2 m reflector antenna.
Planar near-field antenna measurements have developed into a mature science. Nonetheless, unique difficulties arise when measuring some modern antennas, such as high gain satellite antenna systems. In a typical planar near-field measurement, the antenna under test (AUT) has a collimated beam in the near-field which is perpendicular to the scan plane (i.e. the AUT boresight is parallel to the normal of the scan plane). On the other hand, the scan plane is positioned close to the AUT to maximize the valid angular range in the far-zone patterns. Unfortunately, it is not always possible to place the AUT very close to the scan plane and keep the near-field beam perpendicular to the scan plane. An investigation of the benefits and pitfalls of a planar near-field measurement where the AUT beam is not perpendicular to the scan plane is presented. The measurements of antennas tilted 45 degrees with respect to the scan plane normal are used as examples. With this atypical arrangement, some of the usual errors in a near-field measurement are emphasized. Procedures to identify and reduce these errors will be presented.
The standard planar near-field to far-field transformation method requires data points on a plane-rectangular lattice. In this paper we introduce a transformation algorithm in which measurements are neither required to lie on a regular grid nor are strictly confined to a plane. Computational complexivity is O (N log N), where N is the number of data points. (Actual calculation times depend on the numerical precision specified and on the condition number of the problem.) This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable for other measurement approaches, such as plane-polar scanning, where data are collected on a non-rectangular grid.
This paper reports on the results of computer simulations of planar near-field test-zone-fields. Techniques for the improvement of the quality of the test fields are presented and demonstrated. These techniques include the use of larger scan areas and the use of window functions applied to the measured near-field data. Test-zone-field quality is measured by the angular spectrum of the error of the test-zone-field as compared to an ideal plane wave test-zone-field. This investigation sought the minimum scan length, L, for a given critical angle, ?c and separation, S. It is shown that significant improvements in test-zone-field quality can be realized if the test zone is extended from the standard length, Ls=D+2S(tan(?c)) by an amount 20?/cos(?c). This scan length is approximately 30? larger, for a critical angle of 50 degree and 60? larger, for a critical angle of 70 degrees, than the standard length. A raised cosine amplitude/quadratic phase window applied to the measured near-field data can significantly reduce scan length requirement while maintaining the increased accuracy of the extended scan length. The recommended scan length with window is given by Lw=D+2S(tan(?c))+2W, where W is the length of the window applied to each end of the scan measurements. The window description and required length are presented.
This paper summarizes the state of the art for using one-dimensional and two-dimensional arrays of modulated scattering elements to rapidly measure the near-field electromagnetic fields 1) radiated by antennas with or without radomes and 2) scattered by targets located in free-space or buried in lossy dielectric media. The application of rapid near-field scanning via measurement arrays based on the Modulated Scattering Technique (MST) in both France and the U.S. is discussed in this paper.
In this paper, a near-field time domain radiation measurement is described, similar to the traditional frequency domain near-field radiation measurement. This time domain measurement approach borrows many of the principles developed in the frequency domain and is ideally suited for the measurement of broadband devices. The goal of determining the radiated far-fields of an antenna is accomplished by the transformation of near-field data collected over a planar sampling surface. The near-fields are generated with an antenna excited by a short duration transient pulse. In particular, the near-fields of an aperture antenna are collected using a digital sampling oscilloscope. The bandwidth of the excitation pulse is approximately 10 GHz.
At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.
This paper describes a technique for determining the magnitude of a radiating field from measurements taken using an Infrared (IR) camera. A thin resistive screen of low conductivity is positioned in the radiating field. The resistive screen absorbs energy from the radiating field and converts this energy into heat within the screen. An IR thermal picture is then taken of the heat distribution on the screen. The resulting 2D image is called an IR thermogram, i.e. and iso-temperature contour map of the data which is a representation of the electric field. The thermogram is then processed to determine the intensity (magnitude) of the radiating field at each pixel location in the thermal image. We describe the technique and show comparisons made between standard probe measurements and results from measured IR thermographs.
This paper describes the application of the plane-to-plane (PTP) iterative Fourier processing technique to infrared (IR) thermographic images of microwave fields for the purpose of determining the near-field and far-field patterns of radiating antennas. The PTP technique allows recovery of the phase by combining magnitude-only measurements made on two planes, both in the radiating near field of the antenna under test. We describe the PTP technique and show excellent comparisons between the predicted results and results from measured IR thermograms of the field of a 36 element patch array antenna operating at 4 GHz.
This paper describes the application of the plane-to-plane (PTP) iterative Fourier processing technique to infrared (IR) thermographic images of microwave fields for the purpose of determining the near-field and far-field patterns of radiating antennas. The PTP technique allows recovery of the phase by combining magnitude-only measurements made on two planes, both in the radiating near field of the antenna under test. We describe the PTP technique and show excellent comparisons between the predicted results and results from measured IR thermograms of the field of a 36 element patch array antenna operating at 4 GHz.
Accurate EIRP measurements are possible to make on a near-field range but require great care and attention to detail. NSI has recently implemented a near-field test range for the Globalstar satellite program which makes automated EIRP and gain measurements. Automation for this program is extremely important since the production cycle requires testing many antenna systems per month, each of which has two antennas with 16 separate beams per antenna. Among the various range measurements, EIRP is the key parameter of the Transmit antenna’s performance. This paper reviews the measurement theory of EIRP measurements and presents some of the results of this automated activity.
For indoor antenna measurements, compact ranges or near-field/far-field techniques are most frequently used. One of the major problems is the handling of physically large antennas. Compact ranges will in general provide test-zone sizes up to approximately 5 meters in diameter. Applying the planar NF/FF technique, even larger test-zone sizes can be realized for certain applications. On the other hand, requirement of real-time capability, for instance in production testing, will exclude NF/FF techniques. It has been shown previously that single-plane collimators have a pseudo real-time capability which makes these devices comparable to compact ranges. Furthermore, the physical test-zone sizes which can be realized when compared to compact ranges are approximately 2-3 times larger for the same size of the anechoic chamber. Finally, it will be shown that the accuracy in sidelobe level determination, gain and cross polarization is considerable higher than with other indoor techniques, even at frequencies below 1 GHz.
The Hughes Space and Communications Company (HSC) is in the process of completing the construction, installation and validation of two large horizontal near-field antenna measurement ranges. These new measurement systems are located in the existing HSC satellite factory building. These ranges will be used to measure various types of directive satellite antennas both at a unit level and at spacecraft level. The facility will accommodate mechanical integration of the test articles as well. This facility is the result of Hughes committing the time and money to create a state of the art antenna measurement facility that will be highly efficient and accurate. A detailed description of this facility’s configuration, design and current status will be discussed herein.
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 measurement control and data preprocessing system has been implemented at the TUD-ESA Spherical Near Field Antenna Test Facility. This facility is located at the Technical University of Denmark (TD) and operated in cooperation with European Space Agency (ESA). The measurement control system as well as a flexible information system is described. The data collected during measurements are passed through a preprocessor before the data are stored on disc. By taking advantage of the band-limited nature of the measured near field the preprocessor is able to detect RF leakages and to correct for the non-ideal sampling that is caused by non-zero integration time of the receiver.
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.