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.)
In near field antenna measurements, knowledge of the the [sic] probe antenna’s pattern, polarization and gain are of vital interest. To calibrate a probe for near field measurements is a delicate task, especially if the probe is small, i.e. low gain. The near field probe and the parameters general to a probe calibration are presented. The delicate task of obtaining an accurate gain for small aperture antennas as well as the problem of transfering [sic] the calibration from the facility where the probe is calibrated to the facility where it is to be used are focussed [sic] upon For a small aperture, the pattern is that of the radiating aperture. The unwanted scattering may be removed by filtering in the spherical mode domain thus obtaining the true aperture radiation. The gain derived from this may however be of little use in reality since the aperture always needs some form of mounting. Such a mounting may be covered with absorber which may reflect and diffract and thus affect the gain value.
In this paper, an antenna measurement technique based on modified cylindrical NF/FF transformation will be presented. In conventional cylindrical near-field scanning techniques, the near fields are probed on a cylindrical surface surrounding the test antenna. This required extensive data acquisition and processing time which can be reduced substantially if the antenna under test is illuminated by a cylindrical wave. In this hybrid approach, cylindrical wave illumination is generated using a single parabolic reflector in combination with a (point) source. The far-field pattern is then computed by a powerful one-dimensional NF/FF algorithm. It is concluded that this alternative approach combines the attributes of the compact-range technique and the classical NF/FF transformation.
The Concept of Compact Test Range has been recently much used for antenna testing facilities, its main characteristic of having far-field conditions in a small and closed place, for a very large frequency band, makes it very attractive. Antenna manufacturers are building them up when the millimetric waves and the spacecraft flight model antennas become part of their activities. The change of the point of view of the antenna characteristics – now, parameters like Gain and Radiation Patterns are replaced by EIRP, Flux Density or Coverage- modifies the classical test philosophy. It makes different the Test Procedures which, in addition, have to take into account the cleanliness and the quality control required for handling flight models, as well. The Compact Payload Test Range (CPTR) in ESTEC shows up a PWZ of 7 x 5 x 5 metres for a frequency range from 1.5 to 40 GHz.; it has been created for testing whole Spacecraft Payloads in space required cleanliness area. The particular properties of the CPTR as such as shielded room, feed scanning, multiaxis test positioner, etc. are used to improve its test possibilities.
Problems exist with the measurement of large aperture antennas due to the far field requirement. This paper discussed a new method to measure a phased array at about 1/10 the normal far field. The basic idea involves focusing the test array at probe antenna a distance R away from the aperture. In the described measurement technique the probe antenna is placed on an arm that rotates 100º on the focal arc given by Rcos(?). This arc minimizes defocusing due to phase aberrations. To minimize the amplitude errors, the pattern of the probe antenna is carefully matched in order to compensate for the 1/R variation induced amplitude error. The application of this technique will enable arrays to be measured in anechoic chambers, allowing convenient classified testing, while avoiding the effects of weather, and will reduce the risks inherent in the high power testing on transmit. The results of a computer simulation is presented that characterizes the validity and limitations of the technique.
Beamspace techniques are usually employed to synthesize phased array antenna patterns of arbitrary shape. In this paper a beamspace method is used to calibrate the pattern of a 32-element linear array with a conventional array taper. By measuring the antenna pattern in specific directions the beamspace technique permits the actually applied excitation function to be determined with little mathematical effort. Iterative corrections can then be made to the excitation function to maintain low sidelobe performance, or to compensate for element failures. Since local corrections to the array pattern result in global changes to the excitation function, explicit knowledge of where an element failure has occurred is not required. The beamspace analysis was carried out using antenna patterns obtained by electronically scanning the array past a far-field source. Such pattern measurements offer the possibility of maintaining phased array performance in an operational environment.
When attempting to make accurate Radar Cross Section (RCS) measurements, it is vital to understand the background levels of both the range and the target support fixture. Typically these support fixtures are either foam columns or metal pylons. Determining the RCS levels of the metal pylons requires the installation of a termination device to hide the rotator which has a significantly lower RCS than the pylon being measured. Quite often this is an impossible task, especially at lower frequencies. An algorithm that accurately determines the pylon background levels independent of the RCS contribution of the pylon terminator is presented. This algorithm requires translating the terminator linearly and isolating the background from the resulting interference pattern. Data is included that validates the implementing computer code.
Errors due to the interaction between test body and the Device Under Test are often overlooked in test body design. Interactions which cannot be gated or subtracted can be present even in low RCS test bodies. This paper presents an approach to evaluate the edge interaction errors of a component RCS test body. In order to quantify the interactions, small cylinders were attached to the face of the test body and measured from grazing to 50 degrees. The scattering of the cylinders illuminated the edges so that the interactions could be measured. This data is presented along with the results of several computer models which were used to determine the interactions involved. A method of moments model of the cylinders on an infinite ground plane gave the theoretical level of the cylinders. A pattern of a monopole antenna on a test body shaped ground plane was used to determine the contribution of each edge; and a point source model was used to locate the points on the edge where the diffraction occurred. This technique allows the dominant source of error signals to be identified.
Range resolution of a radar image can be obtained by use of wide-band signal (linear FM or chirp waveform) and cross-range resolution by object rotation which synthesized a large antenna aperture (the so called ISAR method, refer [1]). Although both cross-range profiles can be resolved by rotation of the abject about two mutually orthogonal axes, however, the data manipulation would be quite cumbersome and the measurement implementation would require a mechanical support system by which the objet [sic] can be independently tilted and rotated relative to the radar axis. In this paper, the algebraic reconstruction technique (ART)[2] for tomography is used to resolve the vertical cross-range profile (along the axis normal to the ground) while the horizontal cross-range profile still resolved by ISAR method. Applications of the ART to a simple circular pattern and a complicated emblem pattern of the CSIST show that ART is a suitable approach and easier than ISAR method to obtain the second cross-range resolution.
Martin Marietta designed and brought on-line an indoor far-field chamber used for radar cross section (RCS) evaluation. The range has conductive walls on all sides except for the pyramidal absorber covered back wall. The chamber was designed such that wall/floor/ceiling interactions occur with a distance (time) delay allowing for their isolation from the test region. Software gating techniques are used to remove these unwanted signals. This paper presents an analysis of the conductive chamber using Geometrical Optics (GO). The objective was to analyze and evaluate the plane wave quality in the chamber test region. The evaluation of the plane wave was performed using the angle transform technique. The measured results were compared to analytical results and measured antenna patterns.
The properties of a focused scalar horn-lens antenna are presented. The behavior of the field from the lens to the far field is determined from electromagnetic principles and measured antenna patterns at the focal distance are shown.
Recently an antenna pattern measurement technique has been developed which eliminates the effects of the finite ground plane on which the test antenna is mounted. The scattered fields from the edge of the ground plane can often cause perturbations in the total fields, and thus, result in significant differences in the measured patterns as compared the theoretical predictions. This technique consists of the measurement of the edge diffracted fields and their subsequent subtraction from the original pattern. A simple theoretical model is developed to introduce the subtraction technique, and comparisons are made which show the excellent agreement between theoretical (obtained assuming an infinite ground plane) and “corrected” experimental antenna patterns. Experimental results are given from an open-ended waveguide opening into both circular and square ground planes.
A novel differential phase measurement method is developed. No flexible cables or rotary joints are needed in this method. Phase center positions and phase patterns of two corrugated horns are measured at 105-115 GHz and 176-190 GHz by using this method. Good agreement between the measured values and theoretical values, calculated with the modal matching technique, is obtained. Also a new phase error correction method is introduced. This method makes possible to measure the phase error in the cable and then to remove the error numerically from the results. The accuracy of the phase error correction is limited by the phase measurement device in the system. Experimentally this method is verified at 10 GHz.
The aperture opening design of the subreflector chamber for a dual-chamber Gregorian compact range system is presented in this paper. The subreflector is a serrated edge ellipsoidal reflector. The performance of the subreflector chamber and absorber aperture opening has been evaluated in terms of pattern measurements and by cross-range diagnostic techniques. The results of this evaluation have been used to further improve the design of the aperture opening of the subreflector chamber.
A new approach has been developed to achieve an octave bandwidth, reduced size feed fot compact range reflectors. It can provide highly isolate, orthogonal polarizations with a minimal size, suitable for operation at frequencies down to 500 MHz and below. Its construction is relatively simple, with only a few specific dimensions. The beam-width is compatible with compact range reflector feed requirements. The method uses crossed dipoles over a small circular ground plane, with a rim to equalize the E- and H- plane patterns. Parasitic elements are employed to extend the bandwidth with matching provided via a section built into the feed line. The design was optimized using the Numerical Electromagnetics Code (NEC) computer program.
The effects of non-systematic receiver instrumentation errors on precision antenna measurements are investigated. A simple uncertainty model relating dynamic range to random perturbation effects on amplitude measurements is proposed. Examples of measurement uncertainty versus both input level and measurement speed are presented using data taken on modern measurement receivers. Dara are compared with the model to estimate measurement uncertainty at various pattern levels and acquisition speeds. Equivalent dynamic range specifications are deduced from the measures data.
The utility of wideband RCS data for characterizing scattering mechanisms of complex objects has been established by wide-spread applications. The fundamental data from which the final products are derived consist of calibrated scattered fields measured coherently as a function of frequency and aspect angle. By processing these data, one-dimensional range or cross-range reflectivity profiles can be derived; by further processing, two-dimensional images can be derived. Modern RCS instrumentation systems capable of rapidly measuring and processing wideband data provide more object information than is conveyed by the RCS pattern, which has been the traditional descriptor of scattering behavior. The procedures of one- or two-dimensional imaging inherently involve integration processes, constituting many-to-one mappings in which data from a large set are collapsed to produce an individual pixel of the image. For example, a particular pixel of a range response is derived from the total object response “integrated” over a band of frequencies; similarly, a pixel of a two-dimensional image is derived from the object response “integrated” over frequency and angle. The exposure of a local feature of the object signature, obtained by collapsing the fundamental data, comes at the cost of obscuring the global descriptor. This paper explores techniques for presenting large amounts of information on single displays which retain both global and local features of the scattering process. These tools provide to the RCS analyst options for extracting and interpreting significant information from the measured data without arbitrary degrees of integration which can mask essential details represented in the data. The display methods utilize color coding to increase the amount of information conveyed by a single plot. Because color reproduction is not available for the proceedings, the paper is to be distributed at the conference.
This paper examines antenna pattern measurements of RF frequency antennas (300 kHz-3 GHs) using an integrated source/receiver and measurement control software. Current microwave measurement systems provide sufficient measurement capability but are often too expensive to be used on ranges which require test frequencies of less than 3 GHz such as aircraft communications, cellular radio, GPS, and satellite telemetry antenna. Several system block diagrams based on the HP 8753 network analyzer will be examined with respect to system performance, measurement accuracy, and cost. System considerations for outdoor RF ranges such as RFI susceptibility will also be addressed.
A new implementation of the planar near-field back projection technique for phased array testing and aperture imaging is described. In the alignment of phased arrays, the aperture field is treated as a continuous distribution rather than using idealized array concepts. The continuous field is then sampled to obtain element excitations. In this way, nonrectangular arrays can easily be accommodated. The method also produces highly interpolated images of apertures that can offer much insight into their nature. Also, any polarization of the aperture field may be obtained if the probe pattern has been characterized. The technique uses large FFTs which are computed very quickly by a workstation located in the facility. Results from an iterative phase alignment of a 12x18 phased array are presented, as well as highly interpolated images of apertures and results which demonstrate the polarization selection.
A technique for aligning a phased array is described. Array element attenuation and phase commands are derived from far-field patterns measured without calibrations. The technique is based on iteratively forming mulls in the antenna pattern in the directions specified by a uniform array illumination. It may be applied in situations where array elements are not individually accessible, or where an array contains no build-in calibration capacity. The alignment technique was evaluated on a far-field range with a linear, 32-element array operating at L-band. The array containing transmit/receive modules with 12-bit amplitude and phase control. Insertion attenuation and phase measurements were comparable to those obtained by conventional techniques. However, the alignment procedure tends to compensate for the effects of nonuniform element patterns and range multipath. Thus, when used to implement other excitation functions, the array sidelobe performance with adaptive calibrations was substantially better.
To enhance the performance of existing near field techniques the new idea of far field pattern determination from only amplitude distributions of the near field is proposed. In this way the difficulties related to phase measurements are overcome. Some different algorithms are introduced and discussed. In particular, after recalling results for the planar geometry, cylindrical scanning surfaces are considered. The feasibility and the performances of the introduced algorithms are shown through numerical examples.