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A reflector antenna has been designed for the RCS measurements. The antenna is dual linearly polarized and exhibits constant beamwidth over an octave bandwidth. The design principle has been to keep the effective antenna aperture constant in terms of the wavelength over an octave bandwidth. The theoretical design lead into the choice of the antenna and the feed. The reflector was an offset parabolic reflector. The feed was a ring-loaded conical corrugated horn. The measurement results of the designed reflector antenna showed very good agreement with the computer results. The V- and H- polarization characteristics of the antenna are almost identical.
Performance trade-offs are investigated between the use of clustered waveguide bandwidth feeds and the use of one multi-octave bandwidth single aperture feed in a prime focus compact range for dual linear polarization. The results show that feed structure may be used for advantage for the particular test requirements of compact range systems for Radar Cross Section Measurement.
A comparative analysis of different geometries of dual compact antenna test ranges is done looking at the cross-polarization level and the scanning capability of the system. The analysis is based on a very simple and quick computation of the fields over the main refector [sic] projected aperture.
Radar Cross Section (RCS) measurements are typically made at linear polarizations (usually horizontal and vertical) and the transmit and receive polarizations are the same (co-polarized). In addition, however, it is sometimes desirable to measure the cross-polarized RCS of a target (i.e., transmit horizontal, receive vertical or vice-versa). A complete set of both co-and cross-polarized RCS of a target is called a scattering matrix. This paper describes the algorithm used for calibrating a scattering matrix measurement in the McDonnell Douglas Technologies Inc. (MDTI), Radar Measurement Center (RMC). Verification data collected at Ka band on various targets is included to validate the algorithm and implementing computer code.
A bistatic calibration technique for wide-band, full-polarimetric instrumentation radars is presented in this paper. First general bistatic measurement problems are discussed, as there are the coordinate systems, the definition of polarization and the bistatic scattering behavior of convenient calibration targets. In chapter two the new calibration approach is presented. The general mathematical and physical description of errors introduced in the bistatic system is based on the radiation transfer matrix. The calibration procedure is discussed for the application with a vector network analyzer based instrumentation radar. For verification purposes measurements were performed on several targets.
Recently, the theory and computer programs on the Periodic Moment Method (PMM) for scattering from both singly and doubly periodic arrays of lossy dielectric bodies have been developed. The purpose is to design microwave wedge and pyramid absorber for low reflectivity so that one can improve measurements and/or reduce the size of the anechoic chamber. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated on the Cray Y-MP supercomputer at the Ohio Supercomputer Center. Through these studies, some wedge and pyramid absorber configurations have been designed, fabricated and tested in the OSU/ESL Anechoic Chamber. Very good agreement between calculations and measurements has been obtained. In the 1990 AMTA meeting, several wedge absorber designs and results for the TM case and normal incidence were presented. In this paper, the measured and calculated frequency responses of some experimental wedge designs, as well as an 8” and 18” commercial wedge and pyramid absorber panels will be reported for both TM and TE polarizations. Time domain responses will also be shown for both measurements and calculations.
The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.
An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system. This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas. Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.
The experience with near-field scanning at Scientific-Atlanta began with a system based upon a analog computer for computing the two-dimensional Fourier transform of the main polarization component. When coupled with a phase/amplitude receiver and a modest planar near-field scanner this system could produce far-field patterns from near-field scanning measurements. In the 1970’s it came to be recognized that the same advances, which made the more sophisticated probe-corrected planar near field measurements possible, would enable conventional far-field range hardware to be used on near-field ranges employing spherical coordinates. In 1980 Scientific-Atlanta first introduced a spherical near-field scanning system based upon a minicomputer already used to automate data acquisition and display. In 1990, to meet the need of measuring complex multistate phased-array antennas, Scientific Atlanta began planning a system to support the high volume data requirement and high speed measurement need represented by this challenge. Today Scientific-Atlanta is again pursuing planar near-field scanning as the method of choice for this test problem.
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.
New results of wideband polarimetric radar-cross-section-(RCS-) antenna measurements are presented. A special antenna network description including polarization information and multiport feeding offers new insight in antenna behavior. The procedure omits the utilization of a standard gain antenna for absolute gain determination and no RF-feedline is necessary to the antenna under test. Antenna radiation, scattering and feed characteristics are all obtained with one measurement setup. Theory as well as measurements on different dual-polarized antenna types demonstrate the efficiency and uniqueness of this technique.
The spherical probing technique for the angular location of secondary scatterers in antenna measurement ranges is demonstrated for an anechoic chamber far-field range. Techniques currently used for source location use measurements of the range field on a line or plane. A linear motion unit and possible a polarization rotator are necessary to measure the range field in this manner. The spherical range probing technique uses measurements of the range field over a spherical surface enclosing the test zone allowing existing range positioners to be used for the range field measurement. The spherical probing technique is demonstrated on an anechoic chamber far-field range with a known secondary reflection source. The plane wave spectrum of the measured range field is computed and used for source angular location. Source locations in the range correspond to the angular locations of amplitude peaks in the spectrum. The effects of the range field probe on this spherical probing is investigated by performing probe compensation.
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.
This paper will describe new developments in a gated-CW radar that has been designed to improve the productivity and sensitivity of RCS measurements. Improvements in data acquisition speeds result from the design of a fast synthesizer, a data acquisition co-processor and a pulse modulator. Each of these new products have been specifically designed to take advantage of the high speed capabilities of Scientific-Atlanta’s Model 1795 Microwave Receiver. The RF sub-system has also been designed to permit continuous 2-18 GHz, full polarization data acquisitions. Critical RF components are now mounted at the feed in the chamber, improving the sensitivity and ringdown of the system. Productivity in analysis activities has been improved by the use of a multi-tasking system controller which permits simultaneous use of the system for acquisitions, analysis and plotting.
Compact range systems have been widely used for high quality RCS measurements. However the taper and cross-polarization effects can lead to significant measurement errors especially as the target approaches the border of the target zone. The taper error is mainly caused by the feed’s finite beamwidth, and the cross-polarization error by the feed’s cross-polarized radiation and the offset configuration of the reflector. A method to correct these errors is presented. In order to perform taper and cross-polarization error corrections, one has to be able to predict the target zone fields and determine the locations and complex strengths of the various scattering centers associated with the target. The correction can then be done by compensating for the taper and cross-polarization effects for each localized scattering center. Several measurements have been taken, corrected and then compared with the theoretically expected results to validate this technique.
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.
The reflective properties of a flat circular plate and a long thin wire are discussed in connection with the quality and calibration of the quiet zone (QZ) of a compact antenna test range. (CATR). The flat plate has several applications in the CATR. The first is simple pattern analysis, which indicated errors as function of angle in the QZ, the second uses the plate as a standard gain device. The third application makes use of the narrow reflected beam of the plate to determine the direction of the incident field. The vertical wire has been used to calibrate the direction of the polarization vector. The setup of an optical reference with a theodolite and a porro prism in relation to the propagation direction of the incident field is presented as well.
A novel and very powerful measurement technique is presented which allows the determination of antenna radiation and scattering by radar-cross-section (RCS-_ measurements. The antenna under test is treated as a loaded scatterer using a polarization dependent network model that allows a complete antenna description in terms of scattered, radiated and absorbed waves. A load variation principle is used to determine the network model parameters and all commonly used antenna parameters like gain, antenna polarization, axial ratio, polarization decoupling, input impedance and also structural scattering can be derived from the backscatter measurement without using any additional standard antenna. With the antenna network description it is furthermore possible to examine the antenna behavior for arbitrary excitation or loading on their waveguide or radiation port.
Accurate near-field cross-polarization measurements on circularly polarized (CP) antennas at millimeter-wave frequencies require well-characterized probes with low axial ratios. We have recently obtained and calibrated dual-port CP horns for use as near-field probes at frequencies of 40-50 GHz. These horns have axial ratios which are 0.3 dB or less over a 10% frequency bandwidth. With these good axial ratios the difference between vector and scalar probe correction is usually small. Additional advantages of the dual-port probes are the need for only a single alignment, more accurate knowledge of the relative phase between two ports of the same probe, and the ability to obtain both main and cross polarized data during one scan. The axial ratios of the dual port CP probes are also better than those of single-port CP Probes. In this paper we present some gain, axial ratio, and pattern measurements for these probes and show that they give accurate cross-polarization measurements.
A spherical range probing technique for the location of secondary sources in far-field compact and spherical near-field antenna measurement ranges are presented. Techniques currently used for source location use measurements of the range field on a line or plane to locate sources. A linear motion unit and possibly a polarization rotator are necessary to measure the range field in this manner. The spherical range probing technique uses measurements of the range field made on a spherical surface allowing the range positioners to be used for the range field measurement. The plane wave spectrum of the measured range field is used for source location in the spherical probing technique. Source locations in the range correspond to the locations of amplitude peaks in this spectrum. Source resolution limits of this technique is illustrated using simulated range measurements. Obtaining a plane wave spectrum from measured data is discussed.