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Data collection for Synthetic Aperture Radar (SAR) antenna measurements is increasingly making measurement stages very time consuming. This paper presents the capabilities of fast Planar Near Field (PNF) instruments using a linear modulated probe array. It demonstrates the possibilities to decrease the classical near field mechanical scan time by a factor ranging from 100 to 1000. Emphasis is given to the advantages of this technique for multi parameter antenna measurements.
The radiation characteristics for a dual-frequency, dual-polarized millimeter wave antenna for a radar operating at 33 and 95-GHz were measured at the Ipswich Research Facility. On-pole and cross-pole radiation patterns were measured using the 2600 foot far field range. In this paper we'll discuss the general design of the antenna feed system and the instrumentation ensemble used to perform the far field characterization of this high performance large aperture dielectric lens antenna.
The far-field radar cross section (RCS) of a conducting sphere is obtained by transforming scattered near-fields measured on a spherical surface. A simple and convenient calibration procedure is described that involves measuring the incident field directly at the target location. Although a non probe-corrected transmission formula was used in this study the importance of prove correction in practice is demonstrated.
An efficient and accurate spherical near field to far field transformation without probe correction is presented. The indices m of the Legendre polynomials is summed up analytically, thereby reducing the computation time. Computations with both synthetic and experimental data illustrate the accuracy of this technique.
Characterizing antennas under pulsed RF conditions has focused attention on a class of measurement challenges not normally encountered in CW measurements. The primary problems often include high transmit power, thermal management of the AUT, and a close interaction between the antenna and its transmitting circuitry. This paper presents instrumentation techniques for pulsed RF antenna measurements using the Scientific-Atlanta 1795P Pulsed Microwave Receiver as an example of a commercially available solution applicable to both active and passive apertures. Emphasis is given to measurement speed, dynamic range, linearity, single pulse versus multiple pulse measurements, pulse width, pulse repetition frequency (PRF), frequency coverage, system integration and automation, and suitability of equipment for antenna range applications.
The Precision Airborne Measurement System (PAMS) is a flight test facility at Rome Laboratory which is designed to measure in-flight aircraft antenna patterns. A capability which provides antenna pattern measurements for multiple VHF and UHF antennas, at multiple frequencies, in a single flight, has recently been demonstrated. A unique half space VHF/UHF long periodic antenna is used as a ground receive antenna. Computerized airborne and ground instrumentation are used to provide the multiplexing capability. The new capability greatly reduces time and cost of flight testing. The design, construction, and calibration of the half-space log-periodic ground receiving antenna is discussed and the ground and airborne segments of the instrumentation are described.
In polarimetric RCS measurements, the cross-polarization levels which are required in the test zone, correspond closely to those which are realizable with most Compact Antenna Test Ranges (CATR). On the other hand, such a performance may not satisfy the accuracy requirements in cross-polarization measurements of high performance microwave antennas. These applications include spacecraft antennas, ground stations for satellite communications or microwave antennas for terrestrial applications, where two polarizations are used simultaneously.
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.
This paper deals with the development of an approach to the design of triad steering antenna arrays which are used in anechoic chambers for hardware-in-the-loop testing of monopulse antenna seeker systems. In the design of a large array, such as those used for hardware-in-the-loop of guided weapons, it is important to optimize the array element spacing. An excessively narrow spacing results in an unreasonable number of required antennas and increased cost, while an excessively wide spacing will induce angle measurement errors in the seeker under test which can be significant. The specific objective of this effort is to quantitatively describe the monopulse discriminant efforts which result when a non-planar field, radiated by an antenna triad, illuminates a monopulse seeker under test. The approach to this problem is to calculate the triad field at the aperture of the monopulse seeker assuming various levels of triad element phase and amplitude error. Using this illumination field and the illumination function of the monopulse antenna, the resulting sum and difference patterns are calculated along with the monopulse discriminant. Software has been developed to perform these calculations. The resulting patterns are compared with the ideal far field pattern and the discriminant bias, or angle measurement error, is quantified.
A technique is presented for synthesizing a uniform plane wave at Fresnel zone distances. The method attempts to bridge the gap between compact range techniques and far field techniques, in the sense that one may potentially perform antenna or scattering measurements when a compact range reflector is electrically too small and the available far field range length is also too small. Similar to a far-field range, the distance to the test zone region generally varies with the side D of the test item and the frequency of operation being proportional to D2/X. Similar to a compact range, the test zone is confined to a localized region, and the quality of the test zone field does not improve with distance as it does for a far field range. The method is implemented by compensating the phase taper associated with a single radiator by employing a uniformly excited, concentric ring array. The quality and transverse extent of the test zone fields may be adjusted by varying the relative amplitude and phase excitation of the array. Syntheses of a test zone region characterized by a 1 dB amplitude ripple over 70% of the disk defined by the projected ring aperture is demonstrated.
A technique is presented for synthesizing a uniform plane wave at Fresnel zone distances. The method attempts to bridge the gap between compact range techniques and far field techniques, in the sense that one may potentially perform antenna or scattering measurements when a compact range reflector is electrically too small and the available far field range length is also too small. Similar to a far-field range, the distance to the test zone region generally varies with the side D of the test item and the frequency of operation being proportional to D2/X. Similar to a compact range, the test zone is confined to a localized region, and the quality of the test zone field does not improve with distance as it does for a far field range. The method is implemented by compensating the phase taper associated with a single radiator by employing a uniformly excited, concentric ring array. The quality and transverse extent of the test zone fields may be adjusted by varying the relative amplitude and phase excitation of the array. Syntheses of a test zone region characterized by a 1 dB amplitude ripple over 70% of the disk defined by the projected ring aperture is demonstrated.
The field present in the test zone of an antenna measurement range can be calculated from the range field measured on a spherical surface containing the test zone. Calculated test zone fields are accurate only within a spherical volume concentric to the measurement surface. This paper presents a technique for determining the probing radius necessary to create a volume of accuracy containing the test zone of the range. The volume of accuracy radium limit is caused by the spherical mode filtering property of the displaced probe. This property is demonstrated in the paper using measured field data for probes of differing displacement radii. This property is used to determine the volume of accuracy radium from the probing radius. This is demonstrated using measured far-field range data.
Phaseless measurements are going to represent a viable and less expensive alternative to standard near field techniques since they allow to reduce to a very large extent the complexity of an indoor set-up. In fact, they require "scalar" receivers, probe positioning systems with less strict mechanical requirements, and present no cabling problem. Furthermore the anechoic environment extension can be reduced and low dynamic range receivers used as "truncated" data can be managed. In this paper we outline the main advantages of an approach to the solution of the problem of the far field reconstruction from phaseless near field measurements. Conditions to reliably process the collected data can be put forward so circumventing the main difficulties of most solution algorithms for non linear inverse problems. Experimental results are also included for the planar geometry.
The Hughes Aircraft Company Compact Range facility for antenna and RCS measurements, scheduled for completion in 1993, is described. The facility features two compact ranges. Chamber 1 was designed for a 4 to 6 foot quiet zone, and Chamber 2 was designed for a 10 to 14 foot quiet zone. Each chamber is TEMPEST shielded with 1/4 inch welded steel panels to meet NSA standard 65-6 for RF isolation greater than 100 dB up to 100 GHz, with personnel access through double inter locked Huntley RFI/EMI sliding pneumatic doors certified to maintain 100 dB isolation. While Chamber 1 is designed to operate in the frequency range from 2 to 100 GHz, Chamber 2 is designed for the 1 to 100 GHz region. Both RCS measurements and antenna field patterns/gain measurements can be made in each chamber. The reflectors used are the March Microwave Dual Parabolic Cylindrical Reflector System with the sub-reflector mounted on the ceiling to permit horizontal target cuts to be measured in the symmetrical plane of the reflector system.
The new antenna measurement facility in C.A.S.A. Space Division is described. The system, designed and installed by Grupo de Radiación of the Polytechnic University of Madrid , provides antenna measurement set-up for Far Field and both Planar and Spherical Near Field.
A method has been developed by which the fair-field RCS of a target can be evaluated from its RCS measured in the near field. The method can compensate for the nonuniformity of the antenna pattern which can be a function of the angle, the frequency, and the target distance. A correction transform is evaluated which depends on the antenna pattern, the frequency, the target distance and the target size. The correction transform is independent of the target geometry. The RCS of a target is measured in the near field, in a band of frequencies around the frequency at which the far field RCS of the target is desired. The method can practically handle directional scattering elements, shading of the scattering elements by each other, and interactions among the scattering elements. The reconstructed RCS evaluated by this method shows excellent agreement with the actual far-field RCS.
A method has been developed by which the fair-field RCS of a target can be evaluated from its RCS measured in the near field. The method can compensate for the nonuniformity of the antenna pattern which can be a function of the angle, the frequency, and the target distance. A correction transform is evaluated which depends on the antenna pattern, the frequency, the target distance and the target size. The correction transform is independent of the target geometry. The RCS of a target is measured in the near field, in a band of frequencies around the frequency at which the far field RCS of the target is desired. The method can practically handle directional scattering elements, shading of the scattering elements by each other, and interactions among the scattering elements. The reconstructed RCS evaluated by this method shows excellent agreement with the actual far-field RCS.
Increasing use is being made of millimeter-wave systems and there is a need for improved antenna measurement facilities operating at these higher frequencies. Although the practical implementation of compact range and near-field/far-field techniques becomes increasingly difficult, by using a hybrid approach, the attributes of these existing schemes can be exploited and their limitations overcome. The technique uses a linear near-field probe to carry out an instantaneous integration of the field in the date acquisition requirement, together with a quasi-real-time prediction capability. This contribution reviews a number of implementation schemes for the semi-compact antenna test range (SCATR) approach which have been investigated over the past decade and presents the latest results. An implementation of the SCATR with amplitude-only data is presented as an economical and viable method for millimeter-wave frequencies.
A 4 foot by 4 foot near field planar scanner is used to evaluate the performance of a SA5751 compact range in CSIST. Using the far field patterns integrated from the scanned aperture fields, the coming directions of the clutters in the chamber can be determined. Often the clutter level is less than the side lobe level of the far field pattern, the scanned field is multiplied by a certain weighting function before integration to pop out the clutter signal. However the weighting method would broaden the main beam and hence clutters coming close along the reflected wave of the reflector are still can not be seen (sic). In this article, a method called main beam suppression, subtracting a constant filed (sic) on the scanned aperture, is introduced to solve this kind of problem and the result shows it serves well for finding those clutters hidden by the main beam and the side lobes nearer to it.