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The offset fed parabola is one type of reflector used in compact radar ranges. Cross-polarization problems have been noted when a parabola is used in near field applications. A good understanding of the near field cross-polarization effects was needed to evaluate this type of reflector for a compact range. We found that the polarization vector was rotated differently at each location in the "quiet zone." The polarization vector rotation is due to the parabolic curvature. In addition, a mathematical model was derived that compares well with the data. A theoretical study of how the RCS measurements of a wing are affected is presented.
We present a method for determining antenna couplings caused by a near field. These couplings often exist between antennas on tactical aircraft, or any other types of airborne platforms. The limited size and peculiar exterior contour of these platforms make a theoretical calculation of near field couplings difficult. High antenna isolation is critically important to proper avionics functioning. To achieve this higher degree of isolation, the causes of antenna couplings need to be identified and corrected. At present, no diagnostic methods are available. Existing methods only measure the degree of isolation between antennas, which is the ration between the power of the transmitting antenna to that of the receiving antenna. These methods do not provide any clues as to what may be causing the couplings. A diagnostic method using a network analyzer is feasible, and the causes of antenna couplings can be identified.
We present a method for determining antenna couplings caused by a near field. These couplings often exist between antennas on tactical aircraft, or any other types of airborne platforms. The limited size and peculiar exterior contour of these platforms make a theoretical calculation of near field couplings difficult. High antenna isolation is critically important to proper avionics functioning. To achieve this higher degree of isolation, the causes of antenna couplings need to be identified and corrected. At present, no diagnostic methods are available. Existing methods only measure the degree of isolation between antennas, which is the ration between the power of the transmitting antenna to that of the receiving antenna. These methods do not provide any clues as to what may be causing the couplings. A diagnostic method using a network analyzer is feasible, and the causes of antenna couplings can be identified.
Over the last five years a considerable attention has been paid to further developments of Compact Antenna Test Ranges for both antenna and RCS measurements. For many applications, these devices proved to be more attractive than outdoor ranges or near-field/far-field transformation techniques. On the other hand, accurate operation at very low or very high frequencies can cause considerable difficulties. It is the aim of this paper to describe the theoretical limitation of collimating devices, in particular for low frequencies. For this purpose, an idealized collimator will be defined. Using the spectral components analysis a comparison of achievable accuracy will be made between collimators and outdoor ranges. Theoretical limits in the accuracy for RCS measurements will be computed for all applicable frequencies. Finally, a comparison will be made between the experiments on a dual-reflector Compact Antenna Test Range and theoretically achievable limits. Representative targets, like cylinders and rectangular plates have been used for experimental investigation. These data will also be presented.
Precision rotary vane attenuator calibrations are required in both the planar near-field method for determining antenna parameters and in the extrapolation method for determining on-axis gain of standard gain horns and probes. These attenuator calibrations are used to measure the linearity of the receiving systems and also to provide a precise offset capability used in insertion loss measurements. The Antenna Metrology Group of the National Bureau of Standards has utilized the i.f. substitution method to calibrate millimeter wave precision attenuators using equipment available in their measurement laboratory. The technique will be described along with the problems encountered. Results will be presented. In addition to mm wave attenuator measurements, the first calibrations of mm wave antennas and probes has resulted in tests to determine waveguide flange to flange connection errors for insertion loss measurements where repeated connections are necessary. The effects of these measurements on the overall error budget for the determination of the gain of an antenna will be presented and the effects of methods to reduce these errors will be discussed.
A general theoretical approach is formulated to describe the complex electromagnetic environment of an N-element array. The theory reveals the element-to-element interactions and multiple reflections within the array. To experimentally verify some features of the theory, measurements on experimental array panels in various configurations were made. These array panels consisted of 256 microstrip radiating elements. In each of the configurations both the near-field and portside signals were measured to study the interactions between these panels. In particular, the effects of open-circuited array panels on the radiation pattern of a single panel are observed both in the near field and in the far field. It is found that internal scattering is the main mechanism of interaction between panels, rather than reradiating of signals received from adjacent panels. The effects of scattering are observable at the -50 dB level.
A large, tapered anechoic chamber exists at the NASA Johnson Space Center (see Figure 1). This chamber has been used to test antennas mounted on full-size replicas of the Apollo moon lander. Also, antennas mounted on a scale model of the Space Shuttle have been tested in this facility. The chamber will have extensive utilization in the future for testing proposed Space Station antennas and other satellite antennas.
Conventional pattern measurements are difficult to apply when the aperture is very large (250 lambda or more), particularly in the case of a relatively fragile antenna structure intended for a space application. Near field techniques can offer a solution, but may need a relatively large R.F. enclosure and custom instrumentation. This paper examines various alternative approaches in the case of the 15 m planar array under development at CAL for Radarsat. Specifically, the techniques under consideration include planar probing, cylindrical probing, planar cylindrical probing, intermediate range spherical probing, and some special variants. It is shown that the fact that the Radarsat antenna generates shaped beams as opposed to pencil beams impacts the relative accuracies achieved by these techniques to a very significant extent. The data collection and processing time, the size of the anechoic chamber needed, and the instrumentation requirement are also important considerations.
A planar and cylindrical near field facility is described. The facility was designed, constructed and integrated at RAFAEL as an extension to its veteran S-A 2020 Antenna Analyzer. The system utilizes open loop stepper motors for linear motion. Operation modes include cartesian, plane-polar and cylindrical measurements. All measurement control and data acquisition functions are performed by the 2020 computer. Conversion routines are being run on a host CDC CYBER mainframe computer and include a new algorithm for polar probe correction.
Fundamental research in the behaviour of the electromagnetic field in the immediate vicinity of radiators, scatterers or diffracting objects, indicates the need for equipment capable of scanning the three dimensional volume surrounding such objects. Such equipment requires the ability to determine the amplitude and phase of the vectorial components of the fields in a variety of ways, such as on a three dimensional grid of uniformly or otherwise spaced points, on regular (such as spherical, paraboloidal) or on arbitrarily defined surfaces or along various loci.
Near-field measurement techniques are widely used today for antenna far-field pattern characterization. Since the 60's, much has been done concerning accuracy. The three main coordinate systems, planar, cylindrical, and spherical have been investigated. probe corrections have been introduced [1] - [6].
The planar scanning system is commonly used in the near field testing of high gain antennas, where the rectangular measurement grids are used. The polar grids are also used, which are more convenient when the antenna aperture is circular. In the planar scanners the measurements are carried out in the x-y plane in increments of both x and y. The result of the measurement is an mxn matrix of the near field data consisting of m cuts with n data points per each cut. The far field patterns may then be calculated, using the near field data, by the aperture field integration or the modal expansion methods [1]. In this paper the aperture field integration method is studied, where the far field components can be calculated from [1] - [2].
Near field ranges have been used extensively to measure antenna parameters. These ranges have been shown to be very accurate for measuring absolute gain, polarization, and gain patterns. Most antennas are intended to be used with a receiver, a transmitter, or both. In many cases, it is important to characterize the antenna and active electronics as a system.
The National Bureau of Standards, in Boulder, has been doing various types of swept frequency antenna measurements for a number of years. Included in these are swept gain measurements on the Extrapolation range and fixed point Near-Field swept measurements.
This paper discusses some of the applications of high-resolution coherent radar image processing techniques in unimproved indoor facilities. The techniques are particularly useful in situations where traditional indoor range chambers are unavailable or impractical. Experiments in an 18-foot-high warehouse building have shown that useful measurements can be made at close quarters, in a high-clutter environment.
The planar scanning system is commonly used in the near field testing of high gain antennas, where the rectangular measurement grids are used. The polar grids are also used, which are more convenient when the antenna aperture is circular. In the planar scanners the measurements are carried out in the x-y plane in increments of both x and y. The result of the measurement is an mxn matrix of the near field data consisting of m cuts with n data points per each cut. The far field patterns may then be calculated, using the near field data, by the aperture field integration or the modal expansion methods [1]. In this paper the aperture field integration method is studied, where the far field components can be calculated from [1] - [2].
The NBS has developed a test for estimating the effects of multiple reflections between the probe and antenna on the far field using a near-field measurements. The essence of this test is to take near-field data at more than one separation distance. For each separation distance the far field is obtained using a Fourier transform. The different far fields are then averaged in a complex manner. The difference between the average far field and each of the other far fields is due to multiple reflection effects.
Near-field testing was conducted on the 30 GHz TRW proof-of-concept (POC) Multibeam Antenna (MBA). The TRW POC MBA is a dual offset cassegrain reflector system using a 2.7 m main reflector. This configuration was selected to assess the ability to create both multiple fixed and scanned spot beams. The POC configuration investigated frequency reuse via spatial separation of beams, polarization selectivity and time division multiple access scanning at 30 GHz.
This paper reports on the measurements portion of an ongoing research program at the Georgia Institute of Technology into the design, analysis and measurement technologies of radomes. Specifically this paper reports on a technique for the near-field measurement of radome performance. The motivation for the development of the near-field measurement technique for radomes is to identify the types of interactions which take place between the radome and the transmitted electromagnetic field. It is postulated that such phenomena as coupling to the radome wall, tip scattering, internal reflections and bulkhead reflection would be easier to identify through near-field measurement than far-field measurement. * This work was supported by the Joint Services Electronics Program and Northrop Corporation
In this paper a theoretical study is reported of the electromagnetic performance of the new Scientific-Atlanta compact range reflector system. The reflector consists of a 15-foot semicircular parabolic reflector with a 5-foot blended rolled edge added to the circular section and skirt mounted on the base. The performance of this system is examined in terms of its probed near field data at the center (36-feet) and back end (50-feet) of the target zone. The calculated results are for the three-dimensional reflector and include the skirt and blended rolled edge diffracted field as well as the aperture blockage scattering caused by the feed and associated feed/mount structure. The potential target zone size based on these parameters is presented as a function of frequency and desired ripple level requirement. *This work was supported by the National Aeronautic and Space Administration, Langley Research Center, Hampton, Virginia under Grant NSG-1613 with The Ohio State University Research Foundation.