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For over a decade the National Bureau of Standards has utilized the Planar Near-field Method to accurately determine antenna gain, polarization and antenna patterns. Measurements of near-field amplitudes and phases over a planar surface are routinely obtained and processed to calculate these parameters. The measurement system includes using a cw source connected to an accessible antenna port and a two channel receiver to obtain both amplitude and phase of the measurement signal with respect to a fixed reference signal. Many radar systems operate in a pulsed-cw mode and it is very difficult if not impossible to inject a cw signal at a desired antenna port in order to calibrate the antenna. As a result it is highly desirable to obtain accurate near-field amplitude and phase data for an antenna in the pulsed-cw mode so that the antenna far-field parameters can be determined. Whether operating in the cw or pulsed-cw modes, one must be concerned with calibrating the measurement system by determining its linearity and phase measurement accuracy over a wide dynamic range. Tests were recently conducted at NBS for these purposes using a precision rotary vane attenuator and calibrated phase shifter. Such tests would apply not only to measurement systems for determining antenna parameters but also to systems for radar cross section (RCS) measurements. The measurement setup will be discussed and results will be presented.
A 15-meter diameter self-deployable antenna has been developed which utilizes the hoop-column structural concept with a gold-plated molybdenum mesh reflector. This antenna was developed to determine if a system could be designed and built with the dimensional tolerances necessary for in-space operational performance and for use as a test article in a ground based technology development program. One feature of the design is the provision for reflector surface shape control by cable adjustment. The antenna was deployed and tested at the Martin Marietta Denver Aerospace Near-Field Test Laboratory to measure its surface shape and its electromagnetic performance. RF test results show very good agreement between predicted and measured radiation patterns. The antenna is currently undergoing modifications which will allow automated surface adjustments and adaptive feeds to be utilized for further improvement in the electromagnetic performance. Controls, structural, and simulated thermal deformation tests will be integrated with future electromagnetic tests.
In any type of electromagnetic measurements, the ideas of "precision and accuracy" and "low cost" tend to be mutually exclusive. At Scientific-Atlanta, for instance, production testing of antenna products is conducted in low cost miniature "anechoic chambers" which are fabricated in-house. These "chambers" are actually medium-sized to large (64-200 cubic feet) rectangular boxes with absorber attached to their walls. They are usually equipped with single axis positioners at one or both ends, and their usefulness is limited to the measurement of axial ratio on low gain small antennas.
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.