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The direct far field pattern measurement of an aperture antenna becomes more difficult as the size of the aperture increases. Recent measurements on reflector antennas with 2D2/? =1500’ at The Ohio State University ElectroScience Laboratory have demonstrated the usefulness of the compact range in obtaining the complete far field pattern of antennas with large far field distances.
The direct far field pattern measurement of an aperture antenna becomes more difficult as the size of the aperture increases. Recent measurements on reflector antennas with 2D2/? =1500’ at The Ohio State University ElectroScience Laboratory have demonstrated the usefulness of the compact range in obtaining the complete far field pattern of antennas with large far field distances.
When performing antenna pattern measurements on far-field antenna test ranges or in anechoic chambers, one of the main problems concerning the pattern accuracy is range reflections. Previous works dealing with this have been limited to the one-dimensional case.
A technique has been developed to characterize the illuminating signal present within an antenna test zone. Information of angular multi-path distribution as well as relative signal amplitudes of various paths can be ascertained by transforming phase and amplitude data measured at numerous intervals across the lineal aperture probe apparatus. An experiment was carried out to test the technique using a ten-foot linear aperture probe installed to probe an antenna test zone located at one end of a one-half mile range. During the experiment several measurements were carried out at two different locations within the far-field antenna range and at two different frequency bands. This paper discusses the results of the experiment as well as the practical aspects of this technique.
This paper describes a novel technique for acquisition of far-field antenna patterns from a very low side-lobe antenna. The low side-lobe requirement imposes stringent multipath restrictions on the measurement range and to accommodate this requirement a vertical range configuration is employed rather than the more conventional range which is parallel to the earth's surface. To assure accurate measurement of side-lobe levels, multipath levels were specified at minus seventy dB (-70 dB) relative to the direct-path, peak-of-the-beam level. In this novel range configuration, an Antenna Under Test (AUT) is oriented to face skyward and operated in a receive mode with E-Field illumination provided from an airborne source. An optical tracker provides data of airborne source location and time-division multiplexing of both frequency and antenna beam position enable optimization of data acquisition efficiency. Post-acquisition processing provides de-interleaving of the desired beam(s)/frequency(s). This paper will present a discussion of the problems encountered and the techniques employed to overcome them in the design of this range. A description of the range will also be presented.
The principle of time-gated antenna measurements is differentiation of signals by time-of-arrival. On a far-field antenna range, all reflected stray signals arrive later in time than the direct path signal. A pulse-modulated waveform from the source antenna can be gated at the receiving antenna-under-test to produce a response to the wanted signal only.
Near-field antenna measurements have many advantages, but also some limitations, which can be mainly attributed to the need for costly facilities or severe environmental effects. Although anechoic chambers are widely employed, absorbing material is very expensive and the whole construction becomes a considerable project, especially if it is required to accommodate various size antennas over wide frequency ranges.
A pulsed, coherent radar system was used in the inverse synthetic aperture radar mode to obtain 1-way high resolution images of simple antennas. These high resolution images display the amplitude and phase distribution of the received wave. The images were then edited and reconstructed using System Planning Corporation's Image algorithms contained in the SPC RPS software package. The 2-D (range vs. cross range) image data is very useful for detecting defects in antennas and can also 0be applied to modification of illumination conditions such as wavefront sphericity (phase taper) and/or amplitude variabilities (taper, ripple). This technique offers an alternate approach to near field/far field transformation. The technique involves rotation of the antenna under test at a controlled, uniform rate. The antenna port is connected to the radar receiver and the radar transmitter attached to an illuminating antenna. The radar transmits a step chirp wave form. The received signal is recorded to tape and processed off-line on the SPC Image Reduction Facility. A calibration technique was developed using simple wide bandwidth horn antennas. The downrange and cross range resolution of these 1-way ISAR antenna images is half as large as with 2-way radar ISAR for the same bandwidth and angular integration interval. Image data will be shown on reflector-type antennas to illustrate the technique.
Microwave holographic metrology is considered to be a key technique for achieving improved performance from large reflector antennas, especially at the shorter wavelengths. An important benefit of microwave holography is that the mathematically transformed data yields precise information on panel alignments on a local scale [1-5]. Since the usage of the holographic technique requires both the amplitude and phase data of the measured far-field patterns, one must carefully assess the impact of systematic and random errors that could corrupt the data due to a variety of measurement error sources.
The holographic antenna measurement system developed for the COMSAT Labs far-field range was tested with various antennas including axis-symmetric reflector antennas, offset single and dual reflector antennas, and phased-array antennas. Numerous examples which demonstrate the value of holographic measurement as an antenna diagnostic tool are presented. Microwave holography utilizes the Fourier transform relation between the antenna radiation pattern and the antenna aperture electromagnetic field distribution. Complex far-field date are collected at sample points and a Fourier transform is performed to give amplitude and phase contours in the antenna aperture plane. These contours facilitate reflector antenna diagnosis. The feed illumination and blockage pattern are provided by the amplitude distribution. The aperture phase distribution allows simple determination of deviations in the reflector surface and feed focusing. For phased-array antennas, the contours provide a measure of the complex element excitation. Measurement system parameters including pointing accuracy, phase stability, and measurement dynamic range were studied and refinements implemented to increase speed, accuracy, and resolution of the contour plots. To prevent aliasing errors, sampling criteria were explored to determine the optimum parameter ranges. For most antenna positioners, the antenna center is displaced from the rotation center. The importance of properly accounting for this displacement is discussed in the final section.
Imaging by microwave holography was initially envisaged as a two dimensional diagnostic technique applicable to a wide variety of objects and environments [1], [2], being particularly relevant to reflector antenna metrology [3]. For electrically large paraboloidal reflectors the radiation is well collimated and can be assumed to arise from an effective aperture field at a specified plane within the antenna volume. Fresnel or far field measurements are then restricted to a small angular range around boresight so as not to violate the assumptions made for reconstruction of the aperture field. The processed image represents the aperture illumination function whose phase can be accurately related to feed position and profile error by comparison with 'a priori' knowledge of the ideal reflector shape [4]. Since the aperture field approximation imposes severe restrictions on the data window size the intrinsic depth resolution of the image is characteristically poor, and wide angle scattering from feed support struts for example is not recorded causing the struts to appear as geometric shadows on the image. Regions of the reflector surface lying beneath these blockages cannot therefore be reconstructed. Moreover, the narrow data recording bandwidth also produces inferior transverse resolution of profile perturbations on the reflector surface.
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.
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.
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.
The Radarsat synthetic aperture radar (SAR) antenna is baselined to be a large slotted waveguide planar array, with a rectangular aperture of dimensions 1.5m x 15m. At the nominal frequency of 5.3 GHz, the dimensions in terms of wavelengths are approximately 26.5 lambda x 265 lambda. The usual 2D(squared) divided by lambda formula yields a far-field range length of at least 7.96 Km, which is far beyond the range lengths currently available to the program. A more conservative design would at least double that number, rendering a far-field measurement concept all but impracticable. (*) This work was carried out for the Radarsat Project Technical Office of the Communications Research Centre, Canadian Department of Communications, under DSS contract OSR84-00175, funded by the Canadian Department of Energy, Mines, and Resources.
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].
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.
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