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In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.
The radar cross section of large targets has previously been measured on large outdoor far field ranges. Due to environmental and security limitations of outdoor ranges, low cost indoor compact ranges are preferred. To optimize compact range performance and to minimize size, careful attention must be paid to the design of feeds which are required for the proper illumination of the reflector. This paper describes a new polarization diversified parasitic multimode/corrugated (PMC) feed for a compact range reflector. The performance attributes of the PMC feed are presented. The PMC feed provides several advantages over other known commercially available compact range feeds.
A far-field antenna range has been assembled on the roof of the Electrical Engineering building at Virginia Tech. Antenna radiation patterns and polarization patterns can be measured. The system consists of two Scientific-Atlanta azimuth positioners, a Scientific-Atlanta 1711 receiver, a Scientific-Atlanta 1832A amplitude display unit, a DC motor controller, a synchro-to-digital converter, an IBM PC, and signal sources. The DC motor controller has been interfaced to the PC along with the synchro-to-digital converter, forming a closed loop positioning control system that can be used with either of the azimuth positioners. One of the positioners is used for the antenna under test while the other positioner controls the polarization of the transmit antenna. The receiver and amplitude display provide a 60 dB dynamic range for antenna measurements. The PC has been programmed in TURBO Pascal to control the antenna positioner, record antenna patterns, store pattern data on disk, and provide antenna pattern plots. This modular approach provides permanent storage on PC disk of all measurements as well as allowing many plot combinations including linear or logarithmic form and rectangular or polar format.
For over a decade the National Bureau of Standards (NBS) has used the planar near-field method to accurately determine the gain, polarization and patterns of antennas either transmitting or receiving cw signals. Some of these calibrated antennas have also been measured at other facilities to determine and/or verify the accuracies obtainable with their ranges. The facilities involved have included near-field ranges, far-field ranges, and compact ranges. Recently, NBS has calibrated an antenna to be used to evaluate both a near-field range and a compact range. These ranges are to be used to measure an electronically-steerable antenna which transmits only pulsed-cw signals. The antenna calibrated by NBS was chosen to be similar in physical size and frequency of operation to the array and was also calibrated with the antenna transmitting pulsed-cw. This calibration included determining the effects of using different power levels at the mixer, the accuracy of the receiver in making the amplitude and phase measurements, and the effective dynamic range of the receiver. Comparisons were made with calibration results obtained for the antenna transmitting cw and for the antenna receiving cw. The parameters compared include gain, sidelobe and cross polarization levels. The measurements are described and some results are presented.
A roof-top antenna range has been installed at the Bellcore facility in Red Bank, New Jersey. This facility is used as a far field range to measure highly directive antennas at millimeter wave frequencies. Theoretical and experimental studies were performed to characterize the range environment and identify reflections. Two computer programs were used to analyze the strength and location of interfering signals at both UHF and millimeter wave frequencies. These programs use Geometrical Optics and the Geometrical Theory of Diffraction to predict the location and strength of diffracted and reflected energy from the surrounding structures. Both singly and doubly diffracted interferences were considered. A bi-static radar, with an 850 MHz carrier, bi-phase modulated by a 40 Mbit/s pseudonoise code, was used to measure the impulse response of the environment. The antenna range measurements are compared with the analysis done at 850 MHz and calculated results are printed for the behavior of the range in the millimeter wave regime.
A method for the calculation of the errors induced through target-wall-target interactions is presented. Both near-field and far-field situations are considered. Far-field calculations are performed both with Fraunhoffer diffraction theory and target antenna analogies. Absorber is considered as both a specular and a diffuse scatterer. The equations developed permit trade studies of chamber size versus performance to be made.
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.