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In order to diagnosis array antennas we implemented the back projection technique in our bi-polar near field laboratory. Using capabilities of microwave holography we investigated actual distribution of excitation coefficient values in a variety of antenna arrays operating at 5 GHz and around 10 GHz. We investigated phase and amplitude alignment of the linear MC-8 phased array with eight elements operated in the band centered at 5000 MHz. The reconstructed phase distribution images reveal phase distributions consistent with the design values. A major technical impairment is that the resolution at the element level can not be easily assured and it is related to the element spacing.
Microwave holography is a popular method for diagnosis and alignment of phased array antennas. Holography, commonly known in the near-field measurement community as "back transformation", is a method that allows computation of the primary (aperture) fields from the secondary (far-zone) fields. This technique requires the far-zone fields to be known over a complete hemisphere and adequately sampled on a regular spaced grid in K-space. The holography technique, while known to be mathematically valid, is subject to errors just as all measurements are. Surprisingly, very little work has been done to quantify the accuracy of the procedure in the presence of known measurement errors. It is unreasonable to think that the amplitude and phase of the array elements can be trimmed to better than the uncertainty of the back-transformed amplitude and phase. This makes it difficult for an antenna engineer to determine the achievable resolution in the measurement and calibration of a phased array antenna. This study reports the results of an empirical characterization of known errors in the holography process. A numerical model of the near-field measurement and holography process has been developed and many test cases examined in an effort to isolate and characterize individual errors commonly found in planar microwave holography. From this work, an error budget can be developed for the measurement of a specific antenna.
ARCS acquisition software for antenna and RCS measurements has been modified such that it is now based on LabWindows/CVI of National Instruments. With open system architecture, industry-standard tools and platform flexibility, new ARCS software delivers all components which are required for an advanced antenna and RCS measurement system. This means tht the portability and modularity of the software is increased considerably. Such a concept has the major advantage of simple adaptation/modification by the user, for instance by adding new menu pages. The virtual instrument concept of CVI guarantees easy adaptation of the newest interface technology, such as USB and firewire. Furthermore, there is a large base of instrument drivers which can be readily used to extend the measurement capabilities of ARCS in a minimum of time Special care is taken in the design of the user interface. This is to avoid complex procedu res for entering measurement parameters. Even less experienced operators must be comfortable with the software and be able to perform complex calibration and data acquisition procedures. Finally, a large number of application programs is written for advanced antenna and RCS calibration, microwave holography, ISAR imaging and frequency extrapolation techniques.
Gabor holography is an appropriate technique for near field measurements at THz frequencies when apertures of the order of thousands of wavelengths are involved. The method permits pattern prediction over a restricted angular range from intensity measurements, providing a direct method of recovering phase which overcomes cable, planarity and atmospheric effects; problematic to conventional near-field phase measurements. We demonstrate the feasibility and convenience of the method with an example planar near-field measurement at 94GHz for a 1.1m Cassegrain reflector and we determine the relationships governing dynamic range and the requirements for sampling. Finally, two-dimensional numerical simulations for a lm antenna at 0.5THz, with a 10m scan distance, will be presented to demonstrate the feasibility of the method for large terahertz antennas.
Radar cross section measurements must be performed in a wide variety of situations throughout development of a new vehicle. In these days of smaller budgets, it is vitally important that the right things get measured, at the right time in the program, with the right accuracy, and that these measurements be integrated into the development process in the right way. After delivery, the measurement system must be confidently usable by the user organization, with a minimum of outside to ensure that the vehicle is maintained. Many of the key programs in this area were begun before modern measurement technology was known to be capable of providing detailed diagnostic measurements. Consequently, specifications did not consider what can be easily measured with today's modern diagnostic radars. This paper addresses how mcxlern diagnostic radar cross section measurements can be exploite4:l to make the specification, development, pnxluction, and testing phases much more efficient than they have been in the past.
Optical surveying techniques with theodolites have been utilized for many years for static measurements of reflector antennas. This paper reports on updated optical surveying systems used to measure the accuracies and structural deformations of reflector antennas. Deformations of large Cassegrain tracking antennas during elevation rotation and a fixed, billboard-style compact range reflector over time are discussed. A simple surveying method is shown for the integrated measurement of Cassegrain antennas (both primary and secondary reflectors) from near the primary vertex. Other topics covered include accurate prediction of interpolated gravity deformations of rotating reflectors based on a small measurement sample, and a method for taking differences between measurements. The use of EDM (Electronic Distance Measurement) theodolites as well as angle-only devices is described, along with software which manages both the measurement and data-reduction systems.
Traditional range/doppler ISAR techniques have inherent geometric limitations. By using concepts of microwave holography and tomography, a vector-based k space approach allows a more generalized geometry of the sampled Fourier space. By constructing a complete annulus in the polar sampling space, arbitrary apertures up to 360 degrees can be processed for "full body" two dimensional images. This processing also typically exhibits better resolution. The algorithm relies on linear interpolation for potar Cartesian conversion. The general geometric formulation is also readily adaptable to arbitrary antenna configurations.
This paper discusses results of a recent attempt to measure aperture distribution of a small active steerable array antenna at 60 GHz using planar near-field measurements and the back transform. Using a procedure which exercises every phase shifter without steering the antenna beam, it is possible to isolate problems with individual bits in the phase shifters. From calculation of the aperture fields for each case we hope to infer the individual phase shifter bit loss. We will also discuss problems which arose in the measurement because of the short wavelength, signal-to-noise ratio and small number of elements.
Microwave holography is an important technique for analyzing electromagnetic fields in close proximity to objects such as antennas or radomes. In this paper, data measured in the far-field and near-field are transformed to the surface of a spherical radome using spherical microwave holography. Further, the fields are calculated on a sequence of spheres concentric to the spherical radome to display the spatial distribution of the fields as a function of distance from the surface of the radome out to five wavelengths from the radome, a movie. This progression demonstrates the ability to locate radome surface defects is severely limited without the use of microwave holography. Three sets of radome defects are presented and range in size from three-eights of a wavelength to three wavelengths. This paper shows that near-field measurements alone are not generally capable of locating defects directly.
Two techniques of antenna diagnostics based on the knowledge of partial information about the near field are discussed. In the first approach, the problem of the characterization of the source from the knowledge of the near field over a limited scanning domain is formulated as a linear inverse one. The second problem concerns the antenna diagnostics from the knowledge of only amplitude distributions over limited regions of two planar surfaces. In both cases effective and reliable solution procedures are discussed.
Progress in Georgia Tech's research in Near-Field Spherical Microwave Holography (NFSMH) is reported. Previously, the amplitude resolution of Spherical Microwave Holography (SMH) was defined and demonstrated. The definition of resolution has been altered to include phase resolution. The resolution of phase is shown to be equivalent to the resolution of amplitude, and both depend on the highest mode order used in the spherical wave expansion. Previous measurements showed that SMH can easily achieve x/2 phase resolution where X refers to free space wavelengths. Current measurements show that the X/2 resolution limit of planar microwave holography can be surpassed by using evanescent energy in the NSMFH technique. Measurements of small, closely spaced, insertion phase defects placed on a hemispheric ally shaped radome are used to demonstrate the improved resolution. The measurement of evanescent energy is achieved by using a specially designed small aperture probe and a small separation distance between small aperture probe and a small separation distance between the radome surface and the measurement surface. The relationship between measured and theoretical insertion phase of a known radome defect is shown. Given the defect size and the maximum mode order used in the spherical wave expansion, measured insertion phase can be used to predict the actual defects electrical thickness.
Antenna microwave holography is now a well established technique and has for a number of years provided a diagnostic tool for the evaluation and optimization of the electrically large reflector antennas used for satellite ground stations. Increasing interest is being shown in the use of the technique during the development of other complex antenna configurations in order to improve the design, minimize design cycles and, hence, reduce the overall cost. This contribution presents two examples of applications of the technique during the development of high performance antennas at ERA Technology LTD. For a corrugated slot-array antenna operating at 19.95 GHz, a clear improvement in the performance following design optimization based on the results obtained from microwave holography is shown for a 3 Am diamond reflector antenna for SATCOM applications operating at 14GHz, the technique provides a verification of distortions in the surface profile by mapping of the aperture phase distribution.
This paper reports on Far-Field Spherical Microwave Holography (FFSMH), currently being researched at Georgia Tech. Microwave Holography is a technique for evaluating complex electric fields near the field sources. Planewave Microwave Holography involves the use of the planewave spectrum and is the most common technique in use. Spherical Microwave Holography involves the use of a spherical expansion of Maxwell's equations and is the topic of this paper. Spherical Near-Field Microwave Holography (SNFMH) has been successfully used to locate and identify defects in radome walls, and to determine antenna aperture distributions. FFSMH differs from SNFMH only in the location of the measurement surface. FFSMH uses a far-field measurement surface and SNFMH used a near-field measurement surface. Progress in the definition of resolution limits for Spherical Microwave Holography is reported. FFSMH is demonstrated and results are compared to SNFMH and Planewave Microwave Holography
Microwave holography has proven to be a powerful technique for various evaluations, diagnostics, and RF performance improvements for large reflector antennas. The technique utilizes the Fourier Transform relation between the complex far-field radiation pattern of an antenna and the complex aperture field distribution. Resulting aperture phase and amplitude distribution data can be used to precisely characterize various crucial performance parameters, including panel alignment, subreflector position, antenna aperture illumination, directivity at various frequencies, and gravity deformation effects. The methodology of the data processing presented in this paper was developed at JPL and has been successfully applied to the NASA/JPL Deep Space Network (DSN) 34m beam waveguide antennas. The performance improvement of the antenna was verified by efficiency measurements and additional holographic measurements. The antenna performance was improved at all operating frequencies of the antenna (wide bandwidth improvement) by reducing the main reflector “mechanical surface” rms error to 0.43 mm. At Ka-band (32-GHz) the estimated improvement is 4.1 dB, resulting in aperture efficiency of 52%.
This paper reports on a spherical near-field measurement technique currently being researched at Georgia Tech. Spherical Near-Field Microwave Holography (SNFMH) has been successfully used to locate defects in radome walls. Defects with size greater than or equal to .5 free space wavelengths (?0) in diameter were detected [1]. Present efforts have established theoretical resolution limits for the SNFMH process. The SNFMH technique is used to process spherical near-field measurements of a (.23 ?0 x .12 ?0) open-ended waveguide probe. The probe consists of a section of WR92 waveguide, tapered down to one fourth of its standard dimensions. The results are compared to the theoretical mode-limited impulse response. The SNFMH technique is also used to process spherical near-field measurements of a plastic, 8 ?0 radius, hemisphere radome with various plastic defects attached. Results of a case with two .5 ?0 diameter plastic defects are compared to the theoretical mode-limited pulse response. Contour plots of uniquely shaped defects are shown.
A microwave holographic analysis system is shown to have successfully resolved the surface deformations on an 8' symmetric Cassegrain reflector antenna known to have significant surface deformation problems. The technique is based on the Fourier transform relationship between the aperture field of an antenna and its radiated far-zone field. A signal processing technique dubbed "pattern simulation and subtraction" is discussed that increases the resolution in the transformed aperture domain by removing unwanted signals from the aperture distribution. Measurements taken on the Cassegrain reflector at 11 GHz in the OSU-ESL Compact Range provided excellent amplitude and phase stable data to be processed by the holographic analysis system. Surface deformation profiles generated by this system were then compared to an optical measurement of the main reflector surface. Excellent agreement was obtained with a worst case deviation in the adjusted profiles being 0.05 ?.
We describe a portable system for performing microwave holography of reflector antennas. This technique derives the complex (amplitude and phase) aperture current distribution from the measured complex far field of an antenna. The amplitude of the current distribution displays directly the effects of feed and support leg shadowing, and illumination taper. The phase of the current distribution is used to optimize feed and/or sub-reflector location, and to generate a table of recommended panel adjustments.
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.