AMTA Paper Archive
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Configuration of spherical near-field ranges
In principle, spherical near-field scanning measurements are performed in the same way as conventional far-field measurements except that the range length can be reduced. This provides a natural advantage to scanning in spherical coordinates over other coordinate systems due to the steady availability of equipment. However, special considerations must be given to near-field range design because of the necessity for phase measurement capability, mechanical accuracy and the need to handle large quantities of data. Based on experience with spherical near-field measurements carried out during verification testing of a spherical near-field transformation algorithm, we discuss the practical aspects of constructing a near-field range. In particular we will consider range alignment procedure, engineering of the RF signal path and times for data collection and processing.
Mechanical alignment of solid reflector antennas on spacecraft
The mechanical alignment of a reflector antenna involves both the reflector shape and also the relative orientation of the feed and subreflector. The requirements for alignment are derived from the system requirements for antenna functional performance, including pointing. A typical alignment plan includes the following alignment operations: • Component inspection of reflector, subreflector and feed. • Antenna assembly, including a final baseline measurement. • Alignment to a positioner for antenna range tests. • Alignment checks before and after environmental exposures. • Installation on spacecraft, including receiving inspection, adjustment to a specific orientation, and structural distortion checks • Alignment checks on spacecraft. Six tooling balls on the back of the reflector are commonly used as a reference for both pointing and structural distortion. Additional references may be provided by mirrored surfaces, auxiliary tooling balls, machined edges, scribe lines and mounting surfaces. Special fixtures for holding the antenna throughout its test sequence have proved useful. These fixtures are designed to provide a rigid support with a minimum of mounting stresses. They also have provisions for fine angular adjustments on antenna positioners. Analytic aids include: • Calculations of the Best-Fit-Paraboloid to the measured points on the reflector surface. • Use of beam deviation factors to calculate the predicted electrical beam from mechanical measurements. • Transformation of coordinates from one system to another. The measurement methods and analytic techniques that are used for a typical set of alignment operations are described.
A Precision optical range alignment technique
Spherical near-field testing and other specialized antenna measurements require precise range and positioner alignment. This paper presents a method based on optical techniques to conveniently measure and monitor both range alignment and the positioner axis orthogonality and intersection. The hardware requirements consist of a theodolite and a unique target mirror assembly viewable from either side.
An Automated RF signature test facility for multibeam antennas
This paper describes an antenna coupling “hat” and the automated measurement equipment for Multibeam Antenna (MBA) signature tests. The test equipment measures, records, and compares the insertion loss or the “signature” of the MBA prior to and after environmental tests; thereby determining the post-environmental test integrity of the MBA. Repeatable mechanical alignments to within ±0.125 inch and RF measurements to within ±0.5dB are required and achieved. This signature test has achieved substantial cost and schedule improvement by freeing up the heavily demanded compact antenna test range and by reducing MBA test time.
Cylindrical near field test facility for UHF Television Transmitting Antennas
This paper describes a horizontal, cylindrical surface, near-field measurement facility which was designed and constructed in 1984 and is used for the determination of far field patterns from near field measurement of UHF television transmitting antennas. The facility is also used in antenna production as a diagnostic and alignment tool.
Characteristics and Capabilities of the Lewis Research Center high precision 6.7- by 6.7-M planar near-field scanner
The development of advanced spacecraft communication antenna systems is an essential part of NASA’s satellite communications base research and technology program. The direction of future antenna technology will be toward antennas which are large, both physically and electrically; which will operate at frequencies of 60 GHz and above; and which are nonreciprocal and complex, implementing multiple beam and scanning beam concepts that use monolithic semiconductor device technology. The acquisition of accurate antenna performance measurements is a critical part of the advanced antenna research program and represents a substantial antenna measurement technology challenge, considering the special characteristics of future spacecraft communications antennas.
Communication satellite antenna measurement
RCA-Astro Electronics in Princeton, N.J. designs, develops and tests multiple-beam offset reflector antenna systems in the C and Ku frequency bands for satellite communications. Antenna measurements are performed at the antenna subsystem and the system level and on the complete spacecraft to demonstrate that alignment and performance meet their specification. This paper discussed the antenna range designs and test techniques involved in data acquisitions for contour patterns, cross-polarization isolation and antenna gain characterization. A description of the software required to obtain, analyze and present the data will be included in addition to typical test results.
Effects of the alignment errors on ahorn's crosspolar pattern measurements. Application to L-SAT propagation package antennas.
When low crosspolar pattern measurements are required, as in the case of the L-SAT Propagation Package Antennas (PPA) with less than -36 dB linear crosspolarization inside the coverage zone, the use of good polarization standards is mandatory (1). Those are usually electroformed pyramidal horns that produce crosspolar levels over the test zone well below the -60 dB level typically produced by the reflectivity of anechoic chambers. In this case the alignment errors (elevation, azimuth and roll as shown in fig. 1) can become important and its efects on measured patterns need to be well understood.
An Outdoor UHF cylindrical surface near-field range
This paper describes the Harris Corporation, Broadcast Group, Outdoor, Cylindrical, Near-Field Antenna Range. The range is located on a bluff overlooking the Mississippi River flood plain near Quincy. IL and is used for the alignment and testing of UHF-TV transmitting antennas.
Conformal test coupler for measurements through antennas mounted on fuselages
Theory, design, and test results of a conformal test coupler that can be mounted on the exterior of a vehicle for direct on site measurements of a fuselage mounted L-band antenna are presented. When there is a requirement to test vehicle instrumentation for radiated power, signal format, etc., a desired method is to couple the test equipment directly to the dedicated antenna on the vehicle. Cavity test couplers have been traditionally employed for direct measurements at the antenna under test. However, a low-profile conformal cavity has poor performance when there is no match between the energy radiated by the antenna and the received energy in the cavity. To suppress unwanted resonances and a high Standing Wave Ratio, such mismatched cavities are loaded heavily with absorber material inside, and in operation exhibit high sensitivity to surface contact and high insertion loss, yielding nonrepeatable measurements. The coupler presented here is a nonresonant cavity that supports a TEM mode compatible with the radiation from the vehicle antenna and avoids spurious resonance spikes. It exhibits extremely low insertion loss and is not sensitive to mounting misalignment. A circumferential microstrip radiator with multiple feed points and a matching network on the back side of the same substrate is wrapped around the inside of a top-hat cylindrical aluminum container. The particular test cavity was designed for the vertically polarized L-band IFF antenna on the cruise missile; however, the same principle makes testing of other fuselage-mounted antennas easier and more reliable.
Rotated feed horns in a compact range for RCS measurements
A way has been found to utilize the reflector return in a compact range as a source of continuous drift compensation. This is performed by translating receive polarizations 45 degrees with respect to the transmit polarizations to ensure returns in co- and cross-polarizations. An added benefit is the simplicity of alignment for the polarization calibration standard.
Effects of measurement errors on reflector surface reconstruction using microwave holographic metrology
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.
Laser corrected field probe measurements of large compact ranges
As the operating frequencies of compact range antennas increase, the accuracy of the field probes used to characterize their performance must also increase. Obtaining the required accuracy through mechanical design becomes more and more difficult as the size of the area to be probed increases. This paper describes the use of a laser measurement system to sense the probe's mechanical displacements thereby allowing corrections of compact range measurement. The relatively simple laser alignment system is well-suited for compact range probing in which accuracy is much more critical in the Z direction than the X-Y direction.
Millimeter Waveguide Flange Alignment Fixture for Antenna and Other Calibrations
Millimeter band measurements require that care be exercised in the connection and handling of the waveguide flanges and their contact surfaces. When properly connected these flanges can provide many years of reliable and repeatable measurements. Improper use will limit the flange life to just a few connections, and cause measurement errors. These misuses are especially acute in situations requiring repeated connecting and disconnecting of small waveguide flanges, such as in antenna or insertion loss measurements. Some examples of misuse are: 1. using the flange to support heavy devices, 2. rocking the flange to get it on or off, 3. over-torquing multiple sides, and 4. using flanges with non-uniform surfaces. The effect of these misuses is that the flange is no longer usable for measurements requiring repeatability and this results in calibrations with unsatisfactory error bounds. NBS is currently addressing these problems by developing a Mechanical Millimeter Flange Alignment Fixture. The fixture indicates deficiencies in the contact area which need to be corrected. The fixture is then used to ensure that the flanges are mated correctly and repeatably. No twisting, rocking or angular mating of the flanges can occur. The fixture relieves the weight of the device on the flange and makes a versatile mounting fixture for almost any device where repeated connections must be made. The fixture and its use will be discussed in detail.
Comparison of antenna boresight measurements between near-field and far-field ranges
This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
Alignment measurements using a special purpose phased array antenna
A special purpose 80 element linear phased array antenna was aligned using an iterative phase cycling method. First, the array was aligned to yield maximum main-beam power in the reactive near-field zone and then in the far-field zone. A record of the phase-shifters settings achieved for each zone was kept for use as look-up table during operation. In situ electronic main-beam steering was performed to compare sidelobe performance for the two cases. This report describes the measured results obtained using the phased cycling alignment procedure and compares the measured one-way radiation pattern for the two distance conditions.
Automated performance evaluation system for mast-mounted direction finder antennas
At Southwest Research Institute, an automated antenna performance evaluation system has been developed for evaluation of mast-mounted direction finder antennas. This system utilizes a dual-channel receiving system and IF processor with off-line antenna pattern analysis software. Antennas are mounted on a test range which includes computer-controlled antenna positioners, test frequency transmitters, and a data acquisition equipment group. Amplitude and phase data is digitized and recorded for automated off-line antenna performance evaluation. The evaluation software provides a Fourier analysis of the antenna patterns which characterize distortion, alignment, relative phase relationships, amplitude mismatch, and bearing deviations (from theoretical values) for each antenna array.
Target alignment techniques for the compact range
Many targets today exhibit radar cross sections sensitive to the angular orientation of the target. While some of these targets have prominent scattering centers which can be exploited to obtain a relative positional reference, many targets unfortunately do not. In addition, many complex targets have a highly directional scattering behavior requiring careful alignment to the incident planar field. This need for accurate positioning has prompted the development of laser alignment techniques for the compact range. One such system has been under development at the ElectroScience Laboratory, and the designs and results of the first prototype are presented here. Performance goals and design criteria are discussed, and future improvements are considered. In addition, similar systems for feed and pedestal location reference systems are presented.
Projection Moire interferometric measurement of panel edge misalignment
An argon laser source in conjunction with an interferometric fringe generation technique allowed projection of high contrast fringes on to the surface of an antenna over a height of 20 to 60 feet. The projected beam, located at the base of the antenna, made an angle of ~ 15 degrees with the surface. The viewer was placed near the central axis of the antenna ~ 80 feet away where the illuminating antenna surface was imaged on a Ronchi grating. A low light level video camera viewed the moire contours through the Ronchi grating. The spacing between two contours represented a surface height variation of ~ .050". Panel edge misalignments of .005" were readily discernible. Applications of this technique are illustrated with photographs.
A New implementation of the planar near-field back projection technique for phased array testing and aperture imaging
A new implementation of the planar near-field back projection technique for phased array testing and aperture imaging is described. In the alignment of phased arrays, the aperture field is treated as a continuous distribution rather than using idealized array concepts. The continuous field is then sampled to obtain element excitations. In this way, nonrectangular arrays can easily be accommodated. The method also produces highly interpolated images of apertures that can offer much insight into their nature. Also, any polarization of the aperture field may be obtained if the probe pattern has been characterized. The technique uses large FFTs which are computed very quickly by a workstation located in the facility. Results from an iterative phase alignment of a 12x18 phased array are presented, as well as highly interpolated images of apertures and results which demonstrate the polarization selection.
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