Accuracy Estimation of Microwave Holography from Planar Near-Field Measurements

Author: Christopher A. Rose

Microwave holography is a popular method for diagnosis and alignment of phased array antennas. Holography, commonly known in the near-field measurement community as “backtransformation”, 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.

 

An Architectural Framework for a Universal Microwave Measurement System

Author: Syed I. Tariq

The complexity of modern antennas has resulted in the need to increase the productivity of ranges by orders of magnitude. This has been achieved by a combination of improved measurement techniques, faster instrumentation and by increased automation of the measurement process. This paper concentrates on automated measurement systems, and describes the requirements necessary to make such systems effective in production testing, and in research and development settings. The paper also describes one such implementation – the MI Technologies Model MI-3000 Acquisition and Analysis Workstation - that was designed specifically to comply with these requirements.

The paper discusses several important factors that must be considered in the design of automated measurement systems, including: (1) Enhancing range productivity; (2) Interfacing with instrumentation from a large number of suppliers; (3) Providing a uniform front-end for the measurement setup and operation that must be largely independent of the choice of the hardware configurations or the type of range (near-field or far-field); (4) Making the test results available in a format that simplifies transition to external commercial and userprogrammed applications; (5) Providing powerful scripting capability to facilitate end-user programming and customization; (6) Using a development paradigm that allows incremental binary upgrades of new features and instruments. The paper also discusses computational hardware issues and software paradigms that help achieve the requirements.

 

Application of the NIST 18 Term Error Model to Cylindrical Near-Field Antenna Measurements

Authors: Allen C. Newell, David Lee

This paper describes error analysis and measurement techniques that have been developed specifically for cylindrical near-field measurements. A combination of analysis and computer simulation is used to show the comparison between planar and cylindrical probe correction. Error estimates are derived for both the probe pattern and probe polarization terms. The planar analysis is also extended to estimate the effect of probe position errors. The cylindrical measurement geometry is very useful for evaluating the effect of room scattering from very wide angles since scans can cover 360 degrees in azimuth. Using a broad beam AUT and scanning over a large y-range provides almost full spherical coverage. Comparison with planar measurements with similar accuracy is presented.

 

Digital Beam-Forming Antenna Range

Authors: Masahiro Tanabe, Davd S. Fooshe

Toshiba Corporation, working with Nearfield Systems Inc., has developed a fully digital antenna measurement system for digital beam-forming (DBF) antennas. The DBF test facility is integrated with the large 35m x 16m vertical near-field range installed at Toshiba in 1997, and includes the NSI Panther 6500 DBF Receiver as the primary measurement receiver. The DBF system was installed in March 1999 and has been used extensively to test and characterize a number of complex, high performance DBF antennas.

A DBF antenna typically incorporates an analog-to-digital (A/D) converter at the IF stage of the transmit/receive (T/R) module. The digital IF signals are transferred to a digital beam-forming computer, which digitally constructs, or forms, the actual antenna pattern, or beams. Since the interfaces to the DBF antenna are all digital, the usual microwave mixers and down-converters are incompatible.

The NSI Panther 6500 is designed to interface directly with DBF antennas and allows up to 8 channels of I and Q digital input (16 bits each) with 90 dB dynamic range per channel. The NSI DBF receiver solves the DBF interface problem while providing enhanced performance over conventional microwave instrumentation.

 

How To Specify An RF System for Antenna Measurements

Authors: David S. Fooshe, Michael Schultz

Antenna measurement systems have unique requirements, which must be properly evaluated and understood in order for the antenna engineer to be successful in specifying an RF system that meets his needs. Antennas are characterized by specific operating and performance parameters that will determine the requirements for a measurement system. Aperture size, frequency range, bandwidth, side-lobe nulls, beamwidth, and polarization characteristics are a just few of the more important parameters. As with most engineering problems, system performance often requires a trade-off of equally important, but conflicting characteristics. Sensitivity and measurement time are well-known examples of this trade-off. Other examples include local vs remote mixers, receiver speed vs sensitivity, range size vs system dynamic range, and there are many others. The antenna engineer must be able to identify his most important system performance parameters in order to make compromises with confidence, since they are inevitably required. Once the system performance requirements have been determined, the antenna engineer can begin to select equipment, cables and components with the desired performance characteristics for his range.

This paper will describe the process for analyzing requirements, performing system trade-offs and specifying equipment and components for several antenna measurement system types.

 

Measured Error Terms for the Three-Antenna Gain Measurement Technique

Authors: G. Todd Park, David Musser, Don Bodnar, Doug Kremer, Jack Snyder

This paper will detail the implementation and results of a gain calculation performed on standard gain horns (SGHs) in the LS and XN microwave bands. The three-antenna method was used to ensure the highest accuracy possible, and extensive efforts were made to minimize the error budget. The measurement was performed in a large anechoic chamber, with the receive and transmit antennas placed 4.6 meters high in opposing corners. The resulting fifteen meters of aperture separation (approximately 10D2/λ for LS band and 15D2/λ for XN band) eliminated all measurable aperture interactions and greatly reduced multipath interference from chamber reflections Rigorous analysis of the error terms proved this method to be both accurate and reliable. Typical values of measured error terms will be presented.

 

Precision Positioner Alignment Techniques for Spherical Near-Field Antenna Measurements using Laser Alignment Tools

Authors: Jeffrey A. Fordham, John Proctor and Douglas Kremer

The majority of precision spherical positioner alignment techniques used today are based on procedures that were developed in the 1970’s around the use of precision levels and auto-collimation transits. Electrical alignment techniques based on the phase and amplitude of the antenna under test are also used, but place unwanted limitations on accurately characterizing an antenna’s electrical/mechanical boresight relationship. Both of these techniques can be very time consuming. The electrical technique requires operator interpretations of data obtained from amplitude and phase measurements. The autocollimation technique requires operator interpretations of optically viewed measurement data. These results are therefore typically operator dependent and the resulting error quantification can be inaccurate.

MI Technologies has recently developed a mechanical alignment technique for Spherical Near-Field antenna measurements using a tracking laser interferometer system. Once the laser system has been set-up and stabilized in the operational environment; the entire spherical near-field alignment may be completed in a few hours, as compared to the much more lengthy techniques used with level/transit or electrical techniques. This technique also simplifies the quantification of the errors due to the inaccuracy of the alignment.

This paper will discuss the effect of the alignment error on results obtained from spherical near-field measurements, and the procedures MI Technologies developed using a tracking laser interferometer system to obtain the precision alignment needed for a spherical near-field measurement.

 
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