Antenna Theory and Applications
Goals
The Antenna Theory and Applications project develops, refines, and extends measurement techniques to meet current requirements and to anticipate future needs for accurate antenna characterization.
Customer Needs
Microwave antenna hardware continues to become more sophisticated. We provide state-of-the-art measurement support for antennas and antenna systems. Current demands include:
Improved Accuracy — High-performance systems, especially those that are satellite-based, require maintenance of tighter tolerances.
Higher Frequencies — Millimeter-wave applications up to 500 gigahertz are being developed.
Low-Sidelobe Antennas — Military and commercial communications applications increasingly require sidelobe levels of 50 decibels below peak (or better), a range where measurement by standard techniques is difficult.
Complex Phased-Array Antennas — Large, often electronically steerable, phased arrays require special diagnostic tests to ensure full functionality.
In Situ and Remote Measurements — Many systems cannot be transported to a measurement laboratory. Robust techniques are needed for onsite testing.
Production-Line Evaluation — Techniques are required that emphasize speed and economy, possibly at the expense of the ultimate accuracy.
RFID Reliability — RF identification (RFID) is being used in new applications such as inventory control and electronic passports. Test methods are needed to assure reliability and security.
Evaluation of Anechoic Chambers and Compact Ranges — A number of widely used measurement systems rely on establishing a well characterized test field. Near-field methods can be used to evaluate and analyze the quality of these test fields.
Technical Strategy
We seek to expand our frequency coverage for antenna calibrations to meet the demands of government and industry. A probe-position correction theory has been developed. Probe-position correction has been implemented at the NIST range by using a laser tracker to dynamically acquire position data. This will help us maintain low uncertainties as we extend the frequency coverage of our calibrations.
The near-field extrapolation method, developed at NIST, is an accurate technique to characterize the on-axis gain and polarization properties of antennas. Further improvement is still possible. We plan to extend the extrapolation software to take full advantage of phase information and to analyze the conditioning of the algorithm.
A thorough uncertainty analysis for planar near- field measurements has been previously developed. A similarly comprehensive uncertainty analysis is needed for spherical near-field measurements. We have completed a preliminary analysis and are working to refine the bounds.
In-situ near-field measurements of antenna systems are problematic because of the mechanical difficulties in maintaining position tolerances and because full spherical scans are typically not physically feasible. We have made significant progress in overcoming both these challenges and are working on the practical implementation of a deployable measurement system.
Accomplishments
- Proximity Effects in Microwave Radiometer Calibrations — Microwave radiometers are calibrated by observing sources of known brightness temperature, assuming the source is in the far field. However, such sources are often actually in the near field. A simulation for estimating the uncertainty due to the source being in the near field of the radiometer has been completed and documented.
- Comparison of Gains Determined from the Extrapolation and Pattern Integration Methods — The gains of three antennas (one a dual-port probe) were determined with both the extrapolation and pattern integration methods and the results compared. For antennas with gains greater than about 15 decibels, the uncertainties of the two methods were comparable. For gains less than 15 decibels, the extrapolation method was found to be superior in the NIST facility.
Setup for the 13.56 megahertz load modulation test, which is used to |
- RFID Test Bed — An RFID test bed has been established at 13.56 megahertz for testing RFID cards for load modulation (return signal strength) and electromagnetic durability when the cards are exposed to electric discharge or to high strength alternating electric and magnetic fields.
Short Courses
NIST and the Georgia Institute of Technology annually offer an introductory course on antenna measurements. Every other year NIST presents an in-depth technical course restricted to near-field methods that were pioneered at NIST.
Software
Planar, cylindrical, and spherical near-field scanning applications algorithms are currently available. Probe position-correction software is available for the planar and spherical methods. The constrained least squares algorithm for partial sphere data is also available. Quiet-zone evaluation and imaging programs will be available soon.
External Recognition
- Mike Francis chairs the Antenna Standards Committee of the IEEE Antennas and Propagation Society.
- Mike Francis has been appointed to the Antenna Centre of Excellence (European Union) Scientific Council for 2006 and 2007.
- Mike Francis received the Antenna Measurement Techniques Association Distinguished Service Award in October 2004.