NRL Researchers Develop Methods for Ultra-Near-Field Microwave Holography

2/14/2000 - 59-99r
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Researchers at the Naval Research Laboratory (NRL) have developed novel experimental and analytical methods for Ultra-Near-Field Microwave Holography. The research team reports the new methods provide an exciting research tool for use in the development of new materials for radar cross-section reduction and ship-board antenna/array isolation. This work was sponsored in part by the Office of Naval Research (ONR).

The research team includes: Drs. Douglas Smith, Peter Moore, Louis Medgyesi-Mitschang, of the Systems Directorate; Dr. Douglas Taylor and Mr. Mark Parent, of NRL's Radar Division.

Dr. Medgyesi-Mitschang, principal investigator of the project reports, "Our work is an outgrowth of research conducted earlier by Dr. Earl Williams and his colleagues of NRL's Acoustics Division. They pioneered the use of near-field holography in acoustics to study the dynamics of acoustic wave sources in underwater applications. We have extended this holographic technique to the microwave regime."

"While the original ONR-sponsored work focused on the non-materials area, our work is providing an exciting new research tool to clarify the complex physics of material boundaries and multi-layered media encountered in Navy applications. In principal, it may also lead to materials and structures for novel array-beam-forming and control."

According to the research team, historically, electromagnetic phenomena such as the radar cross section (RCS) of targets or the beam patterns and gain of antennas were obtained from far-field measurements. In these cases the data came from sensors placed many wavelengths distant from the object under test. Specialized ranges have been developed and a whole instrumentation industry grew up around these techniques.

While these techniques are robust and well established, the measured data contains only part of the physics involved, namely, the far-field electric and magnetic fields. But demonstrably other field entities can also be important. These are the subliminal, or surface-trapped electromagnetic fields. These may be observed only very close to the object. In acoustics, the analogous quantities are termed subsonic fields. These surface-trapped fields die out rapidly with distance from the surface of the radiating or scattering object. In many Navy applications the effect of these vanishing fields can have a critical impact such as in the control of scattering from ship topside structures or isolation of shipboard antennas.

"Conventional near-field microwave measurements have been used before, such as in the design and validation of large arrays for telecommunication satellites," says Dr. Smith, co-Principal Investigator of the project. "Traditional far-field measurements of these arrays at outdoor ranges proved to be technically challenging and costly; near-field techniques provide an alternative. With these, the electromagnetic fields are measured with sensors usually a few wavelengths from the face of the array, i.e. (in the Fraunhofer or Fresnel region of the object). From this data, the fields are then projected or transformed to the far-field, yielding the array beam patterns or gain of the deployed satellite. This approach allows highly accurate measurements to be made in an anechoic chamber under carefully controlled conditions. However, in these measurements, the trapped-surface fields are generally neglected since the fields are sampled at some distance from the array surface.

This approach is inappropriate for many important Navy applications since the principal coupling mechanisms and physics of interest are embedded in the surface-trapped fields reports the research team. This shortcoming of conventional microwave near-field technique motivated the current microwave holography investigation.

An experimental facility was constructed at NRL to provide proof-of-principal demonstrations of ultra-near-field microwave holographic techniques. The current facility allows field scans over a large 5- by 10-foot region, near the object under test. The key elements of this research facility are an Orbit/FR robotic positioning system built to NRL specifications, together with a vector network analyzer and associated data acquisition/processing software. Using this facility, the research team was able to obtain ultra-near-field holograms for a variety of targets and antennas. Concurrent with the experiments, computer simulations are underway at NRL to further elucidate the physics of ultra near-field microwave holography.

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