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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