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pass through foggy, dusty, snowy,
or otherwise visually occluded
environments, making them
ideal for rugged and hazardous
conditions, such as storms. A more
recent application of RF imaging is
millimeter-wave imaging technology,
which is used in security scanning
systems to detect concealed
objects under clothing by analyzing
RF energy reflections, rather than
visually seeing through clothing.
A radio telescope is a type of RF
imaging that passively captures
radio signals from outer space
for scientific study. A possible
future US National Aeronautics
and Space Administration (NASA)
mission is the Lunar Crater Radio
Telescope (LCRT), a proposed
radio telescope constructed of
wire mesh and installed inside a
crater on Earth's moon (Figure 3).
Radio telescopes allow operators
to measure radio signals that are
otherwise invisible to other types
of telescopes, and these signals
may be the key to learning about
aspects of the ancient universe and
other astronomical phenomena.
RF Material Characterization
As RF signals pass through
or by materials, the signals
are altered. Moreover, placing
materials with certain dielectric,
conductive, semiconductive, and
magnetic properties near exposed
waveguides, transmission lines, or
resonant cavities leaves indelible
marks on reflected signals. These
changes to RF signals passing
through materials and reflected
from them can be used to perform
measurements if the setup
is adequately controlled and
calibrated. The key measurements
using RF signals are complex
dielectric permittivity, complex
magnetic permeability, conductivity,
and resistivity. With non-contact
material measurement methods,
RF material characterization
apparatuses can be used on
extremely cold or hot materials and
under a variety of other conditions.
Figure 3: A lunar radio telescope concept. (Source: NASA/Vladimir Vustyansky)
3
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