  Conventional microwave remote sensing/imaging of through-the-wall objects
made of different materials is usually performed at frequencies below 3 GHz
that provide relatively low spatial resolution. In this paper, we evaluate the
ability and sensitivity of high-frequency microwave or millimeter wave standoff
sensing of through-the-wall objects to achieve high spatial resolution. The
target under study is a sandwich structure consisting of different object
materials placed between two wall blocks. An Agilent PNA-X series (model
N5245A) vector network analyzer is used to sweep over the entire Ka-band (26.5
GHz to 40 GHz). The beam is then directed to a standard rectangular horn
antenna and collimated by a 6-inch-diameter Gaussian lens towards the sandwich
structure (wall block/object/wall block). The reflected electromagnetic wave is
picked up by the same system as the complex S-parameter S11. Both amplitude and
phase of the reflected signal are used to recognize different materials
sandwiched between the cement blocks. The experimental results are compared
with the theoretical calculations, which show satisfactory agreement for the
cases evaluated in this work.
