  Huygens' metasurfaces are electrically thin devices which allow arbitrary
field transformations. Beam refraction is among the first demonstrations of
realized metasurfaces. As previously shown for extreme-angle refraction,
control over only the electric impedance and magnetic admittance of the
Huygens' metasurface proved insufficient to produce the desired reflectionless
field transformation. To maintain zero reflections for wide refraction angles,
magnetoelectric coupling between the electric and magnetic response of the
metasurface, leading to bianisotropy, can be introduced. In this paper, we
report the theory, design, and experimental characterization of a
reflectionless bianisotropic metasurface for extreme-angle refraction of a
normally incident plane wave towards 71.8$^\circ$ at 20 GHz. The theory and
design of three-layer asymmetric bianisotropic unit cells are discussed. The
realized printed circuit board structure was tested via full-wave simulations
as well as experimental characterization. To experimentally verify the
prototype, two setups were used. A quasi-optical experiment was conducted to
assess the specular reflections of the metasurface, while a far-field antenna
measurement characterized its refraction nature. The measurements verify that
the fabricated metasurface has negligible reflections and the majority of the
scattered power is refracted to the desired Floquet mode. This provides an
experimental demonstration of a reflectionless wide-angle refracting
metasurface using a bianisotropic Huygens' metasurface at microwave
frequencies.
