RCS of a coated circular waveguide terminated by a perfect conductor

The radar cross section (RCS) of a circular waveguide terminated by a perfect electric conductor is calculated by the geometrical theory of diffraction (GTD) for the rim diffraction and by a physical optics approximation for the interior irradiation. The interior irradiation is generally more than 10 dB higher than the rim diffraction for
a/\lambda \geq 1
(
a
is the waveguide radius,
\lambda
is the free-space wavelength). At low frequencies (
a/\lambda \sim 1
), the interior irradiation can be significantly reduced over a broad range of incident angle if the interior waveguide wall is coated with a thin layer (1 percent of the radius) of lossy magnetic material. Our theoretical prediction is confirmed by measurements. At higher frequencies (
a/\lambda \sim 3
), a thin layer of coating is effective for the case of near axial incidence, provided that a good transition of the
TE_{11}
mode near the waveguide opening to the
HE_{11}
mode inside the waveguide is made. A thicker layer of coating is required for the RCS reduction over wider incident angle.