Abstract
High-frequency diffraction from isosceles triangular cylinders with two faces soft and one face hard is investigated for backscattering case and novel first-order physical theory of diffraction asymptotics are derived. Only primary edge waves are taken into account. It is shown that, by changing the apex angle of the cylinder, PTD results of the soft-hard triangular cylinder agree-well with those of the PTD results of Soft-Hard Strip.