Directivity of rectangular aperture electromagnetic radiation

On the basis of an analytical solution for a reflectance and a gain of the open-ended waveguide having cross-section a×b with wave H₁₀ is shown for a≥0, 7λ, b≥a or a≥4λ, b>0,4a, where λ is a wavelength in free space, what the gain dependence of the open ended waveguide on a/λ at a/b=const corresponds to generalized gain dependence of optimal plane horns in plains E or H on a dimension of the horn aperture. The calculation of an effective area and an area use factor of open-ended waveguide and optimal plane horn apertures are made.