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/spl times/b with wave H/sub 10/ is shown for a/spl ges/0, 7/spl lambda/, b/spl ges/a or a/spl ges/4/spl lambda/, b>0,4a, where /spl lambda/ is a wavelength in free space, what the gain dependence of the open ended waveguide on a//spl lambda/ 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.