  The generality of the Bethe's two dipole model for light diffraction through
a subwavelength aperture in a conducting plane is studied in the radiation zone
for coated conical fiber tips as those used in near field scanning optical
microscopy. In order to describe the angular radiated power of the tip
theoretically, we present a simple, analytical model for small apertures
(radius < 40 nm) based on a multipole expansion. Our model is able to reproduce
the available experimental results. It proves relatively insensitive to cone
angle and aperture radius and contains, as a first approximation, the empirical
two-dipole model proposed earlier.
