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A technique similar to that of Kirchhoff is used to obtain an analytic expression for mode Iaunching on a semi-infinite rod. The approximation is quasi-optical and only valid for small angles of incidence θ. When θ = 0 only HE1M modes are excited; however, for θ<<1 many modes can be launched. The effect on HE11 mode propagation of small imperfections in a dielectric waveguide is analyzed. At the frequency of interest for optical communication (cutoff for the TM01 mode) the radiated power is 160 times larger than that scattered into the HE11 mode.