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The direction-changing capability of electromagnetic waveguides may be limited not only by mode conversion but also by radiation if the transverse field extends indefinitely into a freely propagating region. This paper gives new, more accurate expressions for the permitted bending radius with respect to mode conversion, using coupled-wave theory to categorize the wide variety of transmission media possible. This paper also makes a suggestion for estimating the permitted bending radius when radiation is a limitation. In single-mode “open” waveguides that have transverse fields extending indefinitely into a freely propagating region (such as a dielectric waveguide), the permitted bending radius is limited by radiation effects, whereas in either the open or completely shielded multimode waveguides, the permitted bending radius is usually limited by mode conversion.
Marcatili et al. (1969) studied this question.