Skip to Main Content
Exact cutoff expressions for hybrid and circularly symmetric modes in three-layer cylindrical dielectric waveguides are derived. It analytically established that whenever the refractive index of the enter medium (n2) is higher than either the refractive index of the core (n1) or of the inner cladding (n2), i.e., n1 >n3 >n2 or n2>n3 >n1, the dominant HE11 mode can have a nonzero cutoff frequency. Inequalities relating the permittivities to the ratio of the cladding radius to the core radius, as conditions for the nonzero cutoff of the HE11 mode, are determined. The cutoff conditions presented in this paper are also applicable to similar structures used io millimeter-wave communications.