Dielectric rod arrays in a metallic waveguide alter the propagation modes and group velocities of electromagnetic waves. We have focused on TE₃₀-to-TE₁₀ mode converters and investigated how their behavior varies with frequency. A mode converter is proposed that passes the TE₁₀ mode at frequencies lower than 2ƒ_c, and converts the TE₃₀ mode into the TE₁₀ mode for frequencies higher than 3ƒ_c.