Tapers are designed for a mode conversion of 0.1% or less over either narrow or broad bands. Narrow-band tapers are specially designed for minimum length.
Many other similar tapers have been made for narrowband and broadband applications. For example, a 2-meter long 31.75 to 63.5 mm waveguide taper in corrugated waveguide was made for under 0.1% mode conversion from 100 to 180 GHz.
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John Doane, GA-MFE Technologies