New formulas are presented for the reflection coefficient at the open end of a rectangular waveguide radiating into air including the effect of wall thickness or flange. Existing formulas require significant amount of numerical calculations and do not cover the practical range of waveguide dimensions. Reflection coefficients of open-ended standard waveguides are simulated using commercial electromagnetic software and curve-fitted to derive new formulas. Proposed formulas include the effects of waveguide wall thickness and broad-to-narrow wall aspect ratio. The accuracy of proposed formulas is compared with existing analytical, numerical and experimental results.
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