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The mathematical model of radiation from open-ended circular waveguide, operating on dominant-mode wave H₁₁ has been developed. The simple approximation formulas allowing to calculate the power pattern for dominant-mode range on the whole observation-space, and also for one-mode range, as well as for multimode waveguide up to the level –12 dB have been obtained in E- and H- plane. It has been shown that the approximation relative error for the first case does not exceed 3% in the main lobe of radiation pattern and for the second case it does not exceed 4%.