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Current practical literature for HF NVIS communication places significant emphasis on foF2 as being the maximum frequency for vertical propagation. This, however, fails to consider the extraordinary wave. This paper presents the analysis of 5 MHz beacon data, showing the relevance of the extraordinary wave in the MUF calculation for NVIS propagation. The results are in full agreement with established scientific theory and ionospheric propagation prediction methods, the detail of which the HF NVIS user community may not be aware of.