Lowest usable high frequency

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The lowest usable high frequency (LUF), in radio transmission, is that frequency in the HF band at which the received field intensity is sufficient to provide the required signal-to-noise ratio for a specified time period, e.g., 0100 to 0200 UTC, on 90% of the undisturbed days of the month. The amount of energy absorbed by the lower regions of the ionosphere (D region, primarily) directly impacts the LUF.

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Federal Standard 1037C