What does LUF stand for?

In radio wave propagation, LUF stands for the Lowest Usable Frequency. It specifically refers to the lowest frequency that will support communications between two specific geographic points at a given time. If you attempt to use a frequency below the LUF for a specific path, the radio waves will typically be completely absorbed by the D layer of the ionosphere rather than refracted back to Earth. Distractors that suggest the LUF is based on a general radius, a 24-hour period, or a past 60-minute window are incorrect because LUF is constantly changing and depends entirely on the unique path between two specific locations.