Editing Propagation Basics (section)

== The frequency window: LUF and MUF ==
For any skywave path at any moment there is a usable frequency window:
* Above the '''maximum usable frequency (MUF)''' the ionosphere no longer bends the signal back; it escapes into space.
* Below the '''lowest usable frequency (LUF)''' absorption in the lower ionosphere eats the signal.
The window moves constantly:
* The MUF rises with solar activity and during daytime, and is generally higher on paths toward the equator than at high latitudes.
* The LUF rises during daylight (absorption is a daytime phenomenon) and drops at night, when very low frequencies open up.
* Frequencies just below the MUF usually propagate best; deep inside the window absorption grows toward the low end.

Rules of thumb that follow directly:
* '''Low bands (below about 5 MHz)''': night-time and short-to-medium range in daytime. Daytime long-distance work is largely absorbed.
* '''Middle bands (5 to 12 MHz)''': workable day and night; the default 4625 kHz falls here.
* '''High bands (above about 14 MHz)''': daytime and high solar activity; they close at night.
* Around dawn and dusk (the grey line), low and middle band paths along the terminator are enhanced.