|Definition:|| the highest frequency by which a radio wave can propagate between given terminals below the ionosphere, on a specified occasion, by ionospheric refraction alone
NOTE 1 – Where the basic MUF is restricted to a particular ionospheric mode of propagation, the values may be quoted together with an indication of that mode, for example 2F2 MUF.
NOTE 2 – If the extraordinary wave is involved, then this is noted, for example, 1F2 MUF(x). Absence of a specific reference to the magneto-ionic component implies that the quoted value relates to the ordinary wave.
NOTE 3 – If it is desired to quote the ground range for which the basic MUF applies, this is indicated in kilometres following the indication of the mode type, for example 1F2 (4000) MUF.