"NVIS only works at frequencies from 2 MHz
to 10 MHz. The signal must penetrate the D
layer of the ionosphere, and bounce off the F
layer. Lower-frequency signals will not
penetrate the D layer; higher frequencies will
not bounce off the F layer at these sharp angles and just goes out into space. Remember the
Maximum Useable Frequency (MUF? For amateur radio operators, we're looking at 40 and 80
meters primarily for NVIS use."