b. In general, because of the variations of ionization during the
daytime, higher frequencies can be used.
During the night, lower
frequencies are used. The critical frequency for the F2 layer, which
exists only during the day, is higher than that of the F layer. At
night, the F layer is actually a combination of the Fl and F2 layers.
It is common for stations in a net not to receive each other with the
same signal strength.
Layer density varies over the circuit path.
It is common for one station to hear well and the rest don't. There
are times when there is only one-way communication because of layer
density variation. The layers vary in thickness from 6 to 75 miles.