The higher the reflecting height, H, the greater the range (for a given angle of incidence).
-the reflecting layer height goes up with the Sun (and back down), reaching a maximum at local noon.
-the ionosphere absorbs radio waves, having maximum absorption at the lower frequencies and less as the frequency increases
-frequencies above a maximum value are not reflected at all and continue into space.
-Use the maximum frequency that will reflect (called maximum useable frequency, MUF)
-As the layer height increases, the MUF also increases.
Frequency schedule: changes the frequency in use during the day to use the highest frequency that will reflect. So highest frequencies are at local noon, lowest at night.