High Frequency (HF) Communications
In the HF range (3 MHz to 30 MHz) radio waves propagate over long distances due to reflection from the ionised layers in the upper atmosphere.
Due to variations in height and intensities of the ionised regions, different frequencies must be used at different times of day and night and for
different paths. There is also some seasonal variation (particularly between winter and summer). Propagation may also be disturbed and enhanced during periods of intense solar activity.
Frequencies chosen for a particular radio path are usually set roughly mid-way between the lowest usable frequency (LUF) and the maximum usable frequency (MUF). The daytime LUF is usually between 4 to 6 MHz during the day, falling rapidly after sunset to around 2 MHz. The MUF is dependent on the season and sunspot cycle but is often between 8 MHz and 20 MHz.
Hence a typical daytime frequency for aircraft communication might be 8 MHz whilst this might be as low as 3 MHz during the night.
Exam Question Tips:
During a flight at night a position has to be reported to ATC. The aeroplane is at a distance of 750 NM from the groundstation and at flight level 350. The frequency to be used is:
a) 17286 KHz
b) 123.9 MHz
c) 5649 KHz
d) 1136 KHz
In this scenario VHF is not going to work so you’ll have to use HF.
b) and d) are out of HF Band.
Out of a) and c) the lowest is c) which is correct, as it is night. “Sun Down – Frequency Down”