HF Communication Systems
HF (3-30 MHz) supports long-range communication via skywave refraction in the ionosphere, essential for oceanic and remote operations. Know the layers (D, E, F1, F2), that the F layer supports long-distance propagation and that the D layer absorbs lower HF during daylight. Understand critical frequency, maximum usable frequency (MUF), lowest usable frequency (LUF) and skip distance. Candidates should know that SSB (single sideband) is used for HF voice because it concentrates power in one sideband and saves bandwidth.
Operational concepts include SELCAL (selective calling) to alert the crew without monitoring the channel continuously, and the use of HF for position reporting and oceanic clearances. Understand that propagation varies with time of day, season and solar cycle, and that frequencies are selected accordingly (higher by day, lower by night).
Traps: assuming HF always works; forgetting D-layer absorption is worst in daylight; confusing MUF with LUF; thinking SELCAL carries voice (it only alerts). Another trap is the belief that HF is line-of-sight.
Rule of thumb: day = higher frequency, night = lower frequency. If communication fails, try the next assigned frequency and advise via alternate means (SATCOM, relay).