Radio Propagation, Antennas, Power Supplies and Basic Maintenance
Propagation
MF uses ground wave for short-range coastal work. HF uses skywave refraction by the ionosphere; the usable band depends on time of day, season, solar activity and path length. Higher frequencies generally suit daylight and longer paths, lower frequencies night and shorter paths. VHF is essentially line-of-sight, so antenna height dominates range.
Antennas and feedlines
A VHF antenna should be mounted high, clear of metal obstructions and separated from other antennas. MF/HF whips and backstay antennas need a good earth/ground system and an antenna tuner. A high SWR indicates mismatch, a damaged feedline or a poor ground; it wastes power and can damage the transmitter. Use the correct coaxial cable, waterproof connectors and strain relief.
Power and maintenance
Check battery voltage, charger output, fuses and connections regularly. The reserve source must be capable of supplying the GMDSS installation for the required period, and the emergency battery should not be discharged by non-essential loads. Keep logs of tests and defects.
Worked example: a VHF antenna is 9 m above sea level and the coast station antenna is 16 m. Radio-horizon range is about 2√h nautical miles per antenna: 2×3 = 6 nm plus 2×4 = 8 nm, about 14 nm. The reasoning: VHF range is set mainly by antenna height and line-of-sight, not transmitter power alone.
Traps
Do not assume a higher HF band is always better. Do not transmit into a disconnected or damaged antenna. A battery that shows 12 V may still have no capacity under load. Poor grounding causes both weak transmission and interference.