Power supplies, batteries and antennas
Reserve energy
GMDSS installations need a reserve source of energy independent of the main and emergency switchboards, capable of supplying the required radio load for the period specified by SOLAS. That period is typically one hour where an emergency generator is provided and three hours otherwise; confirm the ship's particular requirement. The reserve battery must be charged automatically, protected against overcharge and deep discharge, and installed with ventilation because lead-acid cells can release hydrogen. Battery condition is checked by voltage, specific gravity where applicable, and a load test.
Antennas and feedlines
VHF antennas are vertically polarized and cut for the marine band. MF/HF installations use a whip or insulated backstay with an antenna tuner at the base and a good ground or counterpoise. Inmarsat radomes need a clear view of the satellite. EPIRBs need a clear view of the sky and must not be shielded by the superstructure. Coaxial feedlines are typically 50 ohms; high standing-wave ratio indicates a mismatch, damaged coax, water ingress, or a disconnected antenna.
Worked example
A 24 V reserve battery supplies 8 A for three hours. The nominal capacity is 8 A x 3 h = 24 Ah. Add about 25 percent for aging and temperature: 24 x 1.25 = 30 Ah. Check that the charger can recharge the battery within the required time and that the load test is done at the correct discharge rate. The reasoning: capacity falls with age and low temperature, so sizing to the bare arithmetic value leaves no margin for a real emergency.
Traps
Do not use an engine-start battery as the GMDSS reserve battery. Do not test specific gravity immediately after adding water. Do not block ventilation around lead-acid cells. Do not paint or coat an EPIRB or Inmarsat radome. Do not coil excess coax into a tight loop near the antenna. Do not tune an HF antenna on a live distress frequency. Do not ignore a high SWR alarm.
Key facts
Reserve energy is independent and time-rated. Lead-acid charging releases hydrogen. VHF antennas are vertical. HF needs a tuner and ground. High SWR means mismatch or feedline fault.