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Modulation Types Used in GMDSS

• •FM (Frequency Modulation): used for VHF voice, including Channel 16 and the other VHF marine channels. The audio changes the carrier's frequency, not its amplitude, which gives good noise immunity within VHF range.
• •SSB (Single Sideband): used for MF/HF voice, both distress and general traffic. It sends only one sideband of a normal AM signal, so it needs less bandwidth and less transmitter power than full-carrier AM for the same effective range.
• •FSK (Frequency Shift Keying): the basis for DSC and NBDP. Ones and zeros are sent by shifting the carrier between two set frequencies instead of changing its amplitude. That holds up well against the noise and fading you get on MF/HF.

Basic Transmitter and Receiver Blocks

A GMDSS transceiver breaks down into stages like any other radio. To track down a fault, you need to know what each one does:

• •Oscillator: generates the base frequency. It's usually crystal-controlled, because GMDSS distress and calling frequencies need that stability.
• •Mixer: combines two frequencies to make their sum and difference. It's central to building a transmit frequency from a stable reference, and to converting a received signal down to a workable intermediate frequency.
• •Amplifier stages: boost signal power at various points. The Amplifiers chapter covers how they're designed and what goes wrong with them.
• •Antenna and feedline: radiate and pick up the RF signal. A bad connector, damaged coax or mismatched antenna can look exactly like a transmitter or receiver fault, so check them directly.

The Superheterodyne Receiver

Most GMDSS receivers are superheterodynes. The incoming signal is mixed with a local oscillator to produce a fixed intermediate frequency (IF). Amplifying and filtering at one fixed IF works far better than trying to do it directly at every possible receive frequency.

This gives you a useful troubleshooting clue. Because every signal passes through the same IF stage, a fault there can hurt reception across a whole band, not just one channel.

Frequency Stability

GMDSS distress and calling frequencies have to be transmitted accurately and stably. A transmitter that drifts off frequency can miss the receiving station's filter and demodulator window, or interfere with the next frequency over.

Most GMDSS equipment gets its stable reference from a crystal oscillator. Keep in mind that drift, aging or damage in the frequency reference is a real cause of faults. It's less obvious than a dead battery, but it can explain a problem that otherwise looks like a mystery.

Transmission Lines and Antenna Matching

The coax and antenna have to be matched to the transmitter's output impedance (usually 50 ohms) for power to transfer efficiently. A mismatch sends power back toward the transmitter instead of radiating it. You measure this as VSWR (Voltage Standing Wave Ratio).

High VSWR from a damaged antenna, a bad connector or water in the coax cuts your effective transmitted power. If it's bad enough, it can damage the transmitter's final amplifier. That's why a VSWR reading is one of the most useful first checks when range drops off or you suspect the transmitter. It's quick and you don't have to open anything up.