20.3 Marine VHF, MF/HF & SSB-SITOR

20.3 Marine VHF, MF/HF & SSB-SITOR

Quick Answer: Marine VHF: 25 kHz channels, ±5 kHz deviation, Ch-16 distress/calling, Ch-70 DSC. USA–INT flips some duplex → simplex. Check GPS sentences by reading lat/long on the VHF display. MF/HF SSB: watch DC voltage under voice peaks; tuner leads to insulated backstays use GTO-15. SITOR: ARQ = two-way 3-character blocks; FEC = one-way to all; SFEC = one-way to one station.

Topic 3-N (Marine) begins with the bridge and radio-room equipment GROL holders install and repair every day: VHF for coastal line-of-sight, MF/HF SSB for longer range, and SITOR/NBDP for maritime telex.

Marine VHF — channel plan and FM parameters

Channel spacing and deviation

Do not import land-mobile 12.5 kHz narrowband spacing or ±2.5 kHz narrow deviation as the marine VHF pool answers—Element 3’s marine VHF items use 25 kHz and ±5 kHz.

Channel 16 vs Channel 70

Memory split: 16 talks; 70 rings the digital doorbell. Never treat Ch-70 as a survival-craft voice channel (Element 1 already hammered that distinction).

USA–INT control

The USA–INT control or function changes some channels that are normally duplex channels into simplex channels. International (ITU) duplex pairs may be reassigned for U.S. domestic simplex or semi-duplex practices. The control is not a power booster and does not “increase range when set to INT.”

When a radio “works on some channels but not others” after a voyage between regions, verify USA–INT before condemning the synthesizer.

Verifying GPS into a marine DSC VHF

How might an installer verify correct GPS sentence delivery to a marine DSC VHF? Look for latitude and longitude, plus speed, on the VHF display (or the radio’s GPS status page showing valid position fields).

False distress alerts are a career-limiting shortcut. Use display status and manufacturer test modes.

MF/HF SSB — installation and transmit symptoms

Beyond VHF line-of-sight, ships use MF/HF single-sideband for voice (J3E) and data (SITOR/NBDP with F1B/J2B-class emissions depending on equipment).

Voice-test observation

A common occurrence when voice-testing an SSB aboard a boat: voltage panel indicator lamps may glow with each syllable. That flicker tracks speech envelope current demand on a shared DC bus—evidence the transmitter is drawing power on voice peaks, not proof the antenna is perfect.

Apparent low voltage on transmit

What might contribute to apparent low voltage on marine SSB transmitting? Pool answer emphasis: a blown black negative fuse (ground/return path open or high resistance) can make the bus look sick under load. Always verify both positive and negative high-current paths, crimps, and battery interconnects—not only the red feed fuse.

GTO-15 to an insulated backstay

What type of wire connects an SSB automatic tuner to an insulated backstay? GTO-15 high-voltage cable—not RG-8/U or RG-213 coax as the tuner-to-radiator lead in the pool’s framing. Coax belongs on the transmitter-to-tuner side in typical installs; the high-voltage GTO-15 runs from tuner output up to the backstay feedpoint.

SITOR / NBDP — ARQ, FEC, and SFEC

SITOR provides reliable maritime teleprinter traffic over MF/HF. Element 3 tests mode behavior more than typing skill.

ARQ (SITOR-A) — interactive two-way

True statement: ARQ message transmissions are made in data groups consisting of three-character blocks. The information-sending station transmits a short block; the information-receiving station acknowledges (or requests repeat) continuously—not only at the end of a whole message. ARQ is not “time diversity FEC” and not a purely one-way mode.

FEC and SFEC — one-way

The sequence ARQ, FEC, SFEC best corresponds to:

Two-way communications → one-way communications to all stations → one-way communications to a single station.

Receiving FEC without linking first

True statement: Two-way communication with the coast radio station using FEC is not necessary to be able to receive the broadcasts. Ships can copy FEC weather/safety telex without establishing an ARQ circuit first. Contrasting false ideas: that answerbacks must be exchanged before any FEC can be received; that ARQ sends each character twice as its defining method; that weather cannot use FEC because “doubling characters” would make broadcasts too long.

Emission and mode selection reminder

GMDSS-related radio tech at Element 3 depth

You are not sitting Element 7/9 here, but maintainers must see how boxes fit:

Service checklist (085–086)

1. Measure channel spacing / deviation with service monitors when aligning VHF.
2. Prove Ch-16 voice and Ch-70 DSC paths separately.
3. Document USA–INT setting for the vessel’s operating area.
4. Confirm GPS lat/long on the radio display after NMEA install.
5. On SSB: size DC cables, inspect negative fusing, use GTO-15 to backstays, maintain RF ground.
6. For SITOR: know ARQ = 3-char blocks, two-way; FEC = broadcast receive without link; SFEC = selective one-way.

Exam-day checklist (3-N 085–086)

1. VHF spacing 25 kHz; deviation ±5 kHz.
2. 16 = distress/calling; 70 = DSC.
3. USA–INT → some duplex channels become simplex.
4. GPS verify → lat/long (and speed) on display.
5. SSB voice-test → panel lamps may glow each syllable; low voltage → check black negative fuse paths.
6. Tuner to insulated backstay → GTO-15.
7. ARQ three-character blocks; ARQ/FEC/SFEC = two-way / all-stations one-way / single-station one-way.
8. FEC broadcasts receivable without prior two-way contact.

Next section covers survival craft electronics, weatherfax, NAVTEX, and NMEA data that feed and extend these radios.