Module 4 — MF/HF radiotelephony and DSC

MF vs HF propagation, band and frequency selection, DSC and voice operation, distress alerting in sea areas A2, A3 and A4.

MF/HF covers distances that VHF cannot. The student must learn to pick the right band.

MF vs HF

MF (300 kHz – 3 MHz) — medium range (~150–300 miles). Distress DSC on 2187.5 kHz; associated voice on 2182 kHz.

HF (3–30 MHz) — transoceanic range via ionospheric refraction. Distress DSC on 4, 6, 8, 12 and 16 MHz bands.

Propagation

By day the higher bands (12/16 MHz) work better; at night the lower bands (4/6 MHz). Picking the correct band is as important as power.

Typical MF/HF console

TX/RX selector (some sets separate transmit and receive frequency on duplex), mode selector (J3E voice, F1B DSC, F3C facsimile), power control and automatic antenna tuner.

MF/HF-DSC distress alert

The procedure mirrors VHF but the operator must select the correct DSC distress frequency or frequency scan. After the alert, switch to the associated voice frequency (e.g. 2182 kHz for MF).

Watch and listening

In A2/A3/A4 the station must maintain automatic DSC watch on the mandatory frequencies.

Six frequencies, two kinds of attempt (3.2.3.8)

On MF/HF, DSC distress uses six frequencies: MF 2187.5 kHz and HF 4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz. The manual distinguishes two attempt types: single-frequency (five consecutive DSC distress calls on one frequency) or multi-frequency (up to six calls dispersed over all six DSC distress frequencies, one on MF and five on HF). VHF and MF/HF calls may be transmitted simultaneously.

Acknowledgement and repetition timing (3.2.3.26)

A coast station receiving the alert should acknowledge after a minimum delay of 1 minute and normally within a maximum of 2.75 minutes. If the ship in distress gets no acknowledgement, it may repeat the attempt between 3.5 and 4.5 minutes after the start of the initial alert. That window exists to avoid saturation and allow propagation or relay time to work.

Frequency choice by propagation

MF 2187.5 kHz offers reliable range to hundreds of NM by day, extending at night; HF is chosen by time of day and path: 4/6 MHz at night over medium distances, 8/12/16 MHz by day over long distances. The DSC controller generates the attempt, but the operator must verify the associated transceiver is tuned to the corresponding voice frequency for follow-up traffic (2182 / 4125 / 6215 / 8291 / 12290 / 16420 kHz).

Associated distress traffic

After the DSC alert, distress traffic moves to voice on the indicated working frequency, or to NBDP (F1B) when appropriate. The operator builds the standard MAYDAY message: identity, position, nature, number of persons on board, assistance required and any useful information, ending with 'OVER'.

DSC testing with a coast station

The manual states that MF/HF DSC testing with a coast station should be performed once a week but not more often, to avoid channel saturation. Testing on dedicated distress frequencies is forbidden except under specific authorization and procedure.

Emission classes and AM reception (RR Articles 47, 51)

On maritime MF/HF, distress and working voice use J3E (single-sideband suppressed-carrier, USB); F3E is reserved on VHF for FM and is not used on maritime MF/HF. H3E (SSB full carrier) is retained so that legacy AM receivers can still pick up 2182 kHz; that is why some sets offer H3E only on transmission over 2182 kHz. AM broadcast reception (news, commercial nav warnings) is done on the watch receiver in AM / A3E mode and does not interfere with the DSC watch.

DSC scanning and propagation planning

The MF/HF DSC receiver scans the six distress frequencies (2187.5 / 4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz). Scanning should be limited to the frequencies actually relevant to the current area and hour; adding useless frequencies slows the scan cycle and delays detection. The operator chooses the working frequency by local time and distance to the called station, not by habit: 4/6 MHz at night over medium distances, 8 MHz at dawn and dusk, 12/16 MHz by day over long distance. Tuning means taking the transceiver to the chosen working frequency after the DSC handshake; it is not the same as selecting a DSC channel.

Single- vs multi-frequency attempt: operational choice

Single-frequency attempt: five consecutive DSC calls on one chosen frequency based on time and distance; used when the operator knows propagation well (for example a coast-bound ship on 2187.5 kHz at 80 NM). Multi-frequency attempt: up to six calls spread across MF 2187.5 plus the five HF frequencies (4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz); used when time of day, MUF or position make it unclear which band will propagate. Default starting rule: daytime long distance, open with 8 MHz; night at medium distance, open with 4 or 6 MHz; polar or high latitude, try MF 2187.5 first. After the alert the transceiver is tuned on the matching voice frequency (2182 / 4125 / 6215 / 8291 / 12290 / 16420 kHz) for the MAYDAY.

Scan-list composition

The MF/HF DSC receiver must always include 2187.5 kHz plus at least two HF bands appropriate to navigation area and expected time of day. The DSC, NBDP and voice scan lists are independent: adding a frequency to DSC scan does not enable voice reception on it. On a transoceanic day-night transit, keep 8 and 12 MHz in scan; in coastal water, a single HF band close to the area's MUF is enough. Adding irrelevant bands only stretches the scan cycle (typically ~2 s per frequency) and delays detection of an incoming alert.

HF band selection and relay authority

By day typically 8/12/16 MHz; by night, 4/6/8 MHz; where possible compute the current grey line to fine-tune the dawn or dusk transition. (b) If no acknowledgement is heard within 3-5 minutes on the chosen band, retry on an adjacent band (for instance step up from 6 to 8 MHz, or down from 12 to 8 MHz) rather than insisting on the same band: propagation will not open by repetition. (c) If the set supports a multi-frequency attempt, use it when time of day, position or MUF make prediction uncertain.

Relay (MAYDAY RELAY) of a received HF distress alert: a ship does NOT automatically relay. Procedure: listen 5 minutes on the associated voice frequency (4125 / 6215 / 8291 / 12290 / 16420 kHz depending on band), check whether a coast station or RCC has already acknowledged, and only relay if (1) no coast acknowledgement is heard after a reasonable time AND (2) the ship is in a position to assist OR has relevant information (visual witness, more accurate position). HF relay carries far wider implications than VHF because it reaches thousands of miles: a premature relay confuses SAR coordination and duplicates traffic. MF/HF DSC testing with a coast station is limited to once a week.

STCW Bridge Watch Lens

Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.

Build the traffic picture with sight, hearing, radar/ARPA and chart context. Do not let AIS or one isolated bearing replace systematic observation.

After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.

Exam Focus

Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.

If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.

Key Takeaways

HF band choice depends on time of day, distance and propagation.

MF-DSC: 2187.5 kHz. Associated voice: 2182 kHz.

On duplex, verify TX and RX frequencies separately.

Position must be current before any alert.

Six DSC distress frequencies: MF 2187.5 kHz and HF 4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz.

Coast-station acknowledgement expected between 1 and 2.75 minutes; repetition between 3.5 and 4.5 minutes.

Multi-frequency attempts improve reception probability when propagation is uncertain.

DSC tests with a coast station are weekly; more often saturates the network.

Maritime MF/HF voice uses J3E (USB); H3E survives for compatibility with AM receivers on 2182 kHz.

DSC scan covers the six distress frequencies; irrelevant bands in the scan list delay detection.

Propagation: 4/6 MHz at night, 8 MHz intermediate, 12/16 MHz by day and long distance.

Selecting a DSC channel is not tuning the transceiver: they are two separate steps.

Daytime long-haul default = 8 MHz; night medium distance = 4/6 MHz; high latitude = MF 2187.5.

The DSC scan list must always include 2187.5 kHz plus ≥2 relevant HF bands.

Associated HF voice after DSC: 4125 / 6215 / 8291 / 12290 / 16420 kHz (SSB J3E).

Common Mistakes

Alerting on an HF band with closed propagation.

Not tuning the antenna (high VSWR) before transmitting.

Confusing 2182 kHz voice with 2187.5 kHz DSC.

Repeating the alert before the 3.5-minute window expires and clogging the coast station.

Picking an HF frequency without considering local time and MUF: wrong propagation hides the alert.

Moving to voice traffic before tuning the transceiver to the correct working frequency.

Running DSC tests on distress frequencies more than once a week.

Treating F3E as an MF/HF emission: F3E is VHF FM; maritime MF/HF uses J3E.

Leaving the DSC receiver scanning bands that will not propagate at the current hour.

Believing that selecting the DSC channel automatically tunes the voice transceiver.

Stuffing every HF band into the scan 'just in case' and lengthening the cycle enough to miss an incoming alert.

Using multi-frequency when propagation is known and wasting cycles; or single-frequency when propagation is uncertain.

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