| DSC frequency (kHz) | Voice frequency (kHz) |
|---|---|
| 2187.5 | 2182 |
| 4207.5 | 4125 |
| 6312 | 6215 |
| 8414.5 | 8291 |
| 12577 | 12290 |
| 16804.5 | 16420 |
1The lesson in plain words
In one sentence…
With MF/HF, DSC alerts and voice use different frequencies: read the right pair and follow the approved procedure.
What does MF/HF add beyond VHF?
An HF alert may concern a vessel beyond local VHF range, and each frequency has a job in the exchange.
How do you read the table?
Every DSC distress frequency has an associated radiotelephony frequency; read the full pair before changing the set.
How do you choose a band?
Path, daylight, season and solar conditions matter; UTC alone cannot select the band.
How do you test the set?
Follow the approved procedure; never send a live distress alert to check it.
2Step by step
HF alert: choose the band, listen and relay
- 1
Use the approved single- or multi-frequency procedure.
UTC alone cannot select the band.
- 2
Note the received identity, position, time and distress information.
The alert and its context must remain clear.
- 3
Listen for five minutes on the associated voice frequencies.
Subsequent voice traffic uses frequencies different from DSC.
- 4
Inform a coast station or RCC by an appropriate means.
Shore coordination precedes an instructed relay.
- 5
Transmit the DSC relay only if instructed, under the master's authority.
Receiving an alert does not authorize an automatic relay.
3Key data
- DSC 2187.5 kHz
- Voice: 2182 kHz
- DSC 4207.5 kHz
- Voice: 4125 kHz
- DSC 6312 kHz
- Voice: 6215 kHz
- DSC 8414.5 kHz
- Voice: 8291 kHz
- DSC 12577 kHz
- Voice: 12290 kHz
- DSC 16804.5 kHz
- Voice: 16420 kHz
- MF/HF watch
- 2187.5 + 8414.5 kHz
- Additional HF
- At least one of four, by time/position
- Unacknowledged alert
- Five minutes on associated voice
4Key points
- 1.
DSC and voice are paired, but they are not interchangeable.
- 2.
The watch keeps required frequencies and adds a suitable HF frequency.
- 3.
HF depends on path, daylight, season and solar conditions.
- 4.
Unacknowledged alert: listen, inform shore and relay only when instructed.
5Frequent mistakes
Wrong: "I use the DSC frequency for voice traffic."
Right: Subsequent voice uses the associated radiotelephony frequency, not the DSC frequency.
Wrong: "UTC alone tells me which band to choose."
Right: Path, daylight, season and solar conditions also matter.
Wrong: "I send a live alert to test the set."
Right: Follow an approved test; never use a live distress alert to check it.
6Practice case & exam keys
Situation
At 0312 UTC an HF DSC distress alert is received on 8414.5 kHz. In the exercise, nothing is heard on 8291 kHz and no coast-station acknowledgement appears by 0317. A learner immediately prepares an all-ships DSC relay.
What should happen instead, and when may a relay be sent?
Show the answer →
Contact a coast station or RCC by any available means and report the alert; send a DSC relay only if that station asks, addressed to it alone, under the master's authority.
Why: The five-minute listening period on 8291 kHz has ended without any sign of shore action, so the next step is to get the information ashore. An all-ships relay would add copies of the alert to the distress frequency and confuse coordination. Silence on your receiver is information to report, not permission to retransmit.
Exam keys
When you see «DSC» and «voice», look for a frequency pair, not one frequency.
When «8414.5 kHz» appears, do not drop that watch because another band seems better.
For an «unacknowledged HF alert», listen, inform shore and relay only if instructed.
How to read it
Treat each row as a pair: one frequency carries the digital alert and the other carries subsequent voice.
Receiving an alert does not make you an automatic relay: listen, coordinate and respect the stated authority.
7Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
Learn the frequencies in pairs
| Band | DSC distress/safety | Voice distress traffic |
|---|---|---|
| MF 2 MHz | 2187.5 kHz | 2182 kHz |
| HF 4 MHz | 4207.5 kHz | 4125 kHz |
| HF 6 MHz | 6312 kHz | 6215 kHz |
| HF 8 MHz | 8414.5 kHz | 8291 kHz |
| HF 12 MHz | 12577 kHz | 12290 kHz |
| HF 16 MHz | 16804.5 kHz | 16420 kHz |
Choosing the HF band for an alert
ITU-R M.541 allows a distress alert attempt on a single HF frequency or a multi-frequency attempt on up to six DSC distress frequencies; many sets automate the sequence. The point is to raise the chance that some coast station hears you when propagation is uncertain.
8 MHz is a common first automatic choice on modern equipment, but the best band depends on the path. If acknowledgements arrive on more than one band, start distress traffic on one of them; if that fails, try the others.
A ship that receives an HF distress alert
On HF, ship stations do not acknowledge another ship's distress alert by DSC.
- 1Watch for a coast-station acknowledgement.
- 2Set the receiver and transmitter to the voice distress frequency of the same band (8291 kHz for an alert on 8414.5 kHz).
- 3If no coast station acknowledges within about five minutes and no distress traffic follows, contact a coast station or RCC by any available means and report what you received.
- 4Send a DSC distress relay only if that coast station or RCC asks for one, addressed to it alone. Never broadcast a relay to all ships.
SSB voice: what J3E means
MF/HF voice uses J3E: single sideband with suppressed carrier, normally the upper sideband (USB). Classic AM wastes power on the carrier and on a second sideband that carries the same information. SSB sends a single sideband and no carrier, packing useful energy into a narrow channel, which matters on power-limited HF links.
The receiver has to rebuild the carrier locally, so a slight mistuning makes voices sound too low or too high ("Donald Duck").
| Class | Plain meaning | Where you meet it |
|---|---|---|
| A3E | Double-sideband AM with full carrier | Broadcast/compatibility |
| H3E | SSB with full carrier | Specific uses |
| R3E | SSB with reduced carrier | Specific uses |
| J3E | SSB with suppressed carrier | MF/HF maritime voice |
| F3E / G3E | FM / phase modulation telephony | VHF maritime voice |
| F1B / J2B | Digital data, automatic reception | MF/HF DSC |
| G2B | Digital data on a phase-modulated subcarrier | VHF DSC |
| A1A | Morse telegraphy | Historical |
Receiver controls that change readability
- AGC (automatic gain control) keeps the audio steady across strong and weak signals.
- RF gain can be reduced to avoid overload and intermodulation; do not use it at random.
- Filter / bandwidth: narrower can improve SNR and selectivity, but too narrow distorts voice.
- Noise blanker / reduction helps but can add artefacts; it never replaces a good antenna or a hunt for on-board noise.
- Sensitivity is the ability to detect a weak signal; selectivity is the ability to separate it from nearby frequencies.
HF noise: often made on board
Noise comes from atmospherics, thunderstorms, galactic noise, other transmissions and, very often, the ship itself: inverters, switch-mode supplies, LED drivers, variable-frequency drives, motors, chargers and data networks. A professional diagnosis compares noise with suspect equipment switched on and off, and across bands.
Changing band or mode on an MF/HF console
- 1Identify the service: distress, urgency, safety or routine.
- 2Select the band and the frequency or ITU channel from propagation and the publications.
- 3For voice, select J3E/USB.
- 4Press TUNE and let the ATU match after any frequency change.
- 5Wait for TUNED/READY and watch for alarms or high VSWR.
- 6Adjust RF gain, AGC or noise blanker only if reception needs it.
- 7Transmit with the power the link needs.
- 8If the exchange began by DSC, move to the traffic frequency set by the procedure or acknowledgement.
ACS: the automatic connection system
ACS uses DSC to automate setting up an MF/HF link: scanning, calling station, called station and choice of a working frequency are handled by the equipment under ITU-R M.541. It builds on the DSC equipment ships already carry.
GOC exam check
You receive a DSC distress alert on 8414.5 kHz with telephony as the follow-on. Which frequency do you set for the voice traffic?
8Full lesson & sources
Open the full lesson (diagrams and official references)
MF/HF DSC alerts and subsequent voice traffic use different frequencies. The table pairs the six DSC frequencies with their associated radiotelephony frequencies. A compliant MF/HF distress watch includes 2187.5 and 8414.5 kHz plus at least one of the other four HF frequencies appropriate to time and position. It must not omit 8414.5 kHz merely because another band seems better.
HF propagation depends on the path, daylight, season and solar conditions; UTC alone cannot select a band. Use the approved single- or multi-frequency procedure. After receiving an unacknowledged HF DSC alert, listen on associated voice channels for five minutes, inform a coast station/RCC, and transmit a DSC distress relay only if instructed, under the master's authority. Follow approved test procedures; never test by sending a live distress alert.
| DSC (kHz) | Voice (kHz) |
|---|---|
| 2187.5 | 2182 |
| 4207.5 | 4125 |
| 6312 | 6215 |
| 8414.5 | 8291 |
| 12577 | 12290 |
| 16804.5 | 16420 |
Read the frequency pair as a change of task
On MF and HF every distress band has two frequencies: one where the DSC alert is sent and one where people then talk. Always give the frequency and its function together. 2187.5 kHz carries the MF DSC alert; 2182 kHz carries the MAYDAY by voice. On 8 MHz, 8414.5 kHz is the DSC frequency and 8291 kHz the voice frequency. They are 123.5 kHz apart, so tuning the receiver to one tells you nothing about what is happening on the other.
The pairing is fixed per band, so after a DSC alert you never have to guess: an alert received on 12577 kHz means voice traffic on 12290 kHz; one on 6312 kHz means 6215 kHz. There are six pairs, one per band. Learn them that way, and say "DSC" or "voice" every time you say a number.
Separate a distress watch from a transmit-band choice
Keep two decisions apart: the distress watch from a transmit-band choice. The watch is fixed by the rules: an MF/HF ship keeps DSC watch on 2187.5 kHz and 8414.5 kHz plus at least one of 4207.5, 6312, 12577 or 16804.5 kHz. A scanning DSC receiver does this for you. The transmit band is a judgement made for one call: which band will reach a coast station from here, now?
| Situation | Better first choice | Reasoning |
|---|---|---|
| 150 NM from an MF coast station, any time | MF 2187.5 kHz | Ground wave covers this distance reliably |
| 1,500 NM from the nearest HF coast station, early afternoon local time | 12 or 16 MHz | Daytime ionosphere supports high bands; low bands are absorbed by the D layer |
| Same 1,500 NM path at 0300 local | 8 or 6 MHz | At night the usable maximum frequency falls; 16 MHz may skip over the station |
| Unsure of propagation | Multi-frequency attempt | ITU-R M.541 allows one alert sent across up to six DSC distress frequencies |
You may decide that 12 MHz is the best band to transmit on and still keep watching 8414.5 kHz. Choosing a transmit band never removes a watch frequency.
Why propagation beats power
HF radio reaches far because the ionosphere bends the signal back to earth. That creates a skip zone: beyond the ground wave (roughly a hundred miles at sea) and before the first sky-wave return, there can be a gap where a station hears nothing. If you are 300 NM from a coast station on 16 MHz and the signal returns 600 NM away, doubling your power does nothing. Changing band does. Use "higher by day, lower by night" as a first guess, then go by what you actually hear: the band where the coast station's signal is clear above the noise is the band to use.
Work through a received HF alert
Here is how a received HF alert plays out at 0312 UTC. The rule is to listen first, then tell the shore, and relay only if the shore asks you to.
- 10312: the receiver shows a distress alert on 8414.5 kHz from MMSI 538001234 at 12°40'S 064°20'E, nature flooding. You tell the master.
- 20312: you set receiver and transmitter to 8291 kHz, J3E, and listen. You do not acknowledge by DSC: on HF a ship station never does.
- 30317: after five minutes, no coast-station acknowledgement and no voice traffic heard.
- 40319: you call an MRCC by satellite phone and report the identity, position, time and nature you received.
- 50326: the MRCC asks you to relay. You send a DSC distress relay addressed only to that coast station, not to all ships, under the master's authority. You would send it only if instructed.
Keep a record you can rely on later
Log each step with its UTC time: the frequency you received on, the MMSI and position, the frequencies you monitored, whom you contacted and how, and the instructions you were given. Write "no acknowledgement heard on 8414.5 kHz between 0312 and 0317". Do not write "no station received the alert": a coast station 3,000 NM away may have acknowledged on a band you were not monitoring.
Official references · 2026-09-09
9Test your knowledge
What relationship does the MF/HF table show?
Keep studying…
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: