4.1 VHF Equipment Controls
Key Takeaways
- The INT-USA control changes certain international duplex VHF assignments to simplex in U.S. waters for VTS and other purposes—not every channel and not weather-band selection
- Channel 16 is distress, safety, and calling; Channel 13 is bridge-to-bridge in most CONUS areas; Channel 70 is DSC only; Channel 22A is the usual U.S. Coast Guard working channel after initial contact; Channel 06 is intership safety
- Simplex uses one frequency so nearby vessels hear both sides; duplex uses separate ship/coast frequencies—distress, urgency, safety, and VTS traffic must be simplex so third parties can monitor the exchange
- VHF ship transmitters must reduce to 1 watt; maximum shipboard F3E carrier power is 25 watts—use low power near shore and on navigational channels when rules require it
- Set squelch to the minimum level that just quiets background noise; Dual Watch listens on a selected channel while periodically checking Channel 16; Scan steps through all or selected channels
4.1 VHF Controls & Channel System
Quick Answer: Know the INT-USA switch (certain international duplex channels become simplex in U.S. waters for VTS and related uses), the core channel map (16 distress/calling, 13 bridge-to-bridge most CONUS, 70 DSC only, 22A USCG working after call, 06 intership safety), simplex vs duplex, 25 W / 1 W power, and correct squelch, Dual Watch, and Scan settings. Weather channels are receive-only marine weather.
Element 1 Subelement C (Equipment Operations) tests whether you can operate a marine VHF set the way Part 80 and good bridge practice require—not just “pick a channel and talk.” This section covers controls (key topic 13) and channel selection (key topic 14).
INT-USA: why the same channel number is not always the same radio path
Marine VHF channel numbers are international, but frequency pairing is not identical everywhere. Many ship-to-shore public correspondence and some operational channels are duplex internationally: the ship transmits on one frequency and the coast station transmits on another. In U.S. waters, some of those same channel numbers are used as simplex for Vessel Traffic Service (VTS) and other domestic needs so every vessel on the channel hears the same frequency.
Purpose of the INT-USA control: to change certain International duplex channel assignments to simplex in the U.S. for VTS and other purposes.
| Wrong idea | Correct Element 1 idea |
|---|---|
| INT-USA changes all channels duplex ↔ simplex | Only certain international duplex assignments |
| INT-USA selects NOAA weather | Weather is a separate WX receive function |
| INT-USA “forces duplex so everyone hears orders” | U.S. mode makes selected assignments simplex so vessels can hear VTS/orders on one frequency |
Operator habit: when you enter U.S. waters (or leave them), confirm the radio’s USA / INT (or INT-USA) mode matches the region. Wrong mode can put you on a frequency pair that the local VTS or coast station is not using for that channel number.
Simplex vs duplex—and why safety traffic is simplex
| Mode | How it works | Who hears what |
|---|---|---|
| Simplex | Ship and coast (or ship and ship) use the same frequency for transmit and receive | Any third vessel monitoring that channel can hear both sides of the exchange |
| Duplex | Ship TX on frequency A; coast TX on frequency B | A third party on only one of the pair may hear only half the conversation |
Why all VHF distress, urgency, and safety communications (and VTS traffic calls) must be in simplex: so vessels not directly in the exchange can still hear both sides. That shared situational awareness is collision avoidance and SAR coordination, not a technical nicety.
Routine ship-to-ship working channels and bridge-to-bridge navigation are also simplex. Public correspondence (marine telephone) channels are classically duplex ship/coast pairs—use them for phone patch traffic, not for MAYDAY coordination.
Core channel map (memorize cold)
| Channel | Role | Operator rule |
|---|---|---|
| 16 (156.800 MHz) | International distress, safety, and calling | Compulsory VHF-FM vessels monitor when at sea (subject to limited VTS/bridge-to-bridge watch exceptions taught in watchkeeping) |
| 13 (156.650 MHz) | Bridge-to-bridge navigational watch in most continental U.S. areas | Navigational traffic; normal power often 1 W |
| 70 (156.525 MHz) | Digital Selective Calling only | No voice on Ch-70 |
| 22A | Typical U.S. Coast Guard working / liaison channel after contact | Call CG on 16 (or via DSC), then shift to 22A when directed |
| 06 | Intership safety | Safety-related ship-to-ship voice, not a substitute for Ch-16 distress watch |
| 08 (example) | Routine commercial intership working | After contact on 16, move routine traffic here or another authorized working channel |
| 28 (example) | Public correspondence (marine operator / shore telephone) | Shore-telephone style traffic, not distress calling |
Channel 16
- What channel must VHF-FM-equipped vessels monitor at sea? Channel 16.
- Use it for distress, urgency, safety, and initial calling—then move to a working channel so 16 stays clear.
- Do not park long business conversations on 16.
Channel 13
- Required bridge-to-bridge watch in most areas of the continental United States: VHF-FM Channel 13.
- Not Ch-16, not Ch-70 DSC, and not “whatever working channel the vessel prefers.”
Channel 70
- Used only for DSC (distress alerts, safety-related digital calls, individual/group DSC calling).
- Voice on 70 is always wrong on Element 1.
Channel 22A and Channel 06
- After establishing contact with the U.S. Coast Guard on Ch-16 (or via DSC), expect to shift to a USCG working channel—commonly 22A in U.S. practice—for the working exchange and safety information.
- Ch-06 is the classic intership safety channel for safety-related ship-to-ship voice when that role is required.
Weather channels
Marine VHF sets include NOAA weather receive channels (WX). They are for continuous weather broadcasts—not ship transmit channels and not what the INT-USA control is for.
Power: 25 W high / 1 W low
VHF ship station transmitters must be capable of reducing carrier power to 1 watt. Shipboard F3E (FM voice) maximum carrier power is 25 watts.
| Setting | Typical use |
|---|---|
| High (up to 25 W) | Open water, weak stations, longer VHF range when needed |
| Low (1 W) | Harbor, near other vessels, bridge-to-bridge when required, any time low power is enough |
Exam anchor: capability to reduce to 1 W is mandatory for ship VHF (with narrow automation/public-correspondence exceptions you will not invent). Default low when close; raise power only when range or non-response requires it.
Volume, squelch, Dual Watch, and Scan
Volume
Sets loudspeaker/handset audio level. Volume does not change transmitter power or receiver sensitivity to weak RF signals the way RF gain might on an HF set—on VHF it is primarily how loud a opened squelch sounds.
Squelch (manual)
Correct manual squelch setting: adjust to the minimum level necessary to barely suppress background noise.
- Too open → constant hiss, operator fatigue, missed weak calls buried in noise fatigue.
- Too tight → weak but valid stations (including distant distress or VTS) never open the audio.
- Do not leave squelch at maximum “to stay quiet.”
Dual Watch (DW)
Dual Watch lets you listen on any selected working channel while the radio periodically monitors Channel 16. It is not “monitor Ch-70 with Ch-16,” not a four-channel scanner, and not a full-band sweep by itself.
Scan
Scan sequentially steps through all or selected channels, stopping when the squelch opens. Use a scan list that includes required watches without drowning the operator in irrelevant traffic.
Aircraft distress frequency (related channel-selection item)
Element 1 also tests the aviation SAR voice frequency used in maritime/SAR context: 121.500 MHz with A3E (AM voice)—not marine FM Ch-16 emission, and not “F3E on 121.5.” Survival craft and SAR coordination may involve that air-band distress frequency; know the frequency + emission pair.
Operator discipline checklist
- Mode: USA vs INT set for the waters you are in.
- Watch: Ch-16 (and Ch-13 / VTS channels as required); Ch-70 for DSC watch on DSC-equipped vessels.
- Power: 1 W when required or sufficient; ≤ 25 W ship F3E maximum.
- Squelch: just quiet—no tighter.
- Traffic flow: call on 16 → work on 13 / 22A / 06 / 08 / other authorized channel as role requires.
- Never voice on 70.
Master the panel and the channel map, and most Subelement C VHF questions become recognition rather than guesswork.
What is the purpose of the INT-USA control settings on a marine VHF radio?
Why must VHF distress, urgency, safety, and VTS traffic communications be conducted in simplex mode?
Which channel is used only for digital selective calling and must not carry voice traffic?
Which statement best describes the correct manual adjustment of the VHF squelch control?