6.1 VHF Equipment Controls (Key Topic 13)

6.1 VHF Equipment Controls (Key Topic 13)

Quick Answer: The INT-USA switch changes certain international duplex assignments to simplex in U.S. waters for Vessel Traffic Service (VTS) and similar uses. A marine VHF transmitter must be able to reduce carrier power to 1 watt. Dual Watch listens on a selected channel while periodically checking Channel 16. Set squelch to the minimum that barely kills the noise. Scan steps through all or selected channels. Distress, Urgency, Safety, and VTS voice must be simplex so non-participants hear both sides.

Why Element 1 tests the front panel

Subelement C (Equipment Operations) supplies six of the 24 live Element 1 items. Key Topic 13 is the VHF half of that slice. The Marine Radio Operator Permit does not authorize you to redesign a transceiver, but the person on watch is the person who will set INT or USA, tap HI/LOW, open Dual Watch, twist squelch, start a scan, and decide whether a safety conversation belongs on a simplex channel. Independent OpenExamPrep study material covering those Element 1 controls is written for that operator, not as a substitute for 47 CFR Part 80 or the U.S. Coast Guard Navigation Center channel tables on the bridge.

The 25 June 2009 pool still groups six VHF-control facts: the purpose of INT-USA, the required 1-watt carrier reduction, Dual Watch as a selected channel plus Channel 16, the correct squelch threshold, Scan as a sequential sweep of all or selected channels, and the reason Distress, Urgency, Safety, and VTS traffic must be simplex. Later Key Topic 14 names the channels. This section names the knobs and the simplex rule those channels depend on.

INT-USA: certain duplex pairs become U.S. simplex

International VHF (ITU Radio Regulations Appendix 18) assigns some channels as duplex pairs. The ship transmits on one frequency and the coast station transmits on another. That arrangement is convenient for a public-correspondence (marine operator) circuit: the coast transmitter can talk to the ship without occupying the ship's transmit frequency. It is a poor arrangement for a safety or traffic-management circuit, because a third ship listening only on the ship frequency hears the ship and does not hear the coast or VTS replies.

The United States has, since the 1960s, taken selected international duplex ship-transmit frequencies and used them simplex — same frequency in both directions — for VTS, Coast Guard liaison, and other domestic purposes. Those U.S. simplex uses are the familiar A channels (Channel 22A, Channel 05A, Channel 21A, and others). The ITU later gave them four-digit numbers (Channel 22A is Channel 1022, Channel 05A is Channel 1005) so other administrations can recognize the same simplex use. The U.S. Coast Guard Navigation Center's U.S. VHF channel table is the current public listing of those assignments.

The INT-USA (or USA/INT, U/I) control on a marine VHF tells the radio which plan to follow:

• USA: apply the U.S. simplex assignments on those selected channels so a U.S. VTS, Coast Guard, or port-operations frequency is a single frequency that every nearby station can transmit and receive.
• INT: apply the international duplex pairing, which is what you want in many foreign waters where the coast station still expects the duplex pair.

Element 1 tests a narrow purpose statement: INT-USA changes certain international duplex channel assignments to simplex in the United States for VTS and other purposes. Three distractors fail that sentence. The control does not change all VHF channels from duplex to simplex in U.S. waters — Channel 24 through Channel 28 remain duplex public-correspondence pairs, and Channel 16 was already simplex. It does not convert VTS frequencies to duplex so vessels can receive maneuvering orders; VTS needs the opposite, simplex, so every ship hears both the VTS center and the ship being directed. It is not a NOAA weather-channel switch. Weather is a separate WX bank (for example WX1 162.550 MHz) on most marine VHFs.

If the radio has no INT-USA switch, the manufacturer has usually locked the set to the market it was sold for. Confirm Channel 22A (157.100 MHz simplex) actually appears as a simplex channel before you need it for a Coast Guard marine-information broadcast. A radio left in INT may show an international Channel 22 duplex pair instead of U.S. Channel 22A, and you will not hear or answer the Coast Guard on 157.100 MHz simplex.

One-watt carrier reduction

VHF ship-station transmitters must have the capability of reducing carrier power to 1 watt. That is the Key Topic 13 power fact. It is a different number from the Key Topic 1 ceiling: shipboard F3E (FM voice) carrier power may not exceed 25 watts. High power on a typical fixed marine VHF is 25 watts; low power is 1 watt. Handhelds are often 1 watt and 5 watts, but they still must be able to drop to 1 watt.

47 CFR 80.215(g) and 80.80 put the same 1-watt requirement on the control point. Except for listed portables, a 156–162 MHz ship radiotelephone must deliver at least 8 but not more than 25 watts, and all transmitters and remote-control units must be capable of reducing carrier power to one watt or less. Transmitters manufactured after 21 January 1987, or used after 21 January 1997, must automatically drop to 1 watt or less on 156.650 MHz (Channel 13) and 156.375 MHz (Channel 67), with a manual override the operator holds to restore full power. Dedicated Great Lakes public-correspondence duplex transmitters used as extra equipment are a listed exception to the 1-watt reduction; they are not an excuse to run 25 watts on Channel 16 or Channel 13.

Use low power first. One watt is usually enough for a lock, a nearby ship, or a short VTS exchange, and it keeps your carrier off the radios of vessels five harbors away. Raise power when the called station does not answer, when you are in a blind bend that the Channel 13 rules treat as an authorized high-power case, or when distress traffic needs the extra range. Key Topic 7 already tested when Channel 13 may exceed 1 watt (distress and rounding a bend or a blind spot; high power also legal for failure to respond, a blind situation, or an emergency). Key Topic 13 only asks whether the transmitter can reduce to 1 watt. If the HI/LOW button does nothing, the set does not meet the ship-station control rule.

Dual Watch: selected channel plus Channel 16

Dual Watch (DW) means the receiver stays on any selected channel and periodically monitors Channel 16. You might be working Channel 13, Channel 08, or a VTS channel; Dual Watch samples 156.800 MHz often enough that a MAYDAY, a coast-station call, or a safety announcement is not missed while you handle working traffic.

Dual Watch is not three other functions that appear as distractors:

• It is not simultaneous Channel 70 plus Channel 16. Channel 70 is a DSC-only frequency with no voice; a voice Dual Watch cannot listen to DSC bursts as if they were Channel 16 audio.
• It is not a four-channel sequential watch. Some radios offer Tri-Watch (working channel, Channel 16, and Channel 9 or Channel 13). That is a different product feature, not Dual Watch.
• It is not a scan of every VHF channel. That is Scan.

Dual Watch does not replace a required dedicated watch when the rules demand a second receiver. The Navigation Center's radio-watchkeeping page notes that sequential monitoring techniques alone cannot satisfy a required Channel 13 bridge-to-bridge watch: ships that must guard both Channel 16 and Channel 13 need two radios, or one radio with two receivers. Dual Watch is an aid on a single-receiver set, not a legal substitute for a two-receiver bridge-to-bridge installation.

Squelch: the minimum that barely suppresses noise

Squelch mutes the loudspeaker when no useful FM signal is present. Marine VHF-FM without a carrier is white noise. An open squelch (control at minimum, unmuted) fills the bridge with hiss and tempts the operator to turn the volume down — which then hides a weak MAYDAY. A squelch cranked to maximum stays quiet even when a weak station is calling, because the mute threshold is higher than the incoming signal.

The correct manual setting is the minimum level necessary to barely suppress any background noise. Open the squelch until you hear hiss, then close it just until the hiss stops, and leave it there. Do not park it at the maximum stop. Do not leave it at the minimum stop. Do not set it to twice the threshold that just quiets the hiss. Recheck after a generator starts, after you switch antennas, or after Dual Watch or Scan changes the apparent noise floor. 47 CFR 80.305 even treats a properly adjusted squelch as compatible with a Channel 16 cargo watch; a misadjusted squelch is how that watch silently fails.

Scan: sequential all or selected channels

Scan sequentially walks all channels or a selected list and stops, or pauses, when the squelch opens. Memory scan might include Channel 13, a VTS channel, Channel 09, and a working channel. All-scan walks the entire marine band. When the pause ends, scanning resumes.

Scan is not Dual Watch. Scan is not a dedicated Channel 16 distress watch, and it is not a Channel 70 DSC watch. A scanning voice receiver does not decode DSC on 156.525 MHz. If the radio offers a separate DSC receiver, that receiver — not Scan — keeps the Channel 70 watch. Use Scan to find working traffic you are allowed to join. Do not use it as the only ear on Channel 16 while you are the compulsory watchstander on a single-receiver set; Dual Watch, or a second receiver, is the Channel 16 tool.

Why Distress, Urgency, Safety, and VTS must be simplex

Simplex means every station transmits and receives on the same frequency. Duplex means the ship transmits on one frequency and the coast transmits on another. On a simplex channel, anyone who can hear the frequency hears both the calling station and the answering station. On a duplex pair, a third ship listening on the ship's transmit frequency hears only the ship.

Distress, Urgency, and Safety communications, and VTS traffic calls, must be performed in simplex so that vessels not directly participating in the communications can hear both sides of the radio exchange. A ship that is not the distressed vessel still needs to hear the Coast Guard's questions, the position that was given, and the instruction to stand by or to divert. A ship that is not the one the VTS just turned still needs to hear that turn, or it will occupy the same water. Simplex is what makes Channel 16, Channel 13, Channel 22A, and a VTS channel into a party-line safety circuit.

Wrong reasons appear as distractors. Simplex is not primarily a trick to minimize interference from routine traffic — power reduction and moving off Channel 16 do that. It is not a method for an RCC or coast station to hear only the vessel in distress, and it is not a method for the RCC to sort which transmissions are from other ships. Those goals would actually argue for duplex or for a private link. Safety radio is public on purpose.

INT-USA exists largely to create that simplex party line on frequencies that the international table still lists as duplex. Leave the radio in USA in U.S. VTS waters. Leave Distress, Urgency, Safety, and VTS on simplex channels. That is the whole control-and-mode job Key Topic 13 tests.

VHF control settings at a glance

Walk the radio once at the start of a watch: USA in U.S. waters, 1 watt until you need more, Dual Watch on the working channel plus 16, squelch just quiet, Scan off unless you are hunting a working list, and any safety or VTS conversation on a simplex channel. That walk-through is Key Topic 13.