RYA SRC Part 2 — VHF Channels, Frequencies, and Propagation

21 May 2026 PersonalSRCRYAvhf-channels

Syllabus coverage: CEPT/RYA section A1 — types of communication in the maritime mobile service, types of stations, the VHF marine band, channel categorisation, propagation, simplex/semi-duplex/duplex, range calculations.

If Part 1 was about why the radio exists, this post is about the medium it transmits on. You can pass the SRC without much theory, but you cannot pass it without knowing which channel is which.

The VHF marine band

VHF (Very High Frequency) marine radio sits in the band 156–162 MHz. The band is channelised — 56 numbered channels in the original ITU plan, plus national additions. Each channel is a defined frequency (for simplex) or pair of frequencies (for duplex), spaced at 25 kHz intervals.

The exam expects you to know the differences between VHF, MF, and HF:

Band	Frequency	Range	Use
VHF	156–162 MHz	Line of sight, ~30 NM ship-to-shore	Voice + DSC; the SRC band
MF	1.6–4 MHz	Ground wave ~150 NM	A2 voice + DSC (2182, 2187.5 kHz)
HF	4–27 MHz	Sky wave, global with skip	A3/A4 voice + DSC

VHF is line-of-sight: it doesn’t bend over the horizon (much), doesn’t propagate via the ionosphere, and is essentially limited by Earth’s curvature plus a small refraction bonus.

Simplex, semi-duplex, duplex

Three operating modes — the exam asks the difference.

Simplex — one frequency, one direction at a time. Press to talk, release to listen. Most ship-to-ship marine channels are simplex. Examples: CH06, CH08, CH16, CH72, CH77.
Semi-duplex — two frequencies (one for ship, one for shore), but each side still operates in single-direction mode. The ship can hear the shore only when the ship’s PTT is released. Used for some coast station calls.
Duplex — two frequencies, both sides transmit simultaneously, like a phone call. Used historically for public correspondence (ship-to-shore phone calls). The UK no longer offers public correspondence on marine VHF, but the duplex channels still exist in the ITU plan and other countries still use them.

In the UK, small craft VHF is overwhelmingly simplex.

Propagation and range

VHF propagation is line of sight with a small atmospheric refraction bonus. The standard radio-horizon formula (accounting for that refraction) is:

range (NM) ≈ 1.23 × √h₁ + 1.23 × √h₂

where h₁ and h₂ are the antenna heights of the two stations in metres.

Worked examples for the exam:

Two handhelds, 2 m antenna height each: 1.23 × √2 + 1.23 × √2 ≈ 1.74 + 1.74 ≈ 3.5 NM.
Yacht with masthead antenna at 15 m to another yacht at 15 m: 4.76 + 4.76 ≈ 9.5 NM.
Yacht at 15 m to coast station at 100 m: 4.76 + 12.3 ≈ 17 NM.
Coast station at 100 m to a SOLAS ship at 30 m: 12.3 + 6.7 ≈ 19 NM.

A few practical consequences:

Doubling antenna height does NOT double range — range scales with the square root of height.
A handheld at deck level is brutally short-ranged. A handheld held above the head when transmitting genuinely helps.
Coast stations site their antennas on high ground specifically because that’s where range comes from.

Atmospheric “ducting” can extend VHF range dramatically in stable weather (anti-cyclonic conditions), occasionally allowing transmissions hundreds of miles beyond the radio horizon. It is unreliable and not on the SRC exam, but you should know it exists if you ever pick up a distant Dutch coast station from the South Coast on a quiet summer evening.

The channel plan — what an SRC holder must know cold

The CEPT/RYA syllabus calls out specific channels by number. Memorise these.

Distress, safety, and calling

CH16 — 156.800 MHz, simplex. International distress, safety, and calling. Listening watch required for all DSC-equipped vessels. Used for the voice MAYDAY that follows a DSC distress alert.
CH70 — 156.525 MHz, simplex. DSC ONLY. No voice. Ever. Class D sets refuse to key voice on 70 — but never assume.
CH13 — 156.650 MHz, simplex. Bridge-to-bridge navigational safety, intership. Mandatory listening watch in some port areas for vessels engaged in navigation.

Intership

CH06, CH08, CH72, CH77 — primary intership channels, simplex. CH06 is the first choice for ship-to-ship traffic; CH08 the next. CH72 and CH77 are the “yacht channels” in practice.

Port operations and ship movement

CH09, CH10, CH11, CH12, CH14, CH67, CH71, CH74 — port operations and ship movement. CH12 and CH14 are heavily used by VTS in major UK ports.

Safety / coastguard supplementary

CH67 — 156.375 MHz — UK supplementary safety channel. UK Coastguard uses CH67 for routine safety traffic after initial CH16 contact, particularly for small craft safety information broadcasts.
CH10, CH73 — sometimes used for MSI broadcasts.

Marina and private

CH80 — UK marinas. Allocated to UK marinas for berthing/fuel/launch coordination. This is where the exam catches people: CH80 is allocated differently in some other countries (e.g. it’s a public correspondence duplex channel in the original ITU plan). When boating abroad, do not assume CH80 = marina.
M (CH37) and M2 — UK private marina/yacht club channels. Voluntary use, not on the international plan. Some marinas use M instead of 80.

Small craft safety

CH06, CH67, CH73 — small craft safety as needed.

What the exam specifically calls out

The RYA syllabus’s A1.3 section names the channels you should be able to explain the allocated usage of: 06, 08, 13, 16, 67, 70, 72, 77, 80, M (37), and M2. Learn these. The “what’s CH70 for” question lands in every paper.

Priority of communications

Every voice or DSC transmission has one of four priority categories. They are processed in this order:

Distress — grave and imminent danger to life, vessel, or aircraft. Highest priority. Stops all other traffic.
Urgency — very urgent message concerning the safety of a vessel, aircraft, or person. Below distress, above safety.
Safety — message concerning the safety of navigation or important meteorological warnings.
Routine — everything else. Public correspondence (where offered), intership chat, marina logistics.

If you hear a higher-priority transmission start, drop your routine traffic immediately and listen.

Types of station

For exam completeness — the syllabus lists these station types:

Ship stations — radios on vessels. What you operate.
Coast stations — shore-based. In the UK, HM Coastguard is the dominant coast station presence.
Rescue Coordination Centres (RCCs) — covered in Part 1.
Pilot, VTS, and port stations — local, working specific channels in port approaches.
Aircraft stations — SAR helicopters and aero-SAR aircraft. They appear in MAYDAY traffic; their MMSI format is 111-prefixed (covered in Part 4).

Types of communication in the maritime mobile service

Also for syllabus completeness:

Distress, urgency, and safety — covered above.
SAR communications — the operational traffic during a search and rescue.
Public correspondence — ship-to-shore phone calls via a coast station. Largely obsolete in the UK and many other countries.
Port operations and ship movement — VTS, pilots, berthing.
Intership — vessel-to-vessel.
Onboard communications — within a vessel (separate band, not really the SRC’s concern).

What to take into the next post

You should leave Part 2 able to:

State the VHF marine band (156–162 MHz) and the difference between VHF, MF, and HF.
Define simplex, semi-duplex, and duplex.
Recite the allocated use of CH16, CH70, CH13, CH06, CH08, CH67, CH72, CH77, CH80, M, and M2.
Calculate VHF range from antenna heights using 1.23 × √h.
Name the four priority categories in order.

Part 3 leaves the airwaves and looks at the radio itself — the controls on the front panel, the antenna feeding it, and the power supply behind it.