How to Maximise AIS Range on a Cruising Sailboat

AIS range isn’t magic. It’s physics, installation discipline, and a bit of seamanship. When a cruising sailor complains that their AIS range is “poor”, the culprit is almost always one of three things: antenna height, power output, or installation quality. Get those right and even a modest Class B unit can perform far better than most people expect.

What Really Determines AIS Range at Sea
Antenna Height: The Single Biggest Factor
Power Output: Class B vs Class B+ in the Real World
Installation Quality: The Silent Range Killer
Coax, Connectors and RF Losses
Antenna Placement and Obstructions
Splitters: When They Help and When They Hurt
Real‑World Range Expectations for Cruisers
How to Maximise Your AIS Range Step‑by‑Step
Summary: What Cruisers Should Prioritise

What Really Determines AIS Range at Sea

AIS is a VHF‑based system, so its range is governed by line‑of‑sight propagation. That means the higher the antenna and the cleaner the installation, the farther your signal will travel. Power output matters too, but only after the fundamentals are in place.

The major factors are:

Antenna height
Transmit power (Class B vs Class B+)
Coax cable quality and length
Connector quality and installation
Antenna placement and obstructions
Splitter performance
Sea state and atmospheric conditions

Each of these can make or break your real‑world range.

Antenna Height: The Single Biggest Factor

If you change nothing else, put the AIS antenna as high as possible. Height alone can double or triple your range.

A masthead antenna typically sees:

15–25+ miles in good conditions
30+ miles when talking to a ship with a very high receiving antenna

A pushpit‑mounted antenna often struggles to exceed:

5–8 miles
10 miles on a very good day

The maths is simple: every metre of height increases the radio horizon. A masthead mount is the closest thing to a “free upgrade” in AIS performance.

Power Output: Class B vs Class B+ in the Real World

Class B units transmit at 2W using CSTDMA, while Class B+ units transmit at 5W using SOTDMA. That difference matters offshore and in busy shipping lanes.Why Class B+ travels farther

Higher power punches through marginal conditions
SOTDMA reserves time slots, so your signal isn’t “lost” in heavy traffic
Ships detect you sooner and more reliably

If you sail offshore, overnight, or anywhere near commercial traffic, Class B+ is a meaningful upgrade.

Installation Quality: The Silent Range Killer

A perfect AIS transceiver with a poor installation will perform worse than a cheap unit installed well. The most common issues I see on cruising boats include:

Corroded connectors
Water‑ingress in coax
Kinked or crushed cable
Excessive cable length
Cheap RG‑58 coax used for long runs
Antennas mounted next to stainless steel

Any one of these can cut your range in half.

Need help choosing the right AIS unit for your boat and your budget? Click here to see my recommended models and comparison table.

Coax, Connectors and RF Losses

Coax losses increase with frequency and length. AIS operates at 162 MHz, so poor coax is brutally exposed.Recommended coax types

Use:

LMR‑400 for long mast runs
LMR‑240 for moderate runs
RG‑213 as a traditional marine alternative

Avoid:

RG‑58 for anything longer than a few metres
Cheap, unbranded coax

Typical coax loss comparison

Coax Type	Loss per 30m	Suitability for AIS
RG‑58	~12 dB	Poor – avoid for mast runs
RG‑213	~5.5 dB	Good – traditional marine choice
LMR‑240	~4.4 dB	Very good for moderate runs
LMR‑400	~2.2 dB	Excellent for long mast runs

Every 3 dB of loss halves your effective power. That’s why coax matters.

Antenna Placement and Obstructions

AIS antennas hate metal. Anything conductive near the antenna distorts the radiation pattern and reduces range.

Avoid mounting near:

Stainless steel pushpits
Solar arches
Wind generators
Radar scanners
Backstays (especially insulated ones)

A masthead mount avoids almost all of these issues.

Splitters: When They Help and When They Hurt

A good AIS‑certified splitter is perfectly safe and performs well. A cheap or non‑AIS splitter is a disaster.

Use a splitter when:

You want to share your masthead VHF antenna
You choose a certified AIS splitter (e.g., em‑trak, Garmin, Vesper)

Avoid splitters when:

The splitter is not AIS‑certified
You’re using old or unknown coax
You already have marginal VHF performance

A poor splitter can reduce AIS range by 50% or more.

Real‑World Range Expectations for Cruisers

Assuming a good installation:

Class B, masthead antenna: 15–20 miles
Class B+, masthead antenna: 20–25+ miles
Class B/B+, pushpit antenna: 5–10 miles
Ship‑to‑ship (both masthead): 30–40 miles

Atmospheric ducting can extend this dramatically, but you should never rely on it.

How to Maximise Your AIS Range Step‑by‑Step

Mount the antenna as high as possible – masthead if at all feasible.
Use high‑quality coax (LMR‑400 or RG‑213 for long runs).
Keep coax runs short and avoid unnecessary joins.
Install connectors properly – no corrosion, no water ingress.
Keep the antenna clear of metal and other obstructions.
Use an AIS‑certified splitter if sharing a VHF antenna.
Choose Class B+ if you sail offshore or in busy shipping lanes.
Regularly inspect the system – coax, connectors, and antenna mounts degrade over time.

Do these things and your AIS will perform at its absolute best.

Summary: What Cruisers Should Prioritise

If you want maximum AIS range, focus on the fundamentals:

Height is king
Power helps, especially Class B+
Installation quality determines everything else

A well‑installed AIS system with a masthead antenna routinely achieves 15–25 miles, and often more when talking to ships. For a cruising sailor, that’s the difference between spotting traffic early and being surprised at night or in poor visibility.

This article was written by Dick McClary, RYA Yachtmaster and author of the RYA publications 'Offshore Sailing' and 'Fishing Afloat', member of The Yachting Journalists Association (YJA), erstwhile member of the Ocean Cruising Club (OCC) and owner/administrator of 'Liveaboard Sailboats for Sale'.