The Importance of Antenna in the AIS System
AIS Reliability Starts at the Antenna
Automatic Identification System (AIS) only protects you if your transmit and receive paths are healthy. Most AIS problems trace back to the VHF antenna system—coax, connectors, placement, and the antenna itself. Here’s how to keep the link solid and compliant.
What AIS needs to work
- Correct frequencies: AIS uses 161.975 MHz (AIS1) and 162.025 MHz (AIS2)—in the VHF band.
- Clean RF path: low-loss coax, proper connectors, solid bonding/grounding, and a correctly tuned antenna.
- Good GPS position: Class A/B transponders must have a stable GPS feed for valid messages.
- Stable DC power: clean 12/24 VDC with proper fusing; voltage sag will degrade transmit power.
Why antennas make or break AIS
AIS transponders have limited power (Class B: ~2 W; Class A: 12.5 W). Any extra loss or mismatch at the antenna system shrinks range and creates “you see me/I don’t see you” scenarios. A poorly tuned antenna (high VSWR) forces the transponder to reduce power to protect itself—exactly when you need it most.
Installation essentials (do it right once)
- Dedicated or properly split feed: Prefer a dedicated AIS antenna. If you must share with the VHF radio, use a certified AIS/VHF splitter with fail-safe for the VHF.
- Antenna placement: as high and clear as practical; ≥1 m horizontal separation from other VHF/GPS antennas; keep away from radar beams and metal masts.
- Coax & connectors: use low-loss 50 Ω coax (e.g., RG-8X or better on small runs), crimped PL-259/BNC with heat-shrink; avoid sharp bends, water traps, and deck joints.
- Bonding/grounding: follow ABYC/NMEA guidance; bond the splitter/transponder per manual; control corrosion and galvanic paths.
- Labeling & drip loops: label both ends; include drip loops into devices to prevent water ingress.
Verification & maintenance (the disciplined way)
| Checkpoint | Method | Pass Criteria | Interval |
|---|---|---|---|
| VSWR/Tuning | Inline VHF analyzer at 156–162 MHz | VSWR ≤ 1.5:1 (≤2:1 acceptable); stable across AIS1/AIS2 | Commissioning; annually; after any strike/repair |
| Coax loss | Analyzer or cut-back inspection | Loss appropriate for length/type; no water ingress | Annually |
| Connectors | Visual + tug test | No corrosion, full crimp, sealed heat-shrink | Quarterly |
| Power supply | Multimeter under load | Voltage within spec at the unit; fuse/CB sized per manual | Commissioning; annually |
| On-air check | Marine traffic receiver or buddy boat | Your MMSI/position seen at expected range | Sea trial; quarterly |
| GPS feed | AIS diagnostics page | 3D fix, healthy satellites; no “GPS lost” alarms | Each departure |
Tip: Many AIS units expose diagnostics (VSWR estimate, TX count, GPS status) on their web UI/NMEA. Record screenshots in your maintenance log.
Quick troubleshooting cues
- You see them, they don’t see you: likely TX path issue—check VSWR, connectors, power sag.
- No targets at all: RX path or splitter mis-wiring; verify antenna and splitter mode.
- Intermittent range: water in coax/connector, corroded PL-259, or antenna detuning from nearby metal.
- Alarms during transmit: high VSWR or low voltage—measure both.
Documentation that pays off
- Save VSWR and coax-loss readings with date, cable length, and antenna model.
- Keep a simple diagram of the RF path (AIS → splitter → coax → antenna).
- Log on-air checks (who saw you, range). Trends reveal degradation early.
Bottom line: AIS safety is antenna-centric. A tuned antenna, dry coax, proper separation, and periodic VSWR checks turn AIS from “nice to have” into a reliable collision-avoidance tool.