Mastnode AIS receiver — deployment guide

1 · Why placement matters

AIS is a VHF line-of-sight service. Unlike satellite or cellular, VHF waves at 162 MHz travel essentially in straight lines, bending only slightly past the geometric horizon. Two sites a few kilometres apart can have wildly different coverage because one looks straight at the sea and the other looks at a wall.

On the MastChain network you earn rewards per sea hex covered, with bonuses for high uptime. A poorly-placed station earns from a few hexes; a well-placed one from dozens. The investment is the same; the difference is siting craft.

2 · Sea view & line of sight

VHF at 162 MHz needs an unobstructed path from your antenna to the ship's antenna. A solid object — a building, a hill, a stand of trees — in the way will block or severely attenuate the signal.

• Open sea horizon in at least one direction is mandatory. A balcony facing the road and not the water will catch nothing.
• Even thin walls cut VHF significantly. Antennas indoors lose 80–90 % of their range. Always mount outdoors.
• Trees in leaf attenuate VHF by 5–15 dB. A row of pines between you and the sea is enough to halve effective range.
• Coastal vs inland: every kilometre inland from the shore typically costs 1–2 km of effective sea range.
• Aim for the widest sea sector possible — ideally 180° unobstructed. Vessels travel in all directions.

3 · Elevation & antenna height

The radio horizon grows with the square root of effective antenna height above the sea surface. Effective height = terrain elevation + mast height. A 25 m mast on a 100 m hill is the same as a 125 m mast at sea level.

The model formula used by the planner is the 4/3-Earth VHF radio horizon, calibrated against field observations:

radius (km) = 4.12 × (√h_station_ASL + √h_ship) × 1.15
where h_station_ASL = terrain_elevation + antenna_above_ground
      h_ship      = 12 m (typical mixed merchant + leisure)

4 · Antenna choice

For AIS you want a vertically-polarised omnidirectional antenna tuned to 162 MHz. Two common types:

• Vertical polarisation — ships transmit vertically. A horizontal antenna loses 20+ dB.
• Tuned for 162 MHz, not the broader marine VHF band. A “156–163 MHz” antenna works; a CB / amateur 144 MHz antenna will be lossy.
• Gain trade-off — higher gain = flatter pattern = more horizon range, less overhead reception. 4–6 dBi is a good sweet spot for coastal AIS.
• Ground plane — if using a 1/4-wave whip, mount on a metal plate or use radials. End-fed collinears don't need one.

5 · Mast & mounting

The antenna should be the highest metallic object within several metres in every direction. Surrounding metal (sat dishes, other masts, AC units) will detune and shadow the antenna.

• Keep ≥ 3 metres horizontal clearance from any other VHF antenna; sharing a mast causes mutual desensitisation.
• Keep the antenna at ≥ λ/4 above the mounting surface (≥ 0.5 m at 162 MHz). Closer = pattern distortion + ground reflections.
• Mast material doesn't matter electrically (the feed-line is shielded), but stability does. Steel + guy wires for masts > 4 m.
• Vibration shortens cable life — secure the coax along the mast every 30–50 cm with UV-stable ties.
• If a satellite dish or 5G small cell shares the roof, keep at least 2 metres from them and try to put the AIS antenna higher.

6 · Cable & connectors

Coax cable loss is the silent killer. The cable between the antenna and the receiver attenuates the signal in dB per metre; double the length and you double the loss in dB (= one-quarter of the power). At 162 MHz:

• Shortest practical run. Move the receiver toward the antenna if possible, not the other way.
• Sealed N-type or PL-259 connectors at the antenna end; weatherproof with self-amalgamating tape under a layer of vinyl tape.
• Form a drip loop at the connector — a downward U-bend so rain can't follow the cable inside.
• No tight bends. Minimum bend radius for LMR-400 is ~10 cm.
• Avoid running the coax parallel to AC mains or DC power cables — ≥ 30 cm separation, cross at 90° if they must meet.

7 · Bandpass filter (162 MHz)

On a busy urban site, a narrow bandpass filter centred on the AIS frequency is the single cheapest improvement you can make to a marginal install.

An SDR-based receiver front-end is wide-open from below FM up to ~1.7 GHz. Strong out-of-band signals — FM broadcast, paging, DAB, GSM, taxi radios — will desensitise the receiver even though they're not the AIS frequency. A narrow filter (typically 156–163 MHz pass) blocks everything else.

• When to add one: any city site, anywhere within 2–3 km of an FM broadcast tower, or anywhere the AIS message rate drops sharply at certain times of day (paging / commercial radio activity).
• Cost: typically $20–50.
• Effect: 2–3 dB extra sensitivity on busy sites, often a 10–20 % range gain in practice.
• Where it goes: in-line on the coax, as close to the receiver as practical — SMA / N-type connectors on both sides.

8 · RF environment

Urban sites face a noisy 162 MHz floor. Common culprits:

• Switching power supplies, LED-bulb drivers, solar inverters — broadband noise. The same room as your receiver is the worst place for any cheap PSU.
• Plasma TVs, old computers, USB 3 cables — radiate strongly across VHF.
• FM broadcast towers (88–108 MHz) at < 2 km can desensitise even with a filter.
• Marine VHF base stations nearby (port authorities) can overdrive the front end.

Practical diagnosis: turn off all electronics in the room with the receiver, then re-enable one by one. Watch the AIS-catcher signal-level histogram. A culprit will lift the noise floor by 2–10 dB.

9 · Power, network & time

Reward eligibility starts at 80 % uptime; full bonus needs > 95 %. A station that drops out a few hours a week earns nothing, regardless of placement.

• UPS (small one, 300–600 VA) on the receiver + router + LNA. Covers mains outages and brown-outs.
• Reliable upstream — fibre or stable LTE. A home connection is fine if it's stable; sustained mobile hotspot is fragile.
• NTP time sync — AIS messages are timestamped at receive. Sync the receiver host to a public stratum-2 pool. Wrong time = mis-validated messages = no reward credit.
• Watchdog — a script (cron / systemd) that restarts the receiver process if the AIS message rate drops to zero for > 5 minutes.
• Static lat/lon — the receiver reports its fixed position. Don't change it unless you've actually moved the antenna.

10 · Common mistakes

11 · Monitor & optimise

After install, give the site at least 72 hours of baseline operation before judging it. Things to watch:

• Messages per minute at different times of day. Off-peak (3–5 AM local) versus peak (07–10 and 17–20).
• Unique vessels per day — the cleanest health metric. Should grow over 1–2 weeks as more ships pass.
• Max distance heard. If far below the planner's prediction, the install is under-performing — check for blockage and cable loss first.
• Signal-level histogram — should be centred between −60 and −90 dBFS with a long tail. A floor above −80 dBFS means RF noise.
• Validation rate — messages should pass CRC-validation > 95 %. Lower = bad antenna, RF noise, or strong out-of-band interference.

Use the planner's coverage circle as a target. If real reception is 60–80 % of the predicted radius after cleanup, the site is healthy. 100 % is rare; 40 % means something major is wrong.

12 · Pre-deployment checklist

1. Site has unobstructed sea horizon ≥ 90° of arc.
2. Antenna mounting point is the highest local object.
3. Total cable run < 20 m, or LMR-400 used.
4. Connectors weatherproofed with self-amalgamating + vinyl tape, drip loop formed.
5. Receiver on UPS, on a circuit separate from heavy switching loads.
6. Internet upstream tested at > 95 % monthly uptime.
7. Host NTP-synced.
8. Watchdog or systemd-restart in place.
9. Coverage measured against planner prediction after 72 h.
10. Bandpass filter installed if site is urban / has FM tower within 2 km.

A well-prepared site reaches its theoretical coverage, scores > 95 % uptime, and earns the full reward multiplier across nearly all its hexes. Half the network's stations under-earn for one of the reasons above — avoid them and you'll be in the top quartile.

13 · Credits & open-source licences

This site is built on the work of many open-source authors. Below is a summary; the full text of every licence is shipped with the project source.