Water and Coastal Propagation

Water and Coastal Propagation

Water surfaces create some of the most favorable RF propagation conditions at 915 MHz. Coastal and over-water deployments can achieve ranges that far exceed typical terrestrial links.

Over-Water Propagation

Water surfaces reflect RF very efficiently, creating a smooth "mirror" that supports long-range propagation. Unlike land surfaces (which are rough, absorptive, and cluttered), open water provides a nearly ideal reflective ground plane. The effective radio horizon over water extends further than over land due to the absence of ground clutter and the slight atmospheric refraction that bends RF signals beyond the geometric horizon.

The Two-Ray Model Over Water

Over a flat, reflective surface like open water, two signal paths exist between transmitter and receiver:

1. The direct ray traveling line-of-sight
2. The ground-reflected ray bouncing off the water surface

These two rays interfere constructively or destructively depending on antenna heights and distance. For LoRa nodes at 5 m height, the first constructive interference peak occurs at approximately 1 - 2 km. Beyond this, the interference pattern creates nulls and peaks, but average performance over water remains excellent due to the absence of other loss mechanisms.

Raising antenna height shifts the first constructive peak to greater distances and generally improves over-water performance.

Documented Long-Range Links Over Water

LoRa operators regularly achieve 50 - 80 km links across large lakes or bays with elevated antennas. Some documented examples:

• Consistent 50 - 80 km links across large freshwater lakes with 5 - 8 dBi antennas at 10 - 20 m height
• The current LoRa distance record exceeds 800 km (balloon-assisted, high altitude)
• Over-water links of 30 - 50 km with standard hardware and good antenna heights (10+ m) are achievable without special equipment

These distances are not achievable over land with equivalent hardware and height - the over-water propagation advantage is real and significant.

Coastal Network Planning

Islands, peninsulas, and coastal communities benefit greatly from over-water propagation. A node on a bluff or sea cliff can cover:

• Coastal marine traffic (boats, kayaks, vessels with LoRa-equipped trackers)
• Island communities at ranges exceeding typical land deployments
• Adjacent coastal nodes along the shoreline

For coastal networks, prioritize elevated nodes on headlands, bluffs, and sea cliffs. Even modest elevation (10 - 20 m above sea level on a coastal promontory) provides excellent coverage over water.

Marine Environment Hardware Considerations

Coastal humidity and salt air accelerate corrosion of connectors, coax, and metal mounting hardware. Coastal deployments require additional weatherproofing measures compared to inland installations:

• Use marine-grade stainless steel hardware (316 SS or better) for all mounting
• Apply NO-OX-ID A-Special or equivalent anti-oxidant compound to all coax connectors
• Inspect weatherproofing tape, heat shrink, and connector boots more frequently than inland sites - annually at minimum, semi-annually in high-exposure locations
• Use sealed junction boxes (IP67 or better) for any exposed connections
• Consider conformal coating for PCBs in equipment enclosures near the waterline

Troposcatter

At longer distances over water, tropospheric scatter occasionally enables beyond-horizon propagation. Troposcatter occurs when RF energy scatters off irregularities in the troposphere and some portion reaches a beyond-horizon receiver.

Troposcatter is rare, unpredictable, and unsuitable as a network planning basis - you cannot count on it being available when needed. However, it explains occasional unexpectedly long contact distances reported by LoRa operators over open ocean or large lakes. If you observe an anomalously long contact, troposcatter (or ducting under temperature inversion layers) is the likely explanation.