Fresnel Zone: Sea and Marine Antennas
A link can be weak even when two antennas can see each other. The reason is often the Fresnel zone: how close the volume the signal travels through comes to the sea surface determines speed and stability at sea.
Antennas and the Fresnel Zone in Maritime Applications
Communication on yachts, ferries and commercial vessels depends on physical conditions that differ from those on land. At sea there are no obstacles such as buildings; instead there is the reflective water surface, the horizon and a platform that is constantly rolling.
The Fresnel zone and multipath are often confused. Both relate to the sea surface, but they describe different things: the Fresnel zone defines the volume the signal passes through, while multipath describes how many paths the signal takes to reach the antenna.
What Is the Fresnel Zone? The 60% Rule
The Fresnel zone is an ellipsoid-shaped, three-dimensional volume surrounding the line of sight between two antennas. Radio energy is carried not just along a thin line but through this volume.
Obstacles that enter this volume (the sea surface, a building, a hill) weaken the signal even when the line of sight is clear.
The 60% rule: At least 60% of the radius of the first Fresnel zone should remain clear. When this is met, the additional loss caused by obstruction is negligible. The further the sea surface enters the zone, the higher the loss, leading to packet loss and lower speeds.
Visible Horizon and Radio Horizon
The visible horizon is the point where a straight line from the antenna touches the sea surface; it is the furthest point the eye can see.
The radio horizon is a little further away. Because radio waves are bent slightly downwards in the atmosphere, they partly follow the Earth's curvature and reach about 15% further than the visible horizon.
The radio horizon is the limit used in Fresnel zone calculations. To calculate range, use the calculator on our 4G/5G signal range at sea page.
How Is the Fresnel Zone Calculated?
The first Fresnel zone is widest at the midpoint of the path. The radius there is given by:
The radius grows with distance and shrinks with frequency. At sea the Earth's curvature must also be taken into account: at the midpoint the sea surface rises about D² / 68 metres towards the line of sight (D in kilometres).
Fresnel Zone Calculator
How to read the result
- Fresnel radius: the radius of the first Fresnel zone at the midpoint.
- Earth curvature: how far the sea surface rises at the midpoint relative to the straight line joining the bases of the two antennas.
- Clearance ratio: at the most critical point on the path, the gap between the line of sight and the sea surface divided by the Fresnel radius. The target is 60% or more.
Heights are measured from sea level. The calculation assumes standard atmospheric refraction (k = 4/3) and a calm sea surface; wave height and the vessel's rolling reduce the clearance further.
Fresnel Zone Radius by Distance and Frequency
| Distance | 800 MHz | 1800 MHz | 2600 MHz | 3500 MHz |
|---|---|---|---|---|
| 5 km | 22 m60%: 13 m | 14 m60%: 9 m | 12 m60%: 7 m | 10 m60%: 6 m |
| 10 km | 31 m60%: 18 m | 20 m60%: 12 m | 17 m60%: 10 m | 15 m60%: 9 m |
| 20 km | 43 m60%: 26 m | 29 m60%: 17 m | 24 m60%: 14 m | 21 m60%: 12 m |
| 30 km | 53 m60%: 32 m | 35 m60%: 21 m | 29 m60%: 18 m | 25 m60%: 15 m |
Note: Over long distances at sea the 60% condition often cannot be met; the zone reaches a radius of tens of metres while the boat antenna is only a few metres high. This does not mean a link cannot be established, but extra loss and fluctuation should be expected. Raising the antenna is the most effective way to reduce this loss.
Fresnel Zone vs Multipath
The two are linked: the further the sea surface enters the zone, the greater the effect of the reflected signal.
| Condition | Fresnel zone | Effect on the link |
|---|---|---|
| Ideal | The zone is high enough above the sea surface, at least 60% clear. | Minimal multipath, stable link. |
| Critical | As the vessel moves away, the lower part of the zone touches the sea. | Multipath begins and the signal fluctuates. |
| Poor | The zone clearly enters the water. | Internet speed drops or the link is lost. |
Other Factors Affecting Range at Sea
Poynting XPOL Advantages at Sea
In Poynting XPOL (cross-polarised) antennas, two antenna elements are placed at 90° to each other.
The polarisation and phase of a signal reflected from the sea surface change. In a cross-polarised design, when one element receives the signal weakly, the other receives it more strongly. The link is maintained during fading and the two MIMO channels stay separated.
XPOL does not remove the loss caused by the Fresnel zone entering the sea; it makes the link more resilient to the fluctuation this loss causes.
In summary
- Antennas are mounted high on the mast to lift the Fresnel zone away from the sea surface and reduce multipath.
- At lower frequency bands the Fresnel zone is wider; at the same height it comes closer to the sea surface.
- As distance increases, the zone grows and the Earth's curvature reduces the clearance.
- The link should be judged by RSRP, RSRQ and SINR, not just by signal bars.
For manufacturer product information, see the Poynting marine and coastal antennas page.
Fresnel Zone FAQ
Why does the Fresnel zone matter when the line of sight is clear?
Radio energy is carried not along a single line but through a volume surrounding the line of sight. If part of this volume is cut by the sea surface, the signal weakens even though the antennas can see each other. The Fresnel zone defines the size of this volume.
Can the 60% rule always be met at sea?
No. It can be met over short distances and with high antenna positions; as distance increases, the zone grows and the Earth's curvature gets in the way, making it harder to meet. When the condition is not met the link does not drop, but extra loss and fluctuation occur.
How does the Fresnel zone change with frequency?
The Fresnel zone narrows as frequency increases. At 20 km, the midpoint radius is about 43 m at 800 MHz and about 21 m at 3500 MHz. Higher frequencies need less clearance in this respect, but path loss and cable loss are higher.
How does raising the antenna affect the Fresnel zone?
The size of the zone does not change, but the line of sight moves away from the sea surface and a larger part of the zone stays clear. This is why marine antennas are mounted at the highest possible point, usually at the top of the mast.
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