Wireless · 7 min read
Planning a Wireless Bridge: Line of Sight, Fresnel Zone and Link Budget
Plan a reliable point-to-point wireless bridge: line of sight, Fresnel zone clearance, mounting height, link budget, alignment and common mistakes, with Navijin examples.
A point-to-point (PTP) wireless bridge can replace kilometres of trenching in a single afternoon, provided it is planned properly. Most failed links come from the same few mistakes. This guide walks through the essentials.
1. Line of sight is not optional
At 5 GHz and above, radio behaves much like light. If you cannot see the far end, because a building, a hill or a tree is in the way, the link will be unstable or fail. Visit both ends, use binoculars or a drone, and remember that trees grow.
2. Clear the Fresnel zone
A radio link uses more than the straight line between antennas. It uses an ellipse-shaped area called the Fresnel zone. Keep at least 60% of the first Fresnel zone clear of obstacles.
The radius at the midpoint is approximately:
r (m) ≈ 8.66 × √(D / f)
where D is the distance in km and f is the frequency in GHz.
Example: a 3 km link at 5.8 GHz has r ≈ 8.66 × √(3/5.8) ≈ 6.2 m. At least 60% of that, about 3.7 m, must be clear around the line at mid-path. This often means raising antennas on masts.
3. Calculate the link budget
Received signal = transmit power + antenna gains − free-space path loss − other losses. Aim for a fade margin of 15–20 dB above the receiver sensitivity for your target data rate. Navijin engineers calculate this for every link during design.
4. Pick the right model
- NP-CPE512: 5.8 GHz, up to 4 km, up to 1000 Mbps air rate.
- NP-CPE510: 5.8 GHz, up to 3 km, 300 Mbps.
- NP-CPE41M: Wi-Fi 6 outdoor PTP/PTMP.
5. Mount and align carefully
- Use rigid mounts. Pole sway at 3 km means a lot of beam movement. The NB-OACPE-KIT provides J-mast mounting.
- Align one end at a time using the signal indicator, then lock it down.
- Ground the mast and use surge protection on outdoor cables.
6. Test and document
Run a throughput test in both directions, save the configuration and record signal levels. These baselines make troubleshooting easy later.
Common mistakes
- Assuming the air rate equals real throughput.
- Ignoring the Fresnel zone.
- Mounting on unstable poles.
- Choosing a crowded channel without a spectrum scan.
See the full Navijin wireless bridge solution.
FAQ
Frequently asked questions
What is the Fresnel zone?
It is the ellipse-shaped area around the direct line between two antennas that radio energy travels through. At least 60% of the first Fresnel zone should be clear of obstacles.
Do trees block 5.8 GHz links?
Yes. Foliage strongly attenuates 5 GHz signals, so links must clear trees, allowing for growth.
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