Troubleshooting Ubiquiti Lite AP Link: Lessons on Line of Sight and the Fresnel Zone

At Direct Media Pvt. Ltd., we regularly test and deploy wireless solutions for reliable connectivity. Recently, we worked on a case involving Ubiquiti Lite AP 120 antennas, where connecting two sites just 350 meters apart turned out to be more challenging than expected. This case study highlights the troubleshooting process, the unexpected findings, and what we learned about the importance of the Fresnel zone in wireless communication.

The Challenge

We attempted to link two Lite AP 120 antennas between our office and home, separated by roughly 350 meters. On paper, this should have been a simple setup, especially with clear line of sight available. However, the antennas consistently failed to establish a stable connection between the two locations.

Troubleshooting Steps

1. Initial Attempt on the Tower

• One antenna was installed on the office building tower, and the other on the home rooftop.
• Despite apparent line of sight, no connection was established.

2. Short-Distance Testing

• To rule out faulty hardware, both Lite APs were tested just 1 meter apart.
• At close range, the devices connected immediately.
• Further testing at distances up to 14 meters also succeeded.

3. Balcony-to-Rooftop Setup

• One antenna was placed on the office balcony and the other on the home rooftop.
• Again, no link was established despite visible line of sight.

4. Home Experiment

• As a final check, one Lite AP was placed outside on the rooftop while the other was kept inside a room.
• The only visible path was through the doorway.
• The antennas failed to connect, suggesting that obstructions, even partial ones, were disrupting communication.

Key Insight: The Fresnel Zone

This experiment pointed us toward the **Fresnel zone**, an invisible elliptical area around the line of sight path between two antennas. Even if the antennas can "see" each other, any obstruction within the Fresnel zone — such as walls, balconies, or even parts of a building structure — can cause signal loss and prevent connectivity.

This case illustrated that clear line of sight is not enough; ensuring a clear Fresnel zone is equally important for stable wireless links.

Comparison with Other Devices

For context, here are the coverage distances of common devices we use:

TP-Link WL841N: \~45m indoors, \~90m outdoors (with no obstructions)

TP-Link EAP225: up to 300m outdoors (with clear conditions)

TP-Link CPE510: point-to-point links up to 15km

Conclusion

The case of the Lite AP antennas showed us that wireless planning must go beyond just measuring distance. Obstructions in the Fresnel zone can silently disrupt even short links. For WISPs and anyone deploying point-to-point wireless solutions, ensuring both line of sight and Fresnel zone clearance is critical for reliable performance.