How best to optimize the line-of-sight donor signal in over-the-air DAS networks

Suketu Shah, Hubbell Senior Product Manager

April 2026

The over-the-air (OTA) signal source is a common method in public safety ERCES or DAS system. The optimal line-ofsight is needed between the donor antenna and public safety tower for successful OTA ERCES or DAS deployments, as it directly impacts signal strength and drives overall system performance. Consider following good practices to optimize the line-of-sight.

1. Conduct a Donor signal Survey

• Measure donor signal strength on the roof: A comprehensive site survey should include a donor signal survey by walking the building perimeter on the roof with a signal meter to measure and record signal readings (including all channels and frequencies) from different sides.
• Identify the donor signal source and direction: Once the location with the strongest signal is identified, slowly rotate 360 degrees with the signal meter to pinpoint the source and direction of the nearest cell tower, which is critical for orienting the donor antenna. Also, assess nearby radio networks for potential interference as it may affect donor antenna type and placement.

2. Consider the surrounding terrain and topography and understand Fresnel zones

• Clear Line-of-Sight is Ideal: A direct, unobstructed path between the donor antenna and the cell tower optimizes signal reception. Any obstructions such as buildings, trees, and terrain can significantly degrade signal strength due to interference, reflection, and refraction.
• Fresnel zones are critical: Maintain at least 60% of the first Fresnel zone free of obstructions to minimize signal fading. The Fresnel zone is an elliptical area surrounding the line-of-sight between tower and donor antennas, and obstructions within this zone cause signal loss.

3. Choose the right donor antenna type and placement

• Directional antennas are the most common type of antenna used in OTA DAS scenarios, especially in Public Safety ERCES solutions. They typically offer narrow beamwidth and higher gain, essentially reducing the impact of unwanted signals and allowing focused signal reception from distant towers.
• Placing the donor antenna on the side of the building is preferable avoiding any reflective (metallic) body such as HVAC units or elevator penthouse. Elevating to a safer height also helps overcome obstructions and improve line-ofsight, extending the effective range.

4. Account for local regulations

• AHJ requirements: Ensure compliance with local Authority Having Jurisdiction (AHJ) regulations, particularly regarding maximum donor tower input levels to avoid overdriving the system. In addition, many jurisdictions (e.g. PSERN in Seattle area, RWC in the Phoenix metro) are mandating use of high gain panel antenna front-to-back ratio of 26dB or better.

These are some of the good practices to keep in mind for optimal line-of-sight. Note that all scenarios are different and the help of RF professionals may be required to navigate challenging ones for a robust and reliable OTA DAS network.