Installing a GNSSDO timing system. Any tips for antenna placement to avoid multipath?

Great question. For a GNSSDO, your antenna placement is everything—garbage in, garbage out. Multipath (signals bouncing off nearby surfaces) can corrupt your timing solution and degrade your 1PPS pulse. Here's a comprehensive breakdown:

1. Sky Visibility: The Foundation

Your antenna needs an unobstructed view of the sky from roughly 5° above the horizon to the zenith. Use a smartphone app (like "GNSS View") or a physical survey to check for obstructions. Ideally, install at the highest point of your roof. Avoid locations near large HVAC units, chimneys, or satellite dishes that can create signal shadows or reflections. A clear view of the horizon in all directions is your primary goal.

2. Multipath Mitigation: The Main Event

This is where you win or lose. Multipath is the enemy of precise timing.

• Reflective Surfaces: The biggest culprits are metal roofs, gutters, vent pipes, and even wet, flat roofs. Your antenna should be at least 1-2 meters away from any large flat metallic object. If using a metal roof, consider a non-metallic mounting mast to raise the antenna significantly above it.
• Ground Planes: A properly sized ground plane is non-negotiable for a timing antenna. It helps create a consistent radiation pattern and rejects low-angle signals that are most prone to multipath. A 4-6 inch diameter ground plane is common. Never install a timing antenna without one.
• Natural Absorbers: Position the antenna near the edge of the roof, over areas that are not highly reflective (like gravel or shingle areas vs. metal flashing). Grass and soil are excellent absorbers.

3. Cable Length & Quality: Preserving the Signal

GNSS signals are incredibly weak by the time they reach your antenna. Cable loss is your silent killer.

• Use Low-Loss, Double-Shielded Coax: For runs over 10 meters, use LMR-400 or better. Standard RG-58 is only for very short runs (<5m) and is not recommended for timing systems.
• Shortest Possible Run: Plan your cable route to be as direct as possible. Every extra meter adds loss. Measure twice, install once.
• Connector Quality: Use high-quality connectors (like Type-N for LMR-400) and install them correctly. Poorly crimped or loose connectors introduce loss and potential ingress points for noise.

4. Lightning Protection: The Non-Negotiable

A rooftop antenna is a lightning rod. Protecting your equipment and building is critical.

• Ground the Mast: The antenna mast must be bonded to the building's grounding electrode system using a heavy-gauge ground wire (e.g., 6 AWG).
• In-Line Arrestor: Install a coaxial lightning arrestor (like a PolyPhaser) at the point where the cable enters the building. This must also be bonded to the same ground system.
• Surge Protection: Use a surge protector at the GNSS receiver's input as a final layer of defense.

For a GNSSDO, a clean, stable signal is worth every minute of careful planning. Get the antenna placement right, and the rest of your system will thank you with picosecond-level precision.