Line of Sight
Understanding Line of Sight
Line of sight is the fundamental requirement for most RF links above VHF frequencies. Unlike HF signals that reflect off the ionosphere, microwave signals travel in straight lines (with slight atmospheric refraction). The geometric constraint of LOS, combined with Earth curvature, determines the maximum link distance.
LOS Calculations
- Geometric horizon: d (km) = 3.57 x sqrt(h_m) for standard atmosphere (4/3 Earth radius model).
- Link distance: d_max = 3.57 x (sqrt(h1) + sqrt(h2)) km.
- Earth curvature: Earth surface drops 0.0785 m per km^2 of distance.
Fresnel Zone
The first Fresnel zone radius at the path midpoint: r1 = 17.3 x sqrt(d/(4f)) meters, where d is in km and f in GHz. For reliable propagation, 60% of the first Fresnel zone must be clear of obstacles.
d = 3.57 x sqrt(h) km (4/3 Earth)
Link distance (both towers):
d = 3.57 x (sqrt(h1) + sqrt(h2)) km
Example: 30m tower to 30m tower:
d = 3.57 x (5.48 + 5.48) = 39.1 km
First Fresnel zone radius at midpoint:
r1 = 17.3 x sqrt(d/(4f)) meters
For 10 km at 10 GHz: r1 = 2.7 m
Frequently Asked Questions
What is line of sight in RF?
LOS is the unobstructed path between transmitter and receiver. For reliable microwave links, the geometric line and 60% of the first Fresnel zone must be clear. LOS determines antenna height requirements and maximum link distance.
How far can you see with RF line of sight?
Distance depends on antenna heights. From 30m towers on opposite ends: about 39 km (using 4/3 Earth radius for atmospheric refraction). From 100m towers: about 71 km. Earth curvature is the primary limitation for long terrestrial links.
What is the Fresnel zone?
The Fresnel zone is the ellipsoidal region around the direct path where reflected/diffracted signals can interfere with the direct signal. Obstructions within the first Fresnel zone cause signal loss. At least 60% clearance is required for reliable propagation.