Fresnel Zone
Understanding Fresnel Zones
Fresnel zones explain why simply having geometric line of sight is not sufficient for reliable microwave links. Even with a clear direct path, obstructions near the path (ground, buildings, trees) can scatter energy into the Fresnel zone, creating destructive interference that degrades the signal.
Fresnel Zone Calculation
The first Fresnel zone radius at distance d1 from one end and d2 from the other (d1 + d2 = total path d): r1 = sqrt(lambda x d1 x d2 / d). At the midpoint (d1 = d2 = d/2): r1 = sqrt(lambda x d / 4).
Clearance Requirements
- 60% clearance: No significant additional path loss beyond free space.
- 40-60% clearance: 0-3 dB additional loss.
- 0% clearance (grazing): ~6 dB additional loss.
- Blocked: Knife-edge diffraction applies. Rapid increase in loss.
r1 = 17.3 sqrt(d_km / (4 f_GHz)) meters
Example: 10 km path at 10 GHz:
r1 = 17.3 sqrt(10/40) = 8.65 m
At 2.4 GHz same path:
r1 = 17.3 sqrt(10/9.6) = 17.7 m
60% clearance required: 0.6 x r1
Frequently Asked Questions
What is the Fresnel zone?
The Fresnel zone is the ellipsoidal volume around the direct path where wave interactions affect signal strength. At least 60% of the first Fresnel zone must be clear for reliable microwave links. Obstructions in this zone cause additional path loss.
How large is the Fresnel zone?
The radius depends on distance and frequency. At the midpoint of a 10 km path at 10 GHz, the first Fresnel zone radius is about 8.65 m. Lower frequencies and longer paths have larger zones, requiring taller towers for clearance.
Why does frequency affect the Fresnel zone?
Lower frequencies have longer wavelengths, creating larger Fresnel zones. This means lower-frequency links need more clearance (taller towers) than higher-frequency links over the same distance. However, higher frequencies are more sensitive to small obstructions.