Fresnel Zones & LoS
Why Clear Vision is Not Enough
The Ellipsoid of Energy
Named after physicist Augustin-Jean Fresnel, these 'zones' represent areas where the electromagnetic waves are in-phase. The First Fresnel Zone is where the majority of the energy is transmitted.
Fresnel Zone Clearance Simulator
Line of Sight vs. Electromagnetic Obstruction
Engineering Logic: The First Fresnel Zone contains 90% of the energy. Even if you have a perfect "Visual Line of Sight", any object entering the zone (above the 60% line) will cause multipath cancellation. The signal reflects off the obstruction and reaches the receiver out of phase, canceling out the original signal.
Line of Sight (LoS) vs. Fresnel Clearance
There are three types of Line of Sight:
- Visual LoS: You can see the other antenna.
- Near LoS: You can see it, but objects are close to the path.
- Radio LoS: The 1st Fresnel zone is 60-80% clear.
Curvature of the Earth
For long-distance links (over 10-15 km), the Earth's curvature itself becomes an obstruction in the Fresnel zone. To compensate, antennas must be mounted higher than a simple straight-line calculation would suggest.
Conclusion
Never trust your eyes when designing a wireless link. Trust the math. If your Fresnel zone isn't clear, your bridge will fail.