The Fresnel Zone is the area where radio waves spread out into after they are transmitted from your ground control station and your drone. When we talk about radio line of sight, it is not like visual line of sight (dashed black line) where it is like a laser beam shot but an overall area (blue) that needs to be clear of obstruction to maintain optimal signal strength.
It’s important to calculate this so you have optimal signal strength. As obstructions get close to the center, your signal strength will decrease. 2.4 GHz is used as the primary frequency for many drones and those radio waves are absorbed by plants with water. So trees become an issue.
The higher the frequency, the smaller the Fresnel radius. This is why some drones that transmit on both 5.8 and 2.4 are advantageous for low to the ground operations (like powerlines) because even though the 2.4GHz has greater range than the 5.8GHz, 2.4 has a larger Fresnel radius than the 5.8 and could have a weaker signal due to obstruction than the 5.8Ghz. Go ahead and play around with the different frequencies and ranges.
Also, proper antennae orientation is important. Remember there is a dead zone right above the vertical antennae.
Disclaimer Notice: The final number represents an approximation and should only be used as a guide. No guarantees or warranties are implied. This calculator is “as is” and you should use it at your own risk.