Plain-language answer: Fresnel zones are ellipsoidal regions around a radio path where obstruction changes phase relationships and can reinforce or cancel the received signal.
Why it changes the path
A path can have visual line of sight yet suffer loss if terrain, buildings, or foliage intrude into the first Fresnel zone.
What to observe
Plot both endpoints, frequency, terrain, and antenna heights; aim for practical first-zone clearance rather than visual clearance alone.
The common mistake
Treating an unobstructed straight sightline as a fully clear radio path.
Planning boundary
Propagation models and field regions depend on frequency, geometry, terrain, atmosphere, polarization, antenna pattern, and measurement distance. Use them to plan and compare—not to guarantee a contact or installed range.
