What is the Fresnel zone, and why is it not enough even when line of sight is clear?
The Fresnel zone is the ellipse-shaped volume around the straight line between transmitter and receiver in which the signal energy is carried. Its radius at mid-path is found with r = 0.5·√(λ·D): on a 150 m link it comes out as 5.1 m at 433.92 MHz, 3.6 m at 868 MHz and 2.1 m at 2.45 GHz. The practical rule is that at least 60% of the first zone should be clear.
An RF wave does not travel along a thin line like a laser beam. The energy spreads inside an ellipsoid surrounding the straight line between the two points. Every conductive or dense object entering this volume disturbs the phase through reflection and diffraction; the result is that the signal weakens seriously even though the direct path itself is clear.
The general formula is r = √(λ·d1·d2 / D), and at mid-path it becomes r = 0.5·√(λ·D) (λ in metres, λ = 300 / f_MHz).
In the field this means: on a 150 m 433 MHz link you need a clear tube of about 5 m radius at mid-path. A steel beam 1 m above the antenna, the top of a container, a snow pile or a parked truck is inside that tube and takes tens of dB out of the link budget. That is why the criterion "we can see each other" is misleading.
The fix is simple and usually free: raise the antenna. Taking the receiver antenna 1–2 m above the enclosure gains far more than increasing the transmitter power. If you cannot gain height, look for a position that passes beside the obstacle.
Full topic: 433 MHz or 2.4 GHz?