Why does a lower frequency reach behind obstacles better?
The diffraction effect scales with wavelength. The wavelength of 433 MHz is about 69 cm, that of 868 MHz is 34.6 cm, and that of 2.45 GHz only 12.2 cm. As the thickness of a building corner, beam or wall grows relative to the wavelength, the shadow behind the obstacle deepens; this is why a high frequency loses far more behind an obstacle than a low frequency.
A wave can bend around an obstacle whose size is comparable to its own wavelength. If the obstacle is much larger than the wavelength, the bending weakens and a sharp shadow forms behind it.
Concretely: a 40 cm thick concrete beam is below the wavelength for 433 MHz (λ ≈ 69 cm) and the wave largely goes around it. The same beam is three times the wavelength for 2.45 GHz (λ ≈ 12.2 cm); here the wave cannot go around it and you are left in the shadow.
This effect comes on top of the 15 dB difference in free space path loss. So while the difference between 2.4 GHz and sub-GHz in an open area is 15 dB, inside a factory, behind columns and machine bodies, the difference can exceed 30 dB.
Practical conclusions:
- For a link that will work in an enclosed space, behind obstacles, sub-GHz is generally safer.
- If 2.4 GHz is to be used, keeping the line of sight as clear as possible and raising the antenna is essential.
- Where diffraction cannot solve it, the solution is not a change of frequency but a change of geometry: move the antenna above or beside the obstacle.
Full article: 433 MHz or 2.4 GHz?