Diffraction
Diffraction is the bending of a wave around the edge of an obstacle so that it leaks into the shadow region. The strength of the effect scales with wavelength: the roughly 69 cm wavelength of 433 MHz bends noticeably around the edge of a column or wall, while the 12.2 cm of 2.45 GHz leaves a much deeper shadow in the same geometry.
Diffraction explains why a link can still be established where there is no clear line of sight. The region behind an obstacle is not completely dead; energy bent around the edge reaches it, attenuated.
- Frequency effect: lower frequencies diffract better. Much of the advantage sub-GHz systems show in obstructed environments comes from this.
- Geometry effect: the closer you are to the edge of the obstacle, the shallower the shadow. There is signal immediately behind the corner, and none ten metres further in.
- Limit: diffraction cannot get inside a metal box. For a closed conductive volume the relevant mechanism is shielding, not diffraction.
In the field this shows up as follows: a remote control works immediately behind a wall but may not work a few steps further back. This is not instability, it is the diffraction shadow deepening. The fix is to move the receiver antenna not behind the obstacle but to a point that faces the shared field of view.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety