Line of Sight
Line of sight is the condition where there is a direct, unobstructed optical path between transmitter and receiver. Catalogue range figures are given for this condition. However, optical sight is a necessary condition for RF, not a sufficient one: since the signal spreads not like a beam but like a tube with volume, the surroundings of the path must be clear as well.
RF energy is carried around the direct path, in the ellipsoidal volume called the first Fresnel zone. Ground, a roof edge, a container or a crane boom entering this volume attenuates the signal, even if the other side is visible to the eye.
- Optical sight present, RF sight absent: this is a common situation when looking between two points at ground level.
- Practical target: keeping at least 60% of the first Fresnel zone clear.
- Most effective intervention: raising the antennas. A few metres of height takes the ground obstruction out of the zone.
The range figures stated for AXI sets (about 100 m for the SF series, about 150 m for the ST series, 100 m for the HCT400/HCT402) are given with the qualification of open field and line of sight. When a load, a cabin or the machine body comes between the operator and the receiver these distances shorten; that is why placing the receiver antenna at the most open and highest point of the machine matters during installation.
Context where this term is used: Wireless remote control installation: pairing, relay, safety