Why defend the fence line, not the door
An alarm that only reacts once someone reaches the door or a window reports too late for any kind of response — the intruder is already inside the perimeter. Perimeter protection moves that moment back to the fence line itself, while the person is still outside. That's the difference between a warning someone can still act on and a damage report that arrives the next morning.
How IR barriers work
An IR (infrared) barrier works as a pair: a transmitter sends a beam of infrared light, a receiver expects it on the other side. A break in the beam — a body passing through it — raises the alarm. The rated range for these pairs runs from 100 to 700 metres, depending on the model and site conditions, so barriers are chosen against the actual length of fence or line that needs covering.
What shortens the range compared to the spec sheet
The same factors that shorten a camera's IR night-vision range affect perimeter barriers too: fog, heavy rain and airborne dust weaken the infrared beam. Vegetation that grows in over a season can partly block the line between transmitter and receiver, and a poorly aligned barrier loses range before weather even becomes a factor. So the rated 100 to 700 metres should be treated as an upper limit under ideal conditions, not a guaranteed figure for every site.
The tradeoff: false alarms against gaps in coverage
Barrier pairs spaced closer together, with a shorter distance between transmitter and receiver, produce fewer false alarms from animals, leaves or branches, but need more units to cover the same length of fence — a higher cost. Pairs spaced further apart cover a long line more cheaply but leave larger gaps where a smaller object can pass unnoticed. The choice is always a tradeoff between the number of false alarms and the number of blind spots.
Recommendation
A perimeter rarely makes sense on its own — paired with camera verification (see the guide on alarms with or without verification), an IR barrier reports that someone crossed the line, and the camera covering that zone immediately shows whether the cause is an intruder or the fox that passes through every morning. The actual length and layout of the fence, and the realistic range on that specific terrain, get assessed during the site visit.