Microwave Barriers and Infrared Beams
Microwave barriers and active infrared beams are line-of-sight detection technologies that create an invisible detection curtain between a transmitter and a receiver. Anything passing through the curtain — a person, vehicle, or large animal — interrupts or distorts the signal and triggers an alarm.
These technologies protect fence gaps, gate openings, rooftops, corridors, and open ground where fence-mounted sensors are not applicable. They are among the most mature PIDS technologies, with field-proven reliability spanning decades.
Microwave Barriers
Operating Principle
A microwave barrier consists of a transmitter and a receiver mounted on opposite sides of the area to be protected. The transmitter emits a continuous microwave signal (typically at 10.525 GHz or 24.125 GHz). The receiver monitors the signal amplitude and phase. When a person or object enters the detection zone, it absorbs, reflects, and scatters the microwave energy, causing measurable changes in the received signal.
The detection zone is not a thin line but a three-dimensional volume — an elongated ellipsoid between the two units. The width and height of this ellipsoid depend on the operating frequency, antenna design, and distance between units. Typical zone widths range from 1–6 metres.
Bistatic vs. Monostatic
Bistatic systems use separate transmitter and receiver units facing each other. This is the most common configuration for perimeter applications, providing reliable detection over distances from 30 to 500 metres between units.
Monostatic systems combine the transmitter and receiver in one unit, detecting reflected energy from objects entering the monitored zone. These are less common in perimeter applications but are used for short-range protection of walls, gates, and confined areas.
Range and Stacking
Individual microwave links typically cover 60–200 metres, depending on the model and required detection sensitivity. For longer perimeters, multiple links are installed in series, each covering a segment.
Stacking — mounting multiple microwave links vertically at different heights — creates a taller detection curtain. A single link mounted at 1 metre height can be defeated by crawling. Stacking two or three links at different heights (0.5m, 1.0m, 1.5m) creates a wall of detection that is much harder to bypass.
Key Vendors — Microwave
Southwest Microwave — INTREPID Series, the most widely deployed microwave barrier brand globally. Models from 60m to 500m range. US-based. Known for reliability in government, military, and critical infrastructure.
CIAS Elettronica — MARTE series microwave barriers with advanced signal processing. Models cover 30m to 200m. Italian, strong in European and Middle Eastern markets.
SORHEA — G-FENCE and MAXIRIS microwave barriers, plus combination microwave/IR systems. French, specialising in layered perimeter detection.
Senstar — Produces microwave barrier systems as part of their broad PIDS portfolio. Canadian.
Active Infrared Beams
Operating Principle
Active infrared (AIR) beam systems project one or more invisible infrared beams from a transmitter column to a receiver column. When a person or object interrupts one or more beams, the system detects the loss of signal and generates an alarm.
Unlike passive infrared (PIR) motion detectors used in indoor security, active infrared systems use dedicated light sources and aligned receivers, providing precise detection at ranges up to 200 metres regardless of ambient temperature.
Single-Beam vs. Multi-Beam
Single-beam systems create a single detection plane at the height of the beam. They are inexpensive and simple but are easily defeated by stepping over or crawling under the beam.
Multi-beam systems use columns with 2–4 beams at different heights, requiring an intruder to break multiple beams to trigger an alarm. Advanced systems use beam logic — only triggering if beams are interrupted in a sequence consistent with a person (bottom beams first, then upper beams) rather than an animal or falling object.
Curtain beam systems use dozens of beams closely spaced vertically, creating a virtually continuous detection wall. OPTEX's Redscan series uses a laser curtain approach to detect any object passing through the sensing plane, regardless of height or speed.
Environmental Performance
Active IR beams are affected by dense fog, heavy snow, and any condition that attenuates or scatters the infrared signal. Manufacturers compensate by using higher-power transmitters, modulated signals (to reject ambient infrared noise), and automatic gain control (AGC) to adapt to changing atmospheric conditions.
Alignment is critical. Over long ranges, thermal expansion of mounting poles, ground settlement, and wind can shift beam alignment. Self-aligning or wide-aperture designs mitigate this, but periodic alignment checks remain part of maintenance protocols.
Key Vendors — Infrared
OPTEX — The dominant vendor in active infrared perimeter detection. Redscan laser curtain detectors, AX series multi-beam barriers. Japanese, global distribution. Product range from short-range gate protection to 200m perimeter barriers.
Takex — Multi-beam barriers and photoelectric sensors. Japanese, established in Asian and Middle Eastern markets.
SORHEA — SOLARIS series combining infrared beams with microwave detection in a single system. French.
Choosing Between Microwave and Infrared
Both technologies solve similar problems — detecting intrusion across open space — but with different characteristics.
Choose microwave when: the detection zone exceeds 100 metres, the environment is subject to fog or precipitation, covert detection is required (microwave barriers are less visible than IR columns), or the site requires volumetric detection (the elliptical detection zone catches lateral as well as perpendicular movement).
Choose infrared when: the installation requires precise, narrow detection zones (doorways, corridors, fence gaps), when cost per detection point must be minimised, when multi-beam logic is needed to reduce animal-related false alarms, or when the environment is dry and clear.
Choose a combination when regulatory or operational requirements mandate layered detection. Some standards (particularly CPNI requirements for UK critical infrastructure) require two independent detection technologies to achieve the highest performance grades. A microwave barrier paired with an infrared system on the same perimeter segment provides independent, dissimilar detection — reducing common-cause failure modes.
Installation Considerations
Line of sight is mandatory for both technologies. Any obstruction — vegetation, accumulated snow, debris, parked vehicles — between the transmitter and receiver will degrade or block detection. Site design must include vegetation management plans and clear zones.
Mounting stability directly affects reliability. Microwave and IR systems require stable mounting platforms. Tall masts that sway in wind will cause intermittent beam misalignment and nuisance alarms. Concrete-filled bollards or rigid wall mounts are preferred.
Power and communications must reach each unit. Microwave transmitters and receivers require power at both ends. Some IR systems offer battery or solar-powered options for remote locations.
Terrain matters. The ground between transmitter and receiver should be relatively flat and free of undulations that could create dead zones below the detection envelope. On undulating terrain, the detection curtain follows a straight line between units, leaving low ground unprotected.