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Electronic Perimeter Security FAQs

To produce the best possible perimeter security systems requires a combination of a physical barrier, like the Metalith™ earth filled protective barriers, control point bollards, popup wedges and gates, plus electronic surveillance equipment. To assist companies or governmental purchasers understand a little about the various electronic security systems that are available, we have included the following Electronic Perimeter Security FAQs.

What are passive sensors?

Passive sensors detect energy submitted by the target, or detect a change in the environment associated with the presence of the target.


What are active sensors?

Active sensors transmit some form of energy and detect a change in the received energy created by a disturbance caused by a target.


What is a line-of-sight sensor system?

A line-of-sight system usually refers to a clear line-of-sight between the transmitter and receiver of an active sensor.


What are buried line sensors?

There are four types of buried-line sensor systems:

  1. Pressure or seismic - a sensor that consists of geophones made up of conducting coil and a magnet. Either the coil or magnet are fixed and let the other vibrate during a disturbance.
  2. Magnetic field - sensors that respond to changes in the local magnetic field caused by movement of some metallic material. They are ideal for detecting vehicles or intruders with weapons.
  3. Ported coaxial cable – also called leaky coax or radiating cable sensors. These sensors respond to a high dielectric constant or high conductivity near the cable that can be produced by humans and metal vehicles.
  4. Fiber-optic - Fiber-optic cables are made up of long, hair-like strands of transparent glass or plastic that pass light from one end. If there is a disturbance in the zone, this function is changed and causes an alarm.


What are fence disturbance sensors?

Fence disturbance sensors are installed on security fences and typically constructed with chain-link mesh. They use several kinds of transducers used to detect motion or shock caused by an intruder attempting to climb or cut through the fence.


How well do fence disturbance sensors work?

Fence disturbance sensors respond to all mechanical disturbances so noises such and wind and debris blown by wind, rain, and nearby traffic activity can cause nuisance alarms.


What are taut wire fence sensors?

Taut wire sensor fences consist of high tensile strength parallel, horizontal wires that connect to transducers near the midpoint of the wire span. Since these wires are under tension, they are able to detect climbing or cutting of the wires.


How well do taut wire fence sensors work?

Taut wire fence sensors are much less susceptible to nuisance alarms than fence disturbance sensors.


What are electric field or capacitance sensors?

Electric field or capacitance sensors can be set to detect intruders up to 1 meter beyond the actual wire of the fence.


How well do electric field or capacitance sensors work?

Electric field or capacitance sensors are susceptible to lightning, rain, fence motion, and small animals. Also, ice storms may cause damage to the wires.


Do freestanding video motion detection systems offer good perimeter security solutions?

Video motion detection uses existing closed-circuit television (CCTV) cameras to sense a change in the video signal level viewed on the scene. A computer that is set to recognize movement in a particular area, point, or grid on an image usually accomplishes motion detection. When the computer detects a change in the set image under the set parameters, an alarm is generated that indicates there was movement in that set zone.


How well does a freestanding video motion detection system work?

Video motion detection only works with fixed cameras, so pan/tilt/zoom cameras cannot be used since any motion of the camera and the change of the video image produces an alarm. The reliability of the detection can be lowered during conditions of reduced visibility that is caused by fog, snow, heavy rain, or loss of lighting at night.


What are microwave sensors?

Microwave sensor security systems use transmitters and receivers working in the 10 Gigahertz (GHz) or 24 GHz microwave range. If the microwave sensors do not receive a reflection back in a certain time period due to an intruder's movement, an alarm is activated


How well do perimeter security microwave sensor systems operate?

There are some inherent problems using microwave systems in large open areas. First, the ground must be flat so that objects do not shadow the beams. Second, the microwave antennas should not be placed greater than 120 yards between each other since a crawler may not be detected due to dead zones created in the first few meters in front of the antennas. Finally, there should not be any standing water in the area since this will create a moving reflective zone


What are passive infrared security systems?

Passive infrared (PIR) sensors detect electromagnetic radiated energy generated by sources that produce temperatures below that of visible light. The sensors focus on a narrow bandwidth measured in microns with the human body producing energy in the region of 7-14 microns. These sensors use the Rate of Change measurement to process and evaluate an unshielded/unprotected intruder walking through a designated zone. When the radiation change captured by the lens exceeds the set parameters, it triggers an alarm.


How well do passive infrared security systems work?

There are some defeat mechanisms that can be deployed against passive infrared systems. These include shadowing, cloaking, or masking an intruder's heat signature from the passive infrared field of view.


What are active infrared security systems?

Active infrared sensors use two units, a transmitter and a receiver. Unlike the microwave units, the active infrared sensors use a transmitter that generates a multiple frequency straight-line beam to the receiving unit that creates a sort of ‘fence' between these two units. If an intruder passing through this field of detection interrupts the signal, it will cause an alarm.


How well do active infrared security systems work?

First it is critical for the active infrared sensors to align the two infrared towers together to obtain a clear line of sight. Older models had problems sensing everything that moved including animals and vegetation blown by the wind. The newer active infrared sensors can be adjusted for animals and vegetation to pass through without being detected, but still creating an alarm when a human intruder or vehicle pass through the beam. Tunneling and bridging can cause problems for these systems.


Infrastructure Defense Technologies' earth filled protective barriers are designed to stop the intruders attempting to penetrate a facility's perimeter security by using a vehicle or explosives. Call us today at 1-800-379-1822, email us at, or send a fax using 815-323-1317 to get an immediate quote for your perimeter security problems. Stop them before they cause serious security issues, critical infrastructure damage, or disaster.

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The Metalith
A Division of Infrastructure Defense Technologies

6550 Revlon Drive • Belvidere, Illinois 61008
Phone: 1-800-379-1822 • Fax: 1-815-323-1317



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