These sensors are among others that relay signals to alarms when they detect motion. Microwave motion sensors emit microwave energy to detect movement.
Principle of Microwave Motion Detectors
The system emits microwave energy that completely fills its area of detection, much like ultrasonic sensors. A Gunn diode emits energy in the x-range, similar in effect to Doppler frequency shifts. Any disruption caused by movement within the field of detection may activate the alarm system. Despite fears held by many, these sensors do not adversely affect humans and pacemakers.
Motion Sensor Design
These sensor systems may be monostatic, in which the emitter and receiver are contained in one unit or bistatic, in which the emitter and receiver are housed in separate units. Each type features advantages and disadvantages to the user. Monostatic units define a detection area based on the microwave beam that it emits. The detection range may reach up to 400 linear feet, however the user may configure the beam to fit particular situations, with a long, narrow beam or a short, ovular one.
Conversely, a bistatic sensor offers a larger range of detection, with up to 1,500 linear feet, however it does not enable the user to define the area of detection as precisely as the monostatic sensor. Bistatic sensors are also more prone to false alarms. Microwave sensors emit alternating frequencies, activating and deactivating their frequencies in rapid succession, followed by deactivation of the receiver.
The unit utilizes these time intervals to detect motion, noting the location of objects at different moments. The time intervals define the period during which the receiver detects motion, known as the Receiver Cut-Off region. Monostatic sensors. also offer the possibility of programming that detection only occurs within specified time periods, allowing necessary, expected movement to occur within the time frame.
Drawbacks of Using a Microwave Motion Detector
The special properties of microwave beams allow them to penetrate most types of surfaces, however metal is not among them. Therefore, the sensor may detect motion where detection is not desirable and not detect motion where it is desirable. For example, large metal objects in the area of detection shield the area behind them. Objects such as these thus create "dead" zones, areas where the sensor cannot detect motion.
On the other hand, because the beam can penetrate walls, the sensor will detect motion behind walls, even if such motion is normal. As the sensors are extremely sensitive to motion, they are also prone to other false alarms. Objects blowing in wind may trigger alarms. Even fluorescent lighting, which emits detectable light particles, may trigger a false alarm.
Why Burglars Can't Fool Microwave Motion Sensors
Some would-be intruders attempt to utilize the pattern of activation and deactivation, deliberately moving slowly to deceive the sensors and also use the location of the dead zones to evade detection. Because of these drawbacks many experts recommend pairing these sensors with other types. Many pair microwave sensors with passive infrared radiation (PIR) sensors as the PIR sensors are more sensitive to lateral motion, while microwave sensors are more sensitive to forward motion.
These two types of sensors compliment each other with one more sensitive in the area where the other is less sensitive. Therefore, many people program their alarm systems to activate only in the event of both sensors detecting motion. However, you should remember that the two different types of sensors offer different detection ranges and that if the alarm is dependent on both sensors alarm, activation is less probable.
These sensors offer a reliable method of detecting motion. Their detection ability is complete throughout their area of detection. However, the user should understand the drawbacks including false alarms and dead zones. Many experts recommend pairing microwave sensors with PIR sensors, but the user should remember that programming an alarm to activate depending on both sensors reduces the probability that an alarm will sound.