Microwave Detector

Microwave detectors use the Doppler Effect and work in a similar way to the ultrasonic detector. The difference is that microwave detectors employ radio waves that oscillate at extremely high frequencies. The standard for microwave detectors is 10.7 GHz, but 1.48 GHz is also employed.

The microwave detector's receiver and transmitter are in the same unit, and the receiver receives some direct radiation from the transmitter along with that reflected from objects within the protected area. If the reflected frequency is shifted by the Doppler Effect, a beat frequency is produced and the result activates the alarm circuits.

A feature of microwaves is that they will penetrate wood, glass, plaster, and to a limited extent, brick. Microwave detectors also have a much greater range than ultrasonic sensors, and will cover up to 150 feet. Microwave detectors are thus well suited for the protection of large areas, which is why microwave detectors are often used in warehouses, not only because of the size, but also because large stacks of cases and goods could afford shelter to an intruder from other types of volumetric detection.

As microwaves pass through these, there is no shelter from them. Propagation patterns are usually tear-shaped in the horizontal field, with a narrow lobe in the vertical plane. Some microwave detectors are designed with long narrow horizontal lobes to cover similar-shaped areas, or have split beams to give V-shaped coverage. Deflector plates can modify the pattern.

Steel cabinets, shelving or machinery could provide cover, however, because metals reflect microwaves. A solution to this problem is to mount the microwave detector in the center of the ceiling. Some microwave detectors are designed for this purpose and have a hemispherical distribution pattern. The feature of these is they can see over metal obstructions, so there is little cover for any intruder.

Microwave detectors are not as subject to false alarms as ultrasonic detectors. Draughts, air disturbances or turbulence have no effect, nor does any type of sound wave. Fluorescent and neon lights can generate radio signals that could be accepted by the microwave detector receiver, but there are usually filters that remove these electronically.

The main possibility of false alarms with microwave detectors comes from the penetration of microwaves beyond the perimeter walls or, in the case of ceiling-mounted microwave detectors, penetration of the floor to the room beneath. To reduce this possibility, the microwave detector's sensitivity must be adjusted so that the range extends only to the perimeter boundary. Even then it is possible for some overspill, especially through windows or doors. The effect can be avoided by curtailing the sensitivity to range somewhere just short of the boundary, although this would reduce the level of security.