detector is usually combined with a passive infrared detector, or Dual Tec brand or similar alarm. Compared to the monostatic, the bistatic units work over longer distances: typical distances for transmitter-receivers up to 200 m for X-band frequencies and up to 500 m for K-band frequencies. Microwave detectors respond to a Doppler shift in the frequency of the reflected energy, by a phase shift, or by a sudden reduction of the level of received energy. Any of these effects may indicate motion of an intruder. Advantages: low cost, easy to install, invisible perimeter barrier. It is not affected by fog, rain, snow, sand storms, or wind. Disadvantages: May be affected by the presence of water dripping on the ground. Typically need a sterile clearance area to prevent partial blocking of the detection field. Functioning The microwave generator is equipped with an antenna that allows it to concentrate the beam of electromagnetic waves in one preferred location and the beam is intercepted by the receiver, equipped with a similar antenna to the transmitter. The graphical representation of the beam is similar to a cigar, and, when not disturbed, it runs between the transmitter and the receiver and generates a continuous signal. When an individual tries to cross this beam, it produces a disturbance that is caught by the receiver as a variation of amplitude of the received signal. These barriers are immune to harsh weather, such as fog, heavy rain, snow and sandstorms: none of these atmospheric phenomena affect in any way the behaviour and the reliability of the microwave detection. Furthermore, the working temperature range of this technology goes from -35°C to +70°C. Digital analysis of the signal The more recent and higher performance models of these detectors adopt a digital technology with "Fuzzy Logic" analysis of the signals. Those who use this generate a detection whether the intruder is rolling, crossing, crawling or moving very slow within the electromagnetic field. Thanks to this innovative technology, capable of targeting movements, the outcome is a reduction in false alarms. The ellipsoidal shape of the longitudinal section however does not allow a good detection capability close to the receiver or transmitter heads, so we identify these areas as dead zones. A solution to avoid this problem, when installing 2 or more barriers, is to cross the respective transmitter and receiver heads some meters from the respective heads or to use mono-head sensor