Editing Security alarm (section)

===Microwave detectors===
This device emits microwaves from a transmitter and detects any reflected microwaves or reduction in beam intensity using a receiver. The transmitter and receiver are usually combined inside a single housing (monostatic) for indoor applications and separate housings (bistatic) for the protection of outdoor perimeters high-risk sites and critical infrastructures such as [[fuel]] storage, [[petrochemical]] facilities, [[military]] sites, civil and military [[airport]]s, [[Nuclear power|nuclear]] facilities and more. To reduce false alarms this type of detector is usually combined with a passive infrared detector or similar alarm. Compared to the monostatic, the bistatic units work over longer distances: typical distances for transmitter-receivers up to 200{{nbsp}}m for [[X band|X-band]] frequencies and up to 500{{nbsp}}m for [[K band (IEEE)|K-band]] frequencies.<ref>{{Cite journal |last=The National Institute of Justice |date=1998 |title=Perimeter Security Sensor Technologies Handbook |url=https://www.ojp.gov/pdffiles1/Digitization/206415NCJRS.pdf |journal=Defense Advance Research Project Agency |pages=NIJ: 2.12–2.13}}</ref>
[[File:Microwavebeam.jpg|thumb|266x266px|[[Microwave]] detection system used to protect a solar farm]]
Microwave detectors respond to a [[Doppler effect|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. Microwave detectors are low cost, easy to install, have an invisible perimeter barrier. and is not affected by fog, rain, snow, sand storms, or wind. 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 (radio)|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 [[Dust storm|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&nbsp;°C to +70&nbsp;°C.<ref>{{Cite journal |date=2012 |title=Guide to Perimeter Intrusion Detection Systems (PIDS) |url=https://www.cpni.gov.uk/resources/perimeter-intrusion-detection-systems-guidance-document |journal=Centre for the Protection of National Infrastructure |number=5/12 |pages=24–38}}</ref>

==== Digital analysis of the signal ====
The more recent and higher performance models of these detectors generate a detection whether the intruder is rolling, crossing, crawling or moving very slow within the [[electromagnetic field]]<ref>{{Cite book |last1=De Astis |first1=Vincenzo |title=Security technology handbook |last2=Gasparini |first2=Bruno |publisher=Assosicurezza |year=2002 |location=Milano |pages=100–102 |language=en}}</ref> reducing false alarms. The [[ellipsoid]]al shape of the longitudinal section however does not allow a good detection capability close to the receiver or transmitter heads, and those areas are commonly referred to 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 to cover the dead zones.<ref>{{Cite journal |date=2012 |title=Guide to Perimeter Intrusion Detection Systems (PIDS) |url=https://www.cpni.gov.uk/resources/perimeter-intrusion-detection-systems-guidance-document |journal=CPNI - Centre for the Protection of National Infrastructure |number=5/12 |pages=24–38}}</ref>