Bistatic Microwave barrier
A practical application of Microwave technology can be found in the field of intrusion detection. The electromagnetic field generated by the microwave is in fact able to perceive the passage of objects above a certain size. On the basis of the previously set detection thresholds, it will be possible to get an alarm signal when the object of interest passes inside the protected area. Since the technology in this case is used for perimeter security, the object of interest is primarily a man. There are two types of use of the microwave for this purpose: bistatic or monostatic devices. In the first case we have a transmitting and a receiving unit, which together form a so-called "barrier": the invisible electromagnetic field is generated between them. In the second case, i.e. monostatic sensors, both the transmitting and the receiving units are housed inside the same casing. The microwave signal will be sent by the transmitter and then received by the rebound receiver once the moving object is encountered using the doppler effect.
Operating principle
Microwave sensors convey microwave signals (in the X band); these signals operate within a pre-set limit, so it is not harmful to human health. These sensors are capable of detecting intrusions in relation to the frequency shift of the Doppler radar.[1] If an object falls within the frequency range it will cause the sensor to generate an alarm signal, otherwise, if outside the tuned frequencies, it will be ignored. The sensor operates by radiating a controlled pattern of microwave energy in the area to protect.[2]
Bistatic Microwave Barrier
In the field of perimeter protection it is possible to identify the Bistatic Microwave Barrier[3] as one of the most relevant detection technologies, used to protect the perimeters of high-risk sites and critical infrastructures such as fuel storage, petrochemical facilities, military sites, civil and military airports, nuclear facilities and more; it also finds applications in lower risk sites, such as civilian and industrial applications. Microwave barriers are especially used in the outdoors, but are suitable for indoor protection too. 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.
Functioning
The name bistatic derives from the principle of operation: it is in fact composed of two parts, a transmitter, made up of an electromagnetic wave generator, and a receiver.
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.
For smaller sites the receiver can be a few meters distant from the transmitter, while in bigger ones it can reach a distance up to 500 meters.
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 will be able to 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.[4]
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 are able to 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 drastic 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 to cover the above mentioned dead zones.[5]
Microwave and health
Nowadays there has been a keen interest in the health effects on the human body when exposed to microwave fields. Authoritative sources, such as ICNIRP (International Commission on Non-Ionizing Radiation Protection), WHO (World Health Organisation), ILO (International Labour Organisation)[6] and the European Union have studied the effects of human exposure to radio frequencies and have released limits beyond which it might be dangerous to human health. [7]
Radio frequency radiation is a specific component of the electromagnetic spectrum. Specifically, it represents the non-ionising radiation, which includes lower frequencies (ranging from 100kHz to 300GHz) of the electromagnetic spectrum (i.e.; radio wave, microwave, ultraviolet, visible light and infrared).[8] [9]
References
- ↑ "Perimeter Security Sensor Technologies Handbook" (PDF). The National Institute of Justice (NIJ: 2.12–2.13. 1998 – via Defense Advance Research Project Agency (DARPA).
- ↑ "Electronic Security Systems" (PDF). Department of Defence USA: 64–67. 2013 – via Unified Facilties Criteria (UFC).
- ↑ De Astis, Vincenzo; Gasparini, Bruno. Security technology handbook. Milano: Assosicurezza. pp. 100–102. Search this book on
- ↑ "Guide to Perimeter Intrusion Detection Systems (PIDS)". Centre for the Protection of National Infrastructure (CPNI). No. 05/12: 24–38. 2012.
- ↑ "Guide to Perimeter Intrusion Detection Systems (PIDS)". CPNI - Centre for the Protection of National Infrastructure. Publication No. 05/12: 24–38.
- ↑ "Safety in the use of radiofrequency dielectric heaters and sealers". www.ilo.org. 1998-01-01. Retrieved 2022-01-12.
- ↑ "Microwave & Health". CIAS Elettronica. Retrieved 2022-01-12.
- ↑ "Microwave and Radio Frequency Radiation". Unknown parameter
|url-status=ignored (help) - ↑ "Safety in the use of radiofrequency dielectric heaters and sealers". Unknown parameter
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