In the first installment of our Microwave Sensor Installation Tips series (Getting Started: Winter 2007), we highlighted preparation of the detection zone and maintenance of the protected area as two key ingredients to trouble-free sensor operation. In this segment, we examine proper microwave sensor selection as a critical element toward maximizing unauthorized access.

Tom Gibbons of Southwest Microwave’s Technical Sales Group explains that the first step in designing a perimeter intrusion detection system is establishing the threat level of potential intruders and determining the corresponding level of security that is required.

“Understanding what the user is trying to protect and who they are protecting it from will facilitate responsible specification of equipment, configuration and zone length”, he says.

Gibbons defines a low security application as one where the user might be looking to address site vandalism or a potential liability issue, such as preventing trespassing teenagers from getting onto a site and getting injured.

“Here the threat level is low. The user would be looking to detect someone relatively harmless, without the knowledge or experience to try to defeat the security system – a target simply walking or running onto the site after scaling a fence or wall.”

For this type of application, Gibbons recommends Southwest Microwave’s Model 300B Multiple Range Microwave Intrusion Link, which will effectively detect a walking, running or hands-and-knees crawling human intruder. The 300B’s 100-foot (30m) short range, wide angle pattern is ideal for gate protection, while the 350-foot (107m) mid-range or 600-foot (183m) extended range patterns can be used for full perimeter protection.

For a medium security application, Gibbons offers the example of a commercial or industrial site where the user is looking to prevent property theft.

“This scenario has a higher threat level because assets are at stake”, he explains. “We would generally see a more experienced intruder with some degree of knowledge about detection technology. This individual would go to greater lengths to defeat the facility’s detection equipment, perhaps by attempting to crawl under the signal”.

To detect hands and knees crawling Gibbons again recommends the 300B, with strict attention paid to zone length and proper overlaps. He also suggests CCTV evaluation for responsible assessment of disturbances.

“When camera assessment is involved, maintaining zone length - the head to head detection area - at a maximum of 100 meters is optimal for clear visibility of the target."

Gibbons suggests that when considering sensor zone length, users not exceed 50% of the sensitivity (potentiometer) setting so that signal to noise ratio stays at a manageable level to keep nuisance alarms at a minimum.

For applications which may be prone to interference from air and seaport landing systems, air and marine craft radar or other microwave intrusion systems which operate at X-band (10.5 GHz) frequency, Gibbons specifies the Model 310B Long Range Outdoor Microwave Detection Link, which operates at K-band (24.125 GHz) frequency.

For high security applications, where a significant threat of catastrophic destruction or harm is present, the highest degree of protection is warranted.

“In certain sectors - such as the nuclear industry - special maximum-security sensor configurations are mandated," Gibbons advises. “Prevention of sophisticated terrorist activity which could result in widespread disaster has to be factored in to system design.”

These sites will apply significant CCTV assessment tools and a short-zone, stacked microwave sensor configuration. High security sites will often also couple these measures with perimeter fence or buried sensors for added protection.

“In a high security application, the 310B would be paired with the 300B in double, triple or even quad-stacked configuration," says Gibbons, who explains that each sensor is configured for a specific threat.

“The 310B would be on the bottom, and used to detect a prone crawling target. Because the K-band frequency is 2.5 times higher than the X-band, the multipath signal generated by an intruder is also 2.5 times higher, so detection of a slow-moving, belly crawling intruder is correspondingly better with this sensor."

Gibbons outlines that a 300B in the center position would be used to detect walking or running targets and the upper 300B would identify jumping or bridging intruders.

“Because no single link is being asked to detect multiple threats, the owner can reduce the overall sensitivity settings on each link, ensuring an extremely low nuisance alarm rate with an extremely high probability of detection.”

Gibbons assures that there is no danger of mutual interference between stacked sensors because all of Southwest Microwave’s links feature four, field-selectable modulation channels.

“The stacked sensor arrangement provides the ultimate in detection performance and an extremely high resistance to electronic jamming.”