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Security alarm
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===Ported coaxial cable=== [[File:Ported coaxial cable installation.png|thumb|right]] This system employs an electro-magnetic field disturbance principle based on two unshielded coaxial cables.{{sfn|Chen|Young|1984|p=<!-- 1313{{ndash}}1317-->}} The transmitter emits continuous radio frequency (RF) energy along one cable and the energy is received by the other cable. When the change in field strength weakens due to the presence of an object and reaches a pre-set lower threshold, an alarm condition is generated. The system is covert after installation. The surrounding soil must offer good drainage in order to avoid nuisance alarms. Ported coaxial cables are concealed as a buried form but can be affected by RF noise and is difficult to install.
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