Could this possibly be accomplished using extended antennas on readers?
There are several different methods by which RFID systems could be used to establish a perimeter around an area. The right one for your particular application depends on what you are trying to achieve.
One way would be to use an active RFID system in which a tag on an asset, or worn by an individual, would emit a signal at set intervals—say, every two seconds. Reader antennas deployed around the perimeter would pick up that signal and triangulate on the tag. You could then set a boundary and have the system’s back-end software alert you if any tag entered the boundary. Some prisons use this to alert guards if prisoners get too close to the prison walls or fences.
Another way would be to use overhead readers that emit energy in a narrow curtain, or mats that emit energy. If a tag passes over the mat or under the overhead reader, you could be alerted. This could become expensive if you had a large area to cover.
A novel way to create a perimeter was developed by inventor Kenneth Cecil, who patented a method of detecting intruders by monitoring when a signal from a tag is blocked by the human body. The system comprises ultrahigh-frequency (UHF) RFID transponders placed on a wall or fence about 2 feet apart at varying heights. UHF readers are positioned 30 feet apart, 10 to 20 feet from the tagged wall, and set to ping the tags 20 times per second.
The body of someone attempting to scale a fence or wall absorbs the RF energy between a tag and a reader, briefly blocking the signal and preventing the tag from responding to the reader with its unique ID number. If the system detects an interruption in the communication between the tag and reader, an alert would be sent to a security guard. The system could also be used underground to prevent anyone from tunneling under a wall or fence (see RFID Detects Intruders).
I hope I have answered your question.
—Mark Roberti, Founder and Editor, RFID Journal