Perimeter Security using a RF coupled Leaky Coax System

Simon Wong
Electrical and Computer Engineering, Duke University
scw7@duke.edu
Advisors: Dr. Gary Ybarra

April 21 2003

 

Abstract:

For over 30 years the leaky coaxial cable (LCX) has been used as a viable mean of perimeter detection, in particular in military, nuclear and airport installations. Current market designs involve the lining of the perimeter with two parallel LCXs buried 2 inches below ground for detection. I have designed a system to illustrate the possibility of using a series of leaky coaxial cables to detect both intrusions and extrusions of a specified area. Three cables of 11 feet each are placed parallel to each other. An invisible EM field is created by sending a 150 MHz sine wave signal in the central transmitting LCX cable while the two receiving LCX cables, placed 2 meters on either side, couple the field simultaneously. This signal is then measured by a computer for processing and analysis. Detection occurs when the intruder (extruder) disturbs the field and the coupled signal deviates from the stable reference signal.

 

The data acquisition and analysis of the coupled signal is processed by LABVIEW, which is well suited for real time analysis. National Instrument equipment BNC 2110 DAQ board with 12bit A/D conversion has the ability to capture the signal with the necessary rate and precision. I present here various situations of detection, including the time responses of the system, intrusion and extrusion responses, speed of object and limitations. The LABVIEW VI program as well as the user interface design will also be shown.

 

I. Introduction

Security remains one of the most important parts of military facility management, in particular of recent terrorist alerts. For over 30 years US military has employed LCX detection systems such as Perimitrax for its perimeter solution due to its high probability of detection and low defeating rate. LCX cables, initially designed for wireless communications where conventional antenna fails, purposely leak the transmitted signal out into its immediate vicinity. By employing two such cables, one transmitting and one receiving, lined in parallel buried in the ground of the fence line, the leaked signal will be coupled into the receiving cable. This generates a continuous volume of EM field above the ground. The movement of objects with high capacitive properties above the cables will disturb the field, leading to detection. 

 

 [i]

LCX systems are classified as active detection systems because energy is constantly inputted into the 'volume' to be protected. Disturbances in this energy are monitored and would trigger the alarm. While the passage and movement of objects above the receiving cable will cause an alarm, the user will not know where on the length of the cable the movement occurred. There are two types of LCX system design to address this issue: zoning and timed reflection analysis.

Zoning: The aim of this LCS design is to specify which zone in the perimeter the movement occurred. The perimeter can be sectioned into different zone of approximately 50m each. Since the leakage of the signal is determined by the slotting arrangement and geometry, each zone can have different slotting configurations. By sending multiple frequency signals down the transmitted LCX, different zones will leak signals at different strengths. By analyzing the type of signal coupled into the receiving cable when an intrusion occurs, the user can determine which zone the movement is in. In this sort of system it is important to employ coupling mode type LCX for it has the structure to support multiple frequencies and higher bandwidth.

The system designed in this study is the next logical step in improvement of the current design by using three cables instead of the current two, the direction of the movement can be known. While the detection of extrusions may at first seem illogical, the system can be used to detect any unauthorized departure of a certain area, ideal when items of interest wish to be restricted to a certain designated area. This directional improvement can be used, for example, to secure aircraft parking sites, where individual aircrafts can be monitored. Any intrusion into the immediate vicinity of an aircraft and the departure of an aircraft out of its designated parking spot can be known.

Figure 1: Using the improved system the direction of movement can be known and can be used in securing the location of objects such as aircrafts in their designated location. Authorization for individuals to approach the object and to remove it from the location can be monitored.

 

II. Results

Initially a two LCX system was first constructed using Radiax RXL4.5-1 Coupled mode 5/8” cable made from corrugated, welded copper outer conductor over a low-density foam[ii]. The amplitude of the coupled signal when intrusion occurred was recorded. Due to automatic scaling of LabView, amplification of the coupled signal was done automatically and a true voltage signal was only measured by an HP 54600A oscilloscope to be approx. 540mV. LabView was able to amplify the signal to 5V, a 100 in gain.

Figure 2: The signal coupled into the receiving cable using waveform capture.vi as subject walked across the detection zone (y=volt, x=sec).

The results of the coupled signal are sufficient for reliable detection of movement across the detection zone. Coupling signals were similar in the three LCX configurations. Using LabView, the location of the peak of the coupled signals from the two receiving cables can be found, and by comparing the locations, the direction of movement, whether intrusion of extrusion can be determined. The user interface for such system is as follows:

 

Figure 3: The user interface from LabView showing the time and direction of movement (intrusion or extrusion). Visual alarm generated when coupled signal reaches threshold set by user.

 

III Summary

The employment of leaky coaxial cables (LCX) as a means of perimeter security has been proven successful for over 30 years. This study aims to improve the current design by allowing user to determine the direction of movement, necessary in certain applications. Many factors, such as signal coupling and threshold levels are site dependent and therefore initial onsite calibrations must be made. Additionally, the high cost of installation may be a deterrent for some. However, with the invisible and covert detection method, along with its terrain following ability, LCX systems remain a viable option in perimeter security. This study has shown the successful implementation of a three LCX design. The distance of separation of the cables is determined by the speed of the object which we wish to detect. Results have shown the fast time response, in the order of milliseconds and therefore separation distance calibrations can be made accordingly.