Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-02-03
2001-06-26
Lee, Benjamin C. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S567000, C340S539230
Reexamination Certificate
active
06252507
ABSTRACT:
DESCRIPTION
1. Technical Field
The invention relates to intrusion detection systems and methods and, in particular, to intrusion detection systems which comprise an open transmission line or so-called “leaky cable” and are used to determine the presence of objects, things or people moving in the vicinity of the leaky cable.
2. Background Art
Known intrusion detection systems use a leaky cable as a receiving antenna to receive a radio frequency signal transmitted from an associated antenna; or as a transmitting antenna to transmit signals for reception by a separate antenna, which might be another leaky cable. U.S. Pat. No. 3,163,861 and U.S. Pat. No. 5,534,869 both disclose passive systems, i.e which do not include a captive transmitter. Instead, their receivers receive signals from an independent source, i.e. a commercial FM station. In the case of U.S. Pat. No. 5,534,869, the receiver receives the normal transmissions from one or more commercial radio stations so as to improve reliability and to minimize the effects of multi-path signals. An advantage of such systems is that, because they don not transmit signals themselves, they do not require licensing. Unfortunately, it is sometimes necessary to deploy the intrusion detection system in a location where such signals cannot adequately be received, perhaps because the location is geographically remote or shielded. In such cases, it is appropriate to use a more traditional “active” intrusion detection system which has its own captive transmitter, such as that disclosed in Canadian patent number 1,169,939. The latter discloses a system having an RF excited antenna within an area to be protected and a leaky coaxial cable extending around the perimeter. One or more additional leaky cables may be added to avoid the possibility of intruders using a particular path which gives a null angle response.
A common problem with such intrusion detection systems, whether passive or active, is that, in certain circumstances, standing waves may be established along the surface of the leaky cable, resulting in a plurality of null positions along the cable at which the detection sensitivity is reduced and an intruder less likely to be detected. The establishment of such standing waves may be inhibited by burying the leaky cable in an electrically-lossy medium, such as the ground. Changes in soil condition, however, may lead to variations in detection sensitivity. As disclosed in U.S. Pat. No. 5,534,869 (Harman) and in U.S. Pat. No. 5,473,336 (Harman and Gagnon), it is possible to reduce such variations in sensitivity caused by the environment by means of a special cable construction involving a combination of shields. A disadvantage of this approach, however, is the relatively high cost of the cable. Moreover, it is not always convenient to bury the cable. In some cases, for example, it is desirable to leave it upon the surface or position it along the edge of a building roof or along the top of a fence.
One object of the present invention is to overcome or at least mitigate these problems and disadvantages of known systems and to provide an intrusion detection system capable of operation with relatively uniform detection sensitivity with the leaky cable above ground.
SUMMARY OF INVENTION
According to one aspect of the present invention, there is provided an intrusion detection system comprising:
a transmitting antenna and a receiving antenna, one of which is a leaky cable,
a transmitter unit connected to the transmitting antenna; and
a receiver unit connected to the receiving antenna;
wherein the transmitter unit transmits signals, by way of the transmitting antenna, at several disparate radio frequencies;
and the receiver unit receives by way of the receiving antenna signals corresponding to the transmitted signals;
the system further comprising means for detecting perturbations in the received signals caused by an intruder in the vicinity of the leaky cable and in dependence thereupon determining the presence of an intruder.
With such an arrangement, using transmission signals at several different frequencies, standing waves may still occur, one for each frequency, but their null points will be at different places along the leaky cable. Consequently, relatively uniform detection capability is maintained.
The transmitter may use frequency-hopping to transmit signals at the different frequencies, the receiver also using frequency-hopping and being synchronized to the transmitter for reception of the signals. Alternatively, the transmission and reception at said disparate radio frequencies may be achieved by spread-spectrum, pulsing or other suitable techniques.
In preferred embodiments of the invention, the disparate frequencies have a bandwidth of at least five per cent, and preferably about 10 per cent, of their centre frequency.
The leaky cable is a relatively inefficient antenna, so the signal it receives must be relatively strong, implying a very efficient transmitting antenna and/or a transmitted signal level which is relatively high. In many countries, regulations prohibit the use of private systems with a signal level above a prescribed limit. For example, in the United States of America, FCC regulations numbers 15.209 and 15.239 limit signal strength to 150 microvolts per meter at 3 meters and 250 microvolts per meter at 3 meters for “non-intentional” radiations and “intentional” radiations, respectively. Systems with a signal level above these levels must use an Industrial, Scientific and Medical (ISM) band which, being extremely narrow, i.e. from 40.66 MHz. to 40.70 MHz., mitigates against the use of multiple frequencies with a significant bandwidth.
A further object of the present invention, therefore, is to provide a leaky cable intrusion detection system which does not necessarily require buried cables or commercial radio station signals, yet can be used in broadcast radio bands.
According to a second aspect of the present invention, there is provided an intrusion detection system comprising
a transmitting antenna and a receiving antenna, one of which is a leaky cable,
a transmitter unit connected to the transmitting antenna and a receiver unit connected to the receiving antenna; and
a control unit for controlling the transmitter and the receiver,
the control unit controlling the receiver to scan one or more sections of the radio spectrum, with the transmitter not transmitting, and detect one or more relatively quiet portions of the spectrum in which instant received signal levels are lower than a predetermined threshold,
the control unit selecting a plurality of disparate frequencies in said one or more relatively quiet portions and subsequently causing the transmitter to transmit signals by way of the transmitting antenna, at said disparate frequencies;
the receiver unit receiving signals corresponding to the transmitted signals;
the control unit detecting perturbations in the received signals caused by an intruder in the vicinity of the leaky cable and in dependence thereupon determining the presence of an intruder.
The preselected radio band may comprise at least a portion of the FM band from about 87.9 MHz. to about 107.9 MHz. and preferably extends from about 92 MHz. to about 107 MHz.
According to a third aspect of the invention, there is provided a method of detecting intruders using an intrusion detection system comprising a transmitting antenna and a receiving antenna, one of which is a leaky cable, a transmitter unit connected to the transmitting antenna and a receiver unit connected to the receiving antenna, the method comprising the steps of:
(i) using the receiver unit, scanning a preselected radio band, with the transmitter not transmitting, and detecting one or more relatively quiet portions of the band in which instant received signal levels are lower than a predetermined threshold,
(ii) selecting a plurality of disparate frequencies in said one or more relatively quiet portions;
(iii) using the transmitter unit, transmitting signals by way of the transmitting antenna at the disparate frequencies;
(iv) using the
Auratek Security Inc.
Lee Benjamin C.
Thomas Adams
LandOfFree
Intrusion detection system using quiet signal band detection does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Intrusion detection system using quiet signal band detection, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Intrusion detection system using quiet signal band detection will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2526388