Note: Descriptions are shown in the official language in which they were submitted.
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INTRUSION DETECTION SYSTEM USING QUIET SIGNAL BAND DETECTION
DESCRIPTION
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.
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. US patent number
3,163,861 and US patent number 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 US
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 do 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,
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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 conditions, however, may lead to variations
in detection sensitivity. As disclosed in US patent number
5,534,869 (Harman) and in US patent number 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.
DISCLOSURE 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 ::,y an in:.ruder
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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 metre 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
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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 extend 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,
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(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
5 the disparate frequencies;
(iv) using the receiver unit, receiving signals
corresponding to the transmitted signals; and
(v) 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.
In preferred embodiments of either of the second and
third aspects of the invention, the disparate frequencies have
a bandwidth of at least 5 per cent, and preferably about 10
per cent, of their mean frequency.
BRIEF DESCRIPTION OF THE DRAWINGS:
Preferred embodiments of the invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which corresponding items in the
different Figures have the same reference number. In the
drawings:
Figure 1 is a simplified schematic diagram illustrating
the transmitter, receiver and control unit of an intrusion
detection system;
Figure 2A illustrates a typical signal levels within the
FM broadcast band, specifically from 92 MHz. to 107 MHz.;
Figure 2B illustrates selection of quiet portions of the
broadcast band for transmission of signals by the transmitter
of Figure 1;
Figure 3A illustrates an ideal, hypothetical uniform
detection sensitivity along a leaky cable of fixed length L;
Figure 3B illustrates the real detection sensitivity
along a leaky cable of fixed length L when it is subjected to
a single-frequency RF signal;
Figure 3C illustrates the detection sensitivity when the
cable of Figure 3B is surrounded by an electrically-lossy
medium;
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Figure 3D illustrates detection sensitivity along the
cable of Figure 3B for each of four disparate frequencies of
RF signal to which the cable is subjected;
Figure 3E illustrates the combined detection sensitivity
along the cable of Figure 3D;
Figures 4A and 4B are a flowchart illustrating operation
of the intrusion detection system of Figure 1; and
Figures 5 to 10 illustrate various configurations of
intrusion detection systems embodying the invention.
BEST MODE(S) FOR CARRYING OUT THE INVENTION:
Referring to Figure 1, a leaky cable intrusion detection
system comprises a leaky cable 10 having one end connected by
way of a coaxial lead cable 12 to a transmitter 14 and
terminated at its other end by a termination impedance 16. The
leaky cable 10 comprises a transmission antenna for signals
from transmitter 14. An omnidirectional receiving antenna 18
is connected by a downlead 22 to a receiver 24 which detects
the signals radiated by the leaky cable 10. The transmitter
14 and receiver 24 are both coupled to a control unit 26 which
includes matched A-to-D converters 28 and 30 with their
respective outputs connected to a microprocessor 32. At the
input to receiver 24, the downlead 22 is coupled to a 88-108
MHz. bandpass filter 34 which passes signals from the
receiving antenna 18 to a low noise amplifier 36. Amplifier
36 applies the amplified, filtered signals to a mixer 38 which
mixes with them a variable frequency signal from a voltage
controlled local oscillator (VCO) 40 which is controlled by
the microprocessor 32 by way of control line 41. The output
signal from mixer 38 is applied to an automatic gain control
amplifier (AGC) 42 which is controlled by microprocessor 32
by way of a control line 44. The microprocessor 32 adjusts
the gain of AGC 42 to compensate for differences in received
signal strengths resulting from variations in the spacing
between the leaky cable 10 and the reception antenna 18. The
output from AGC 42 is filtered by a 10.7 MHz. bandpass filter
46, which may be a crystal filter, and applied to an in-phase
and quadrature (I and Q) demodulator 48, controlled by in-
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phase and quadrature phase control signals (0 and 900) from
a 10.7 MHz. intermediate frequency (IF) oscillator 50. The
demodulator 48 uses the phase control signals to extract the
in-phase (I) and quadrature (Q) signals from the received
signal and supplies them by way of respective matched low pass
filters 52 and 54, respectively, to the A-to-D converters 28
and 30, respectively, of control unit 26. The low pass filters
52 and 54 remove higher frequency signal components or
harmonics resulting from the mixing process.
The microprocessor 32 controls the operating frequency
of both the receiver 24 and the transmitter 14 by varying the
frequency of voltage controlled oscillator (VCO) 40 within the
range 98.6 MHz. To 118 . 6 MHz., i.e. the range 87.9 - 107 . 9
MHz. plus the intermediate frequency (IF) of 10.7 MHz.
Transmitter 14 comprises a second mixer 56, second 88-108
MHz. bandpass filter 58 and two-state (on/off) amplifier 60.
The microcontroller 32 controls the amplifier 60 by way of a
control line 62. The transmitter mixer 56 mixes the variable
frequency signal from local oscillator VCO 40 and the 10.7
MHz. IF signal from IF oscillator 50. (Although the VCO 40
and IF oscillator 50 are shown as components of the receiver
24, because they are used to control both the receiver 24 and
the transmitter 14, they could well be considered to be part
of the control unit 26). The transmitter mixer 56 mixes the
LO and IF signals to provide a transmission signal and
supplies it to 88-108 MHZ. bandpass filter 58, which removes
harmonic frequencies and supplies the filtered transmission
signal to switched amplifier 60. When turned on by
microprocessor 32, amplifier 60 applies the transmission
signal via lead line 12 to the leaky cable 10. Thus, when the
microprocessor 32 adjusts the VCO 40, it will control the
operating frequency of both the receiver 24 and the
transmitter 14.
As shown in Figure 2A, the FM broadcast band will usually
have clusters of signals, identified in Figure 2A as S1 - S5,
with quiet portions between them where the signal level is so
low as to be insignificant, i.e. below background noise. In
Figure 2A,-these quiet portions are identified as P1 - P4
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where P1 represents the widest quiet portion. In operation,
the microprocessor 32 will perform an initial "set up"
procedure by causing the receiver 24 to scan the operating
range to locate quiet portions or "gaps". The scan actually
covers the range from 92 MHz. to 107 MHz. The portion from 88
to 92 MHz. is not scanned as it is mainly reserved for non-
commercial FM stations or non-profit agencies, such as campus
or church stations, which do not always transmit continuously.
If their assigned frequency was selected while the station was
off-air, interference would occur as soon as the station
started transmitting again. Limiting to 107 MHz. allows a 1
MHz. margin to reduce the risk of interference with aircraft
navigation bands above 108 MHz.
During initial scanning of the operating frequency band
to determine the quiet portions, the microprocessor 32 will
turn off the two-state amplifier 60, so that no signals will
be transmitted by the transmitter 14 during the initial
scanning, and will adjust the gain of AGC 42 to maximum.
Once the initial scanning has been performed, and the
microprocessor 32 has determined the quiet portions which may
be used, the microprocessor 32 will select a set of
transmission frequencies located in the quiet portions, turn
on the two-state amplifier 60, and then repeatedly adjust the
VCO 40 to each of the selected transmission frequencies in
turn. As a result, the transmitter 14 will transmit signals
at those frequencies, identified in Figure 2B as fl, f2, f3
and f4, and the receiver 24 will track the transmitter 14 to
receive signals at the same frequencies. Such frequency-
hopping is a technique known to persons skilled in this art,
so it will not described in detail here. As illustrated in
Figure 2B, the amplitude of the transmitted signals is less
than the limit specified for non-licensed usage.
The use of multiple transmission frequencies provides
relatively uniform detection sensitivity along the length of
the leaky cable. Figure 3A illustrates that, for an ideal,
hypothetical leaky cable, the detection sensitivity would be
uniform throughout the length L of the cable. In fact, this
would only occur with an infinitely long cable. When a cable
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of finite length is subjected to a single frequency signal,
a surface wave propagates along the cable and is reflected at
discontinuities, particularly the ends, though discontinuities
may arise from conductive objects in the vicinity of the
surface wave. As a result, standing waves occur, and the
detection sensitivity exhibits a series of alternating nulls
and peaks, as illustrated in Figure 3B. As discussed
hereinbefore, burying the cable in an electrically-lossy
medium causes the detection sensitivity to become more
uniform, as illustrated in Figure 3C.
As explained earlier, it is not always convenient to bury
the cable. Consequently, the approach taken by the present
invention is to accept the existence of the standing waves
and, rather than bury the cable to ameliorate their effect,
use a plurality of different frequencies with significant
frequency spacing (at least five per cent and preferably about
ten per cent bandwidth). As shown in Figure 3D, standing
waves will still occur, one for each frequency, but their null
points will not coincide. Consequently, as illustrated in
Figure 3E, the detection sensitivity along the cable will be
more uniform. Operation of the intrusion detection system to
locate the quiet portions of the broadcast band, for use by
its transmitter 14, is depicted in more detail in the
flowchart of Figures 4A and 4B. Figure 4A depicts the
detection of the quiet portions and Figure 4B depicts the
selection of the transmission frequencies. Referring first to
Figure 4A, in step 70, the microprocessor 32 performs the
following initialization steps: (i) sets the gain of AGC 48
to maximum, so as to obtain maximum reception sensitivity;
(ii) sets to 92.1 MHZ. the frequency variable f.;,,; (iii) turns
the transmitter 14 off by means of amplifier 60; (iv) sets a
minimum signal threshold to 12 dB. above a noise floor; (v)
sets the minimum gap width to 1 MHz. (which corresponds to the
bandwidth for five FM stations spaced 0.2 MHz. apart); (vi)
sets equal to fm;, an initial frequency variable f for the
receiver; and (vii) sets the number of the instant quiet
portion equal to 0.
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In decision step 72, the microprocessor 32 compares the
amplitude of the received signal with the minimum signal level
threshold of 12 dB. above the noise floor. If it is greater,
decision step 73 determines whether or not the instant
5 frequency f exceeds the upper limit of 107.1 MHz. and, if it
does, exits to step 94 (Figure 4B). It does not, in function
step 74 the microprocessor 32 increments the VCO 46 to
increase the frequency f by 0.2 MHz. (which is equivalent to
the frequency spacing between FM station allocation). The
10 loop comprising steps 72 and 74 causes the frequency f to
increment in steps of 0.2 MHZ. until step 72 indicates that
the received signal level is below the 12 dB. threshold, i.e.
the beginning of the first quiet portion (GAP 1) has been
detected. Thus, when step 72 returns a negative result,
because the signal level at that particular frequency
increment f is below the threshold, in step 76, the
microprocessor 32 sets fn,;j to that value of "f", establishing
the lower limit of the first quiet portion or gap.
Thereafter, step 78 increments the receiver frequency f by 0.2
MHz. and decision step 80 detects whether or not the received
signal exceeds the 12 dB. threshold. If it does not, decision
step 82 determines whether or not the current frequency f
exceeds 107.1 MHZ., the upper limit of the operating band.
If it does not, the microprocessor 32 returns to step 78 and
increments the frequency by another 0.2 MHZ. The loop
comprising steps 80 and 82 increments the frequency f in 0.2
MHZ. steps until either a signal level above the 12 dB.
threshold is detected, or the upper limit of 107.1 MHZ. is
reached, whereupon step 84 sets the variable fm87C, to the last
recorded frequency f minus 0.2 MHZ., calculates the width Afl
of the f irst quiet portion or gap by subtracting fn,;, from
fmaXl, and determines its centre frequency fceas midway
between fm;ni and f max,= It should be noted that fxl is set to
f - 0.2 because the upper limit of the gap is 0.2 MHz. less
than the instant frequency f.
In step 86, the microprocessor 32 compares the width of
the quiet portion with the minimum acceptable width (MG), set
to 1 MHz. If the width is less than 1 MHz., (to prevent
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chance of mutual interference), the microprocessor 32 returns
to step 76 and repeats steps 76 through 84 for further
frequency increments.
Once a quiet portion of the prescribed width has been
found, in step 88, the microprocessor 32 records the
parameters of the quiet portion in memory and in step 90
determines whether or not the instant frequency exceeds the
upper limit of the operating range, vis. 107.1 MHZ. if it
does not, in step 92, the microprocessor 32 increments the gap
number to 2 and returns to step 76. The microprocessor 32
then repeats steps 76 to 92 to detect a second quiet portion,
determine its parameters df2, foiiii2, fõ2 and f,,,t,e2 and record
them with the quiet portion number in memory. If the upper
limit of the second quiet band is less than 107.1 MHZ., the
microprocessor 32 will repeat steps 76 to 92 for a third quiet
portion, and so on until the entire operating band has been
scanned for quiet portions. When that has been done, step 90
returns a positive result and the microprocessor 32 proceeds
to the frequency determination process of Figure 4B.
Referring to Figure 4B, in step 94, the microprocessor
32 accesses its memory and determines whether or not any quiet
portions wider than 1 MHz. were detected. If no such quiet
portions were detected, step 96 will return a warning message
to the effect that the FM spectrum cannot be used and suggest
that the use of an alternative system, such as that disclosed
in US 5,534,869 supra which uses signals from a commercial
radio station.
If only one quiet-portion was detected, as indicated by
a negative result for decision step 94 and a positive result
for decision step 98, in step 100 the microprocessor 32
calculates four transmission frequencies fl, f2, f3 and f4,
where:-
fl is the centre frequency fCC1Ve minus three eighths of
the bandwidth Afl;
f2 is the centre frequency f.minus one eighth of the
bandwidth Ofl;
f3 is the centre frequency f,,,,fe plus one eighth of the
bandwidth Afl;
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f4 is the centre frequency f,u,,,, plus three eighths of the
bandwidth Ofl.
Hence, the four frequencies are spaced approximately equally
across the band, and proceeds to intrusion detection step 114.
If several quiet portions were detected, step 98 returns
a negative result and step 102 determines whether or not two
quiet portions were detected. If so, in step 104, the
microprocessor 32 calculates the four transmission frequencies
so that fl and f2 are in the first quiet portion and
frequencies f3 and f4 are in the second quiet portion. In
particular, fl is calculated as the centre frequency f,.tõe, of
the first quiet portion minus one quarter of the bandwidth OfI
of the first quiet portion and f2 is calculated as fc.uel plus
one quarter of the bandwidth Ofl of the first quiet portion.
The other two frequencies f3 and f4 are calculated as the
centre frequency f,,,te2 of the second quiet portion and third
portion, respectively. Microprocessor 32 then proceeds to
intrusion detection process 114.
If three quiet portions have been detected, step 102
returns a negative result and step 106 sorts the quiet
portions in its memory in decreasing order according to their
respective bandwidths Ofl, Of2, Of3, and so on, i.e. the first
being the widest. In step 110, the microprocessor 32 then
determines whether or not there are three quiet portions or
more. If there are three, in step 108 the microprocessor 32
calculates the four frequencies, the first two, fl and f2, in
the first (widest) quiet portion and the second two, f3 and
f4, in the second and third quiet portions, respectively.
Thus, fl and f2 are calculated as the centre frequency
of the first quiet portion minus and plus, respectively, one
quarter of the bandwidth Ofi of the first quiet portion. The
frequencies f3 and f4 are set to the centre frequencies f,,,,Ve2
and f,,õt,6 of the second and third quiet portions, respectively.
The microprocessor 32 then proceeds to intrusion detection
process 114.
If four or more quiet portions were detected, step 110
returns a negative result and, in step 112, microprocessor 32
sets the four transmission frequencies f 1, f2, f3 and f4 equal
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to the centre frequencies f... ,n,, f..ob,2, f,,,,,3 and f,,,t,A of the
four quiet portions or, if there are more than four quiet
portions, to the centre frequencies of the four widest quiet
portions. The microprocessor 32 then proceeds to intrusion
detection process 114.
In the intrusion detection process 114, the
microprocessor 32 causes the transmitter 14 to transmit at
each of the four frequencies fl, f2, f3 and f4, using
frequency-hopping techniques that are known to persons skilled
in this art and so need not be described here. As described
previously, the receiver 20 tracks the transmitter 14 and
receives signals at the same set of frequencies and the
microprocessor processes them to detect an intruder. The
microprocessor 32 may use known techniques to process the
received signals to detect perturbations caused by an intruder
in the vicinity of the leaky cable 10. For particulars of
such a technique, the reader is directed to US patent
number 5,510,766, the contents of which are incorporated
herein by reference.
The invention is not limited to the intrusion detection
system configuration illustrated in Figure 1. An advantage
of embodiments of the present invention is that they provide
for considerable flexibility in system configuration and
physical layout, examples of which are illustrated in Figures
5 - 10. Thus, in the system depicted in Figure 5, two leaky
cable antennae 10 " are connected to the transmitter 14' and
extend one each side of the reception antenna 18', before
being terminated termination impedances 16''. The receiver
24' and processor unit 26' are located with the transmitter
14'. The transmitter 14' uses time multiplexing to transmit
signals to both of the leaky cables and protect two
corresponding zones.
In the system of Figure 6, the distance between the leaky
cable 10 and the reception antenna 18' is relatively large
and/or the leaky cable 10 relatively short. In order to allow
for the consequent reduction if signal strength, the reception
antenna 18' is a directional antenna pointing towards the
leaky cable.
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In the system of Figure 7, a series of leaky cables 101,
102, 103, and 104, each about 150 meters long, are connected in
tandem between the transmitter 14 and a termination impedance
(not shown). A corresponding plurality of "repeater"
amplifiers 641, 642, 643 and 644 are interposed one between each
pair of leaky cables and serve to boost the signals and
maintain substantially the same transmission signal level in
each of the leaky cables 101 to 104. A corresponding series
of reception antennae 181, 182, 183 and 184 and their associated
receivers 201 to 204 are connected in tandem by coaxial cables
22i to 224, each also about 150 meters long, to receiver unit
24. Each of the reception antennae 181 to 184 is adjacent to,
and monitors, a respective one of the leaky cables 101 to 104.
The antennae 18, to 184 depicted in Figure 8 are
omnidirectional. It would be possible, however, to have a
plurality of directional antennae each pointing to a
corresponding one of the leaky cables, but not adjacent to it.
In the system depicted in Figure 8, a single leaky cable
10 is connected at one end to the transmitter 14, disposed in
a loop around the reception antenna 18 and terminated by
termination impedance 16. The reception antenna is an
omnidirectional antenna which receives signals from the whole
length of the leaky cable 10. As before, in order to
determine that an intruder is crossing the leaky cable, the
processor unit may employ the procedure disclosed in US patent
number 5,510,766. It is envisaged that the configuration
could be modified by omitting the termination impedance 16 and
connecting both ends of the leaky cable 10 to the transmitter
14 by way of a power splitter.
In the system depicted in Figure 9, the leaky cable 10
has its ends both connected to the transmitter 14 and again
forms a loop around the reception antenna 18' which is
electrically-steerable, for example a phased array antenna,
or mechanically-steerable, for example a rotatable dish
antenna. The ends of the leaky cable 10 may be connected in
common to the transmitter by a power splitter (not shown).
Alternatively, they might be connected directly to the
transmitter for time-multiplexed operation. The lobe 68 of
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the antenna 18 rotates through 360 degrees to scan, stepwise,
the whole length of the leaky cable 10. The position at which
the intruder crosses the leaky cable can be determined readily
by determining the position of the antenna using, for example,
5 known plan position indication techniques. Preferably, the
antenna 18' is as small as possible, yet its beamwidth as
narrow as possible. In practice, a four-element phased array
antenna should be satisfactory.
It should be appreciated that the antenna need not scan
10 through 360 degrees but could step through an arc appropriate
to the position and length of the leaky cable. It would also
be possible to have several separate leaky cables, as in the
embodiment of Figure 7, and control the steerable antenna to
scan, in steps, each leaky cable in turn.
15 Finally, Figure 10 illustrates the leaky cable 10
connected to the transmitter 14' as before but the receiver
24' and processor unit 26' are located physically adjacent the
reception antenna 18', i.e. spaced from the transmitter 14'.
In this case, the transmitter and receiver are not
synchronized. Instead, each has its own local and IF
oscillators and the microprocessor does not control the
transmitter, i.e. the control line 62 (Figure 1) is omitted.
In operation, the microprocessor will still cause the receiver
to scan the frequency band and display the frequency spectrum
to the user to guide the user in identifying the quiet
portions. The user will then set the transmitter manually to
the four frequencies, at which it will transmit continuously.
Although the above-described embodiments of the invention
each have the leaky cable(s) connected to the transmitter 14,
it is envisaged that the invention could also be implemented
with the leaky cable(s) connected to the receiver and the
antenna(e) 18 used to transmit the signals from the
transmitter 14, at least where a country's regulations permit
higher radiation levels.
INDUSTRIAL APPLICABILITY
An advantage of embodiments of the present invention is
that the preselection of quiet portions in which to operate
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allows the transmission signal level to be low; so low in fact
that it is below the level at which regulatory permission is
required and permits the system to operate in regular FM radio
bands, such as the usual FM radio band of 88-108 MHz. This
is advantageous because it allows multiple frequencies to be
used, which reduces the effect of standing wave or null
problems and facilitates deployment of the leaky cable without
a surrounding electrically-lossy medium, for example above
ground, while maintaining uniform detection sensitivity.
Further advantages are realised because operation in the
92-107 MHz. band allows readily available components to be
used, which keeps costs down, and because one half of the
wavelength of the signal is close to the height of a human
being, giving better discrimination between human intruders
and small animals and consequent reduction of false alarms.
Moreover, the FM band, by its nature, has quiet portions, with
consequent reduced risk of potential interference.
