Threat detection apparatus

Abstract

An apparatus for detecting a threat object carried by or on the body of a person comprises one or more transceivers for generating and detecting RF at one or more frequencies and one or more pairs of

Description

Technical Field The present invention relates to apparatus for detecting a threat, such as an explosive, carried on the body of a person. The apparatus are particularly suited to deployment at public spaces having large areas. Background Over a number of years terrorists have sought with varying degrees of success to infiltrate gatherings both large and small to cause death and destruction. In some cases the terrorists have been willing to sacrifice themselves. Typically, security relies on a series of small access “gates” or “portals” which filter people past screening equipment. Whilst this approach works it can be very limiting for large events such as outdoor concerts or at venues in which the number of people attending is in the 1,000s or 10,000s. Examples of conventional screening equipment are shown in figures 1(a) and 1(b). Figure 1(a) shows a portal type screening apparatus through which users walk. Portal apparatus such as this are commonly found at airports where the number of people is well-controlled. Such portal type scanners are metal detectors operating on pulse induction or millimetre wave scanning. Figure 1 (b) shows a conveyor type baggage scanner. These scanners use X-rays which are directed at the bags as they pass through. A detector collects the X-rays and forms an image that an operator checks for suspicious objects. X-ray detection may be reliant on the operator noticing suspicious objects to raise an alarm. Attempts have been made to produce improved detection apparatus. WO 2012 / 027317 A1 describes apparatus and methods of using radio frequencies for metal detection in which an RF field is transmitted to an interrogation zone and a receiver measures the power and phase of reflections of the RF from any metal objects in the interrogation zone. The apparatus detects movement of a metal object through the interrogation zone. US 10,261,178 B2 describes a further metal detector based device which uses wireless electromagnetic wave signals and their reflection by metal items. US 10,261,178 B2 transmits the signals across an indoor region and determines any reflected abnormality. US 2020 / 264298 A1 describes a baggage scanner type weapon detection system that uses radar and a magnetometer to detect objects made of ferromagnetic materials. The radar uses RF of a wavelength appropriate for producing a strong retroreflective signal for a common range of weapon sizes. The magnetometer is used to detect ferromagnetic materials based on ferromagnetic hysteresis of the object under test. Nevertheless, it is desirable to provide an improved detection system that can detect other materials that can be used in a threat, such as explosives. Preferably, an improved detection system would be unseen and capable of being used across entrances at large venues and without the people filtering needs of airport style portals and baggage scanners. Summary of the Invention The present invention provide apparatus for detection of a threat object carried on the body of a person. The apparatus comprises: one or more transceivers for generating and detecting RF at one or more frequencies; one or more pairs of antennas, each pair of antennas comprising: a transmit antenna configured to transmit RF at the one or more frequencies to a respective detection zone between that pair of antennas, and a receive antenna configured to be spaced apart from the transmit antenna, the receive antenna configured to receive RF at the one or more frequencies from the respective detection zone; and one or more analysers configured to analyse signals from the receive antenna of each of the one or more antenna pairs, the one or more analysers configured to identify a disturbance to the RF at the one or more frequencies propagated through the respective detection zone indicative of one or more materials indicating the presence of a potential threat object based on one or more characteristics of the signals. For example, each frequency may be selected for detecting a particular material relevant to a threat and may be selected due to a specific response to the presence of the material in the detection zone. Although one frequency may be used, for example, to detect explosives, preferably multiple frequencies are used to detect different explosives or shrapnel. Furthermore, multiple frequencies may be used for detecting a particular material to provide increased accuracy. The antenna pairs are preferably concealed in a structure or furnishing of a building or approach to a venue. Multiple such detection apparatus may be provided in a concealed manner across a wide area of a building or approach to a venue allowing free movement of persons, such as without airport style portal detectors and baggage scanners, while providing threat detection. The detection zone may comprise a first detection zone and a second detection zone, and the one or more pairs of antennas may comprise: a first pair of antennas comprising a first transmit antenna configured to transmit RF at the first frequency to a first detection zone, and a first receive antenna configured to be spaced apart from the first transmit antenna, the first receive antenna configured to receive first RF at the first frequency from the first detection zone; and a second pair of antennas comprising a second transmit antenna configured to transmit RF at the second frequency to a second detection zone and a second receive antenna configured to be spaced apart from the second transmit antenna, the second receive antenna configured to receive RF at the second frequency from the second detection zone. The second detection zone may be the same or different to the first detection zone. The first detection zone may comprise the second detection zone. The antennas may be configured to be disposed or mounted on or in a surface of a public space (such as concealed in a structure or furnishing) so as to form the detection zone(s) for the passing of persons there through. The threat object may be an explosive threat object. The one or more analysers may be configured to detect a person passing through the detection zone based on the disturbance to the RF propagated through the detection zone and identify a characteristic of the disturbance indicative of the threat object carried by the person. The identification of a disturbance may comprise analysing a time-based amplitude and / or phase disturbance of the received RF to determine a feature characteristic of the disturbance. The feature characteristic may be one or more of: an increased or reduced phase and / or amplitude; a number or timing of transients; the shape of a leading edge or a falling edge of the received signal; the amount of ringing; an inflexion point in the response, and any other distinct feature, in comparison to a received RF signal caused by the presence, or passing, of a person not carrying a threat in the detection zones. This list is not exhaustive. The frequencies may be frequencies at which the RF propagated through the detection zone(s) experiences disturbance or distortion when interacting with a first material and a second material. The first material and the second material may be different materials and respectively comprise any of: shrapnel such as metal, hard plastic and / or sharp stones, and explosives such as home-made explosive or commercial explosive. The one or more pairs of antennas may be configured to transmit and receive a plurality of frequencies for respectively detecting the presence of a plurality of materials, such as: three frequencies for respectively detecting shrapnel, home-made explosive and commercial explosive; four frequencies for respectively detecting metal shrapnel, plastic shrapnel, home-made explosive and commercial explosive; or five frequencies for respectively detecting metal shrapnel, plastic shrapnel, sharp stones, home-made explosive and commercial explosive. The one or more analysers may be configured such that when the analysis of the signals from the one or more receive antennas is indicative of the presence of a first material and the first material is shrapnel such as one or more of: metal, hard plastic, sharp stones, and the analysis of the signals is indicative of the presence of a second material and the second material is one or more of: home-made explosive and commercial explosive, providing an alert to a user or system. The one or more analysers may be configured such that when the analysis of the signals from the one or more receive antennas is indicative of the presence of a second material and the second material is one or more of: home-made explosive and commercial explosive, providing an alert to a user or system. The one or more analysers may be configured such that when the analysis of the signals from the one or more receive antennas is indicative of the presence of a first material and the first material is one or more of: metal and hard plastic but the analysis of the signals does not indicate the presence of a second material and the second material is one or more of: home-made explosive and commercial explosive, providing no alert. The one or more analysers may be configured to: compare signals based on the transmitted RF and received RF at a first frequency to detect an amplitude and / or phase disturbance in the received signal; and compare signals based on the transmitted RF and received RF at a second frequency to detect an amplitude and / or phase disturbance in the received signal. The comparing may comprise using phase coherence of the transmitted and received signals to detect the amplitude and / or phase disturbance of the received signals. The apparatus may further comprise a first transceiver configured to generate RF at a first frequency, an optional splitter arranged to split the generated RF at the first frequency and direct a first portion of the RF at the first frequency to the first transmit antenna, and the first transceiver configured to receive a second portion of the RF at the first frequency and receive RF from the first receive antenna. If the splitter is not included, comparison to a reference may instead be comparison to the received signal in a quiescent or baseline state, such as with no person passing through the detection zone. The apparatus may further comprise a bandpass filter configured to pass the first frequency and filter out the second frequency, the bandpass filter arranged to receive the RF from the first receive antenna and after filtering send the received RF to the transceiver. The first frequency and the second frequency may be different frequencies and may be between 30 MHz and 18 GHz such as between 100 MHz and 10GHz, and preferably in the range 200 MHz and 6 GHz. The antennas are preferably substantially planar. The antennas may be inverted F, meandering F or other planar antennas. The transmit antenna and the receive antennas may be spaced apart by between 5cm and 5m or more, such as 10cm to 60cm or 2m to 3m. One pair of antennas may be nested between another pair of antennas. The direction of propagation of RF from a first transmit antenna to a first receive antenna may be parallel to, and in the same or the opposite direction to, the direction of propagation of RF from a second transmit antenna to a second receive antenna. The apparatus may further comprise an array of antennas having multiple first antenna pairs and multiple second antenna pairs spaced transversely or parallel to the propagation direction of RF from the transmit antennas to the receive antennas. The one or more pairs of antennas may be printed on one or more flexible substrates and attached to, or embedded in, a foldable floor mat. The one or more pairs of antennas may be configured to be mounted in a floor of a building or approach to a building such as external entrance, forecourt, plaza, etc with the one or more detection zones in an area above the floor. The one or more pairs of antennas may be configured to be mounted in a ceiling of a building with the one or more and detection zones in an area below the ceiling. One antenna of each pair of the one or more pairs may be configured to be mounted on or in a first wall and the other antenna of each pair may be configured to be mounted on or in a second, opposing wall, such that the one or more and detection zones are between the opposing walls. One antenna of each pair of the one or more pairs may be configured to be mounted on or in a ceiling, gantry or overhead structure and the other antenna of each pair may be configured to be mounted on or in a floor below, such that the one or more and detection zones are between the floor and ceiling, gantry or overhead structure. The present invention further comprises a building comprising any of the apparatus set out herein, wherein a plurality of adjacent detection zones are provided across the path of an entrance to the building. The present invention further provides an apparatus for detection of a threat object carried on the body of a person, the apparatus comprising: a foldable floor mat having embedded therein a one or more pairs of antennas, wherein: the one or more pairs of antennas comprise: a transmit antenna configured to transmit RF at one or more frequencies to a respective detection zone between that pair of antennas, and a receive antenna configured to receive RF at the one or more frequencies from the detection zone, the mat configured such that when flat the transmit antenna and the receive antenna are spaced apart forming the detection zone there between; one or more transceivers for transmitting RF to and receiving RF from the antennas at the one or more frequencies; and one or more analysers configured to analyse signals from the receive antenna of each of the one or more antenna pairs, based on the received signals the one or more analysers configured to identify a disturbance to the RF propagated through the detection zone indicative of a threat object based on one or more characteristics of the signals. The foldable floor mat may comprise a plurality of first pairs of antennas and a plurality of second pairs of antennas forming a plurality of adjacent first and second detection zones. The present invention further provides a building comprising apparatus for detection of a threat carried on the body of a person, the building having a concourse or entrance configured to simultaneously receive a plurality of persons there through, comprising: one or more pairs of antennas mounted in or to one or more surfaces of a wall, floor or ceiling of the building, wherein: each pair of the one or more pairs of antennas comprises a transmit antenna configured to transmit RF at one or more frequencies to a detection zone and a receive antenna configured to receive RF at the one or more frequencies from the detection zone, the antennas mounted in or to the one or more surfaces such that the transmit antenna and the receive antenna are spaced apart forming the detection zone there between in a space adjacent to or between the one or more surfaces; one or more transceivers for transmitting RF to and receiving RF from the antennas at the one or more frequencies; and one or more analysers configured to analyse signals from the receive antenna of each of the one or more antenna pairs, the one or more analysers configured to identify a disturbance to the RF propagated through the respective detection zone indicative of a threat object based on one or more characteristics of the signals. The building may further comprise a plurality of pairs of antennas forming a plurality of adjacent detection zones arranged for monitoring the passage of multiple persons through an entrance or concourse simultaneously. An embodiment of the present invention provides an apparatus comprising two pairs of antennas for detection of a threat object carried on the body of a person, the apparatus comprising: one or more transceivers for transmitting and receiving RF at first and second frequencies; a first pair of antennas comprising a first transmit antenna configured to transmit RF at the first frequency to a first detection zone and a first receive antenna configured to be spaced apart from the first transmit antenna, the first receive antenna configured to receive first RF at the first frequency from the first detection zone; a second pair of antennas comprising a second transmit antenna configured to transmit RF at the second frequency to a second detection zone and a second receive antenna configured to be spaced apart from the second transmit antenna, the second receive antenna configured to receive RF at the second frequency from the second detection zone, and one or more analysers configured to analyse a first signal from the first receive antenna based on the received first RF and a second signal from the second receive antenna based on the received second RF, the one or more analysers configured to identify a disturbance to the RF propagated through the first and second detection zones indicative of a threat object based on one or more characteristics of the first signal and / or second signal. The present invention further provides a method of detecting a threat object carried on the body of a person, the method comprising: generating RF at one or more frequencies; transmitting RF at the one or more frequencies from one or more transmit antennas to a respective detection zone to one or more receive antennas spaced apart from a respective one or more transmit antennas; receiving at the receive antenna RF at the one or more frequencies from the respective detection zone; and analysing signals from the receive antenna to identify a disturbance to the RF at the one or more frequencies propagated through the respective detection zone indicative of a threat object based on one or more characteristics of the signals. Brief Description of the Drawings Embodiments of the invention and aspects of the prior art will now be described, by way of example only, with reference to the accompanying drawings, of which: Figures 1(a) and 1(b) are diagrams of conventional screening equipment, namely a portal-type body scanner and an X-ray type bag scanner; Figure 2 is a schematic perspective diagram of part of a detection apparatus according to an embodiment of the present invention, comprising antennas configured in a mat or floor covering for laying on a floor of a venue; Figure 3 shows an alternative arrangement for a pair of antennas, which are arranged on opposing walls, to form a portal type detection apparatus; Figures 4a and 4b are schematic diagrams respectively showing the RF signal supply circuitry for a pair of antennas and the signal processing of the received signal; Figures 5a and 5b are graphs showing a disturbance to the RF for a person walking through a detection zone of a portal type apparatus, measured at two RF frequencies; Figures 6a and 6b are graphs showing a disturbance to the RF for a person walking through a detection zone of a portal type apparatus carrying home-made explosive simulant material, measured at two RF frequencies; Figures 7a and 7b are graphs showing a disturbance to the RF for a person walking through a detection zone of a portal type apparatus carrying metal shrapnel, measured at two RF frequencies; Figure 8 is a schematic diagram showing a detection zone in relation to a pair of antennas provided at the floor; Figure 9 is a schematic diagram showing three detection zones in relation to three groups of antennas provided at the floor; Figure 10 is a schematic diagram illustrating how multiple pairs of antennas and detection zones may be arranged to cover an open space at the entrance to a large entertainment venue; and Figure 11 is a schematic diagram showing how multiple adjacent systems such as those of figures 9 and 10 may be coupled together. Detailed Description The present invention is directed to detecting threats such as explosives or explosives combined with shrapnel or metal pieces, and is particularly suited to detection across wide areas such as entrances to public venues. The invention uses radio frequencies to detect materials of interest. The invention may be deployed in a concealed manner distributed across such entrances or wide areas of public venues. The RF is propagated through a detection zone using antennas. We now describe in detail the present invention. Figure 2 shows two pairs of antennas configured in a mat or floor covering for laying on the floor of a venue. Figure 3 shows an alternative arrangement for a pair of antennas, which are arranged on opposing walls. Figures 4a and 4b are diagrams schematically showing the RF signal supply circuitry for a pair of antennas and the signal processing of the received signal. Figure 8 is a schematic diagram showing a detection zone in relation to a pair of antennas provided at the floor. Figures 5a-7b describe processing of received signals and detection of materials of interest. Turning firstly to figure 2, there is shown a first antenna pair which may operate at a first frequency, f1. The antenna pair comprises a transmitting antenna, Tx f1, and a receiving antenna, Rx f1. The antennas are shown embedded in a mat. The transmitting antenna, Tx f1, is spaced from the receiving antenna, Rx f1. For example, the transmitting antenna and receiving antenna may be spaced apart between around 20cm and 1.5m, such as 20-40cm. A second pair of antennas may also be provided in the mat which are configured to operate at a second frequency, f2, which is different to the first frequency, f1. The second pair of antennas also comprise a transmitting antenna, Tx f2, and a receiving antenna, Rx f2, which are spaced apart. The spacing of the transmitting and receiving antenna of the second pair of antennas may also be between around 20cm and 1.5m. In the embodiment shown in figure 2 the first pair of antennas, Tx f1 and Rx f1, are nested within the second pair of antennas, Tx f2 and Rx f2. Hence, the second pair of antennas may be spaced apart further than the first pair of antennas. For example, the first pair of antennas may be spaced apart by around 20cm and the second pair of antennas may be spaced apart around 40cm with the first pair of antennas arranged equidistant between the second pair. Alternatively, the antenna pairs may be interlaced together. In the arrangement shown the RF is directed from transmitter to receiver. Arrow A indicates a direction in which a person may approach the mat. The first pair of antennas form a first detection zone z1 between the transmitting antenna, Tx f1, and receiving antenna, Rx f1. The direction of propagation of the RF may be opposite to the direction in which a person walks across the mat. The second pair of antennas form a second detection zone z2 between the transmitting antenna, Tx f2, and receiving antenna, Rx f2. The two detection zones may overlap as shown in the figure, such as at a combined detection zone. Although we have described the propagation direction of the RF between the first and second pairs of antennas as being the same, in other embodiments the direction of propagation between one pair of antennas may be opposite to the direction of the other. In a further alternative, the direction of propagation for one pair of antennas may be transverse or perpendicular to the other pair of antennas. Additionally, the mat may be arranged such that the person may cross the mat, antennas and detection zones at other angles than shown in figure 2. For example, the person may cross the detection zones in a direction parallel to the longitudinal direction of the antennas. In a further different arrangement the different frequency antennas may be arranged in a side-to-side configuration and the detection zones are adjacent to each other. We have described that the antennas may be included or embedded in a mat for positioning on the floor. Alternatively, the antennas may be embedded in other floor coverings or may be embedded in the floor of the building itself such as in the concrete or other material from which the floor of the building is made. An advantage of having the antennas in a mat is that the mat can be used outdoors such as at outdoor concert venues. As mentioned, figure 3 shows an alternative configuration for the antennas. The antennas are located on opposing walls of a building, such as forming a portal arrangement. Like figure 2, figure 3 shows two pairs of antennas. For each pair of antennas one of the antennas is mounted on one wall of the entrance way of the venue, with the other antenna of each pair mounted on the opposing wall. For example, transmitting antennas of each pair, namely antennas Tx f1 and Tx f2 are mounted on one wall, and receiving antennas of each pair, namely antennas Rx f1 and Rx f2, are mounted on the opposing wall. Detection zones may be one above the other although there may be overlap. As discussed in relation to figure 2, the direction of propagation of the RF is from the transmitter to the receiver and so the direction of propagation is the same for the two antenna pairs in the embodiment of figure 3. In other embodiments, one of the antenna pairs may be reversed such that the directions of propagation are opposite for the two pairs of antennas. Arrow B shows the direction persons may enter the building, which may be transverse to the direction of propagation of the RF. Although shown as relatively high up the walls in figure 3, the antennas are preferably spaced between 50 and 150 cm from the ground such that persons carrying bags or persons carrying rucksacks will pass through the detection zones. The distance from one wall to the other wall may be from around 80cm to many metres or tens of metres and hence, this may be the distance between transmit and receive antennas of an antenna pair. In a further alternative arrangement the antennas may be mounted with the one antenna of each pair in, or attached to, an overhead structure such as a ceiling or gantry, and the other antenna of each pair mounted in, or on, the floor such as in a mat, floor covering or built into the floor structure. In this arrangement the antenna pair provides a detection zone between the floor and overhead structure. As for other embodiments, multiple pairs of antennas may be included operating at different frequencies. We have described using separate antenna pairs for each frequency. However, in another embodiment, one antenna may transmit multiple frequencies and the RF transceiver multiplexes the multiple frequencies together. A receive antenna may also receive multiple frequencies and the different frequencies may be separated using filtering. In this embodiment the antennas would need to be designed for operation at multiple frequencies. Figure 8 is a schematic diagram of the location of a detection zone with regard to the surface of a floor when the antennas are embedded in a mat, other flooring or the floor structure of the building itself. Figure 8 shows the detection zone is located in the region above the floor. The actual location of the detection zone will depend on the type of antennas used and their orientation. Figure 4a is a block diagram showing the RF supply and receiving circuitry as coupled to the antennas, with the antennas also shown. In the figure some of the functional blocks and connecting arrows are shown with dashed lines indicating they are optional. For example, the splitter 116 is optional depending on the baseline used for detecting disturbances to the RF in the detection zone(s). We first describe the arrangement including the splitter 116. A transceiver 110 is used to generate RF at the desired frequency, for example at a frequency in the range 30 MHz to 18GHz or 100MHz to 10GHz and preferably in the range 200MHz to 6GHz. The transceiver outputs the RF at the output 112 and sends it to splitter 116. In preferred embodiments a bandpass filter 114 is provided between the transceiver output 112 and the splitter 116. The bandpass filter 114 filters out any unwanted other frequencies that are output from the transceiver. Following the splitter is preferably a power amplifier 118 to increase the power of the RF supplied to the transmitting antenna 120. Transmitting antenna may be either of antennas Tx f1 or Tx f2. Splitter 116 also returns a portion of the RF output from the transceiver back to a reference input 117 at the transceiver. Also at transceiver 110 is received an RF signal at Rx Input 124 received from the receive antenna 130. The receive antenna may correspond to either of receive antennas Rx f1 or Rx f2. Preferably, the signal from receive antenna is passed through a bandpass filter 122 to the receive input of transceiver. Bandpass filter 122 filters out any unwanted frequencies that are not at, or close to, the required frequency. The portion of the output that is returned to the receiver at reference input 117 is used for comparison to the received signal at Rx input 124 to allow phase and amplitude changes resulting from the RF passing through materials in the detection zone to be detected. Commonly, low cost RF generating transmitters are not phase coherent, that is there is not a continuous phase relationship at the RF output of the transmitter with the receiver in the transceiver. Hence, to be able to compare phase and amplitude disturbances to the RF propagating through the detection zone, it is necessary to compare the received signal with the output signal via the splitter 116. The reference signal and the received signal may be compared at the transceiver and the result sent to an analyser 140. Alternatively, the analyser may receive the reference signal and the received signal and perform the comparison. The signals may be passed through an analogue-to-digital converter before sending to the analyser via a local area network. Alternatively, if a software defined radio (SDR) transceiver is used the signals are digital. The RF signals are represented by I and Q data from the RF front end of the transceiver. We describe processing of the received signal further in relation to figure 4b. However, we first describe the alternative arrangement of figure 4a in which the splitter is not included. As described already in relation to figure 4a, the transceiver outputs the RF at the output 112 and sends it to power amplifier 118. In preferred embodiments a bandpass filter 114 is provided between the transceiver output 112 and the power amplifier 118 or transmit antenna 120. The bandpass filter 114 filters out any unwanted other frequencies that are output from the transceiver. Preferably a power amplifier 118 is included before the transmit antenna 120 to increase the power of the RF supplied to the transmit antenna. Transmit antenna may be either of antennas Tx f1 or Tx f2. At transceiver 110 is received an RF signal at Rx Input 124 received from the receive antenna 130. The receive antenna may correspond to either of receive antennas Rx f1 or Rx f2. Preferably, the signal from receive antenna is passed through a bandpass filter 122 to receive input of transceiver. Bandpass filter 122 filters out any unwanted frequencies that are not at, or close to, the required frequency. Without the optional splitter 116 included no reference is returned to the transceiver at reference input 117. Hence, to be able to compare phase and amplitude disturbances to the RF propagating through the detection zone, the received signal is compared with itself in its baseline / quiescent state. The baseline or quiescent state is the signal received at the Rx input 124 when no person or object is passing through the detection zone. This may be recorded when the system is initially set and may be updated periodically when no person or object is passing through the detection zone. The baseline / quiescent signal and the received signal may be compared at the transceiver and the result sent to an analyser 140. Alternatively, the analyser may receive the baseline / quiescent state and the received signal and perform the comparison. In embodiments the transceiver may be based on the Analog Devices AD9361 or similar such as the Adalm-Pluto SDR (software defined radio) reference transceiver from Analog Devices. Such transceivers include an RF front end such as the Analog Devices AD9361 coupled with a FPGA (field programmable gate array) such as the Xilinx Zynq XC 7010 which includes discrete logic and general purpose processors based on the ARM architecture. This combination of RF front end coupled with a FPGA allows for compact transceiver implementation. Figure 4b is a schematic diagram showing how the signal received at the receiver is processed. The comparison between the received signal 124 at RX input to itself in its baseline / quiescent state or to the reference input 117 if optional splitter 116 is fitted is described above as being performed by the transceiver but this may instead be performed externally to the transceiver. In both cases the baseline / quiescent state or the reference and received signal are compared to generate a relative phase difference and relative amplitude or gain difference between the baseline / quiescent state or reference input and received signal. By comparing the received signal to the baseline / quiescent state or reference signal any phase or gain irregularities are compensated for. The gain and phase processing may also include removing residual baseline changes from the data and applying any required gain correction. As shown in figure 4b the relative phase and relative amplitude are sent to a feature analyser which may be analyser 140 of figure 4b. Other outputs may also be provided from the gain and phase processing for analysis by the feature analyser. The other outputs may include: a) A received signal strength indication (RSSI) from the RF front end chip of the transceiver, such as from the Analog Devices AD9361, if available. b) For phase measurement: i. Using the optional reference signal: i. Multiply the reference signal (Ref input from 117) by the complex conjugate of the received signal and then take the arctan of the output; or ii. Subtract the complex received signal from the complex reference signal and take the arctan of the output. ii. Without the optional reference signal: i. Take the arctan of the received signal and compare against the baseline / quiescent value. c) For amplitude or power measurement: i. Subtract the received signal from the optional reference signal and take the magnitude squared to determine the received power, which is preferable to the amplitude, which is just the magnitude; ii. Without the optional reference signal, take the magnitude squared of the received signal to determine the received power and compare against the baseline / quiescent value. ill. Taking the signal from c)i or c)ii an RMS value may be taken to include some level of smoothing. In all of the above, the absolute value is of little interest because the values are uncalibrated and the transceiver is unlikely to be a phase coherent source. It is the difference when something passes through the field and the field is disturbed that is of interest. Figures 5a-7b show measured phase results taken with a portal arrangement of two pairs of antennas. Similar results may be obtained with floor based pairs of antennas, floor-to-ceiling based pairs antennas, or overhead antennas. As described previously, the portal arrangement comprises the antennas of an antenna pair mounted on, or in, opposing walls of an opening or doorway. For the measurements of figures 5a-7b, a first pair of antennas were operating at 1160MHz with a transmit antenna on one side of the doorway and a receive antenna on the other side of the doorway. A second pair of antennas were operating at 2360MHz, again with a transmit antenna on one side of the doorway and a receive antenna on the other side of the doorway. The antennas were planar inverted F type antennas, although other types of antennas may be used such as meandering F type antennas. Other frequencies than those mentioned here may be used, but preferably are chosen not to be exact multiples of each other to avoid harmonic interference. Different materials will respond differently to different frequencies. The actual frequencies should be selected based on optimised response of disturbance to the RF transmitted through a detection zone. Figures 5a-7b show the relative disturbance to the phase at the receive antenna compared to the transmit antenna. The horizontal axis is time in seconds and the vertical axis is change in phase in radians multiplied by 100 from the baseline. The baseline is taken when no person or objects are passing through the portal. Figures 5a and 5b show the disturbance to the RF for a person walking through the detection zone. As can be seen in figure 5a some oscillation or ringing is seen with a single well-defined reasonably sharp trough at the point the person passes directly between the transmit and receive antennas. The ringing continues after the person has passed and then dies away. For figure 5b, which is taken at 2360 MHz, the ringing builds in magnitude of the phase change and again a large trough is seen and the ringing dies away. Figures 6a and 6b show similar plots to figures 5a and 5b but here the person passing through the portal is carrying a backpack containing a material to simulate homemade explosive. In this case the material is fertiliser. At 1160 MHz the plot is reasonably similar to that of figure 5a except that a point of inflexion is seen in the curve as indicated at A. The trough is also wider than in figure 5a due to the presence of a wider object passing through, namely a person with a backpack. This point of inflexion may be used as feature to indicate the presence of a material of interest. In figure 5b, which is at 2360 MHz, the ringing is more pronounced as the person approaches the portal and the trough has a more pronounced double-trough feature as shown at B. This differences between the features of figures 5 and figures 6 may be used in detecting materials of interest such as home made explosives. Figures 7a and 7b show similar plots to figures 5a and 5b but here the person is carrying a back pack containing shrapnel such as metal nails. At 1160 MHz, in comparison to figure 5a, the oscillation or ringing begins to grow as the person approaches and then at the point the person passes directly between the antennas the spike becomes a large oscillation. This increasing ringing and oscillation in the spike may be used as features for detection of materials of interest. At 2360 MHz the main difference here is that the spike does not show as a simple spike but has a large peak as well as a trough. Other frequencies may be used for detection of other materials. For example, a first RF frequency may be used to detect metal shrapnel, a second frequency may be used to detect home-made explosive, a third frequency may be used to detect commercial explosive and optionally a fourth frequency may be used to detect hard plastic shrapnel and / or a fifth frequency may be used to detect sharp stones or other material shrapnel. Hard plastic may comprise one or more of Kevlar (RTM), PLA or PET. Preferably a single frequency is used for each material and that frequency is determined to optimally detect that material. In some embodiments two frequencies may be used to detect a single material and features from the two frequencies are used in identifying the material. Feature analyser or analyser 140 on receiving the processed signals checks for the above-identified features to determine if materials of interest are present and whether a threat is detected. By using two or more frequencies, a more definite match may be obtained. If explosive and shrapnel are both detected then an alert should be sent to a user or system. If explosive is detected then an alert should still be sent. Otherwise an alert is not required. Although we have discussed measurements taken at two frequencies, other frequencies and other numbers of frequencies may be used. Other factors may also be taken into account regarding feature matching. For example, a person walking in the reverse direction to a detection zone may show ringing after the peak / trough as compared to before the peak / trough as shown in figures 5b and 6a for a person walking in a first direction to a detection zone. Another factor for consideration is that if two or more materials are present, the resulting phase change at one frequency may be a combination of the phase changes produced by the two or more materials and these may combine constructively or destructively. When the phase change for two materials of interest has the same direction (advance or retard) the combined effect is additive. When the phase changes are different then the combined effect is subtractive. This may mean that the received signal when materials are present having opposing phase changes will be lower than for either of the materials on their own. This additive effect and subtractive effect may be used as a confirmation that the two materials of interest are present. Preferably, this may be used in combination with other frequencies that also provide some identification of the presence of the materials. The responses may be different depending on how the detection zone is configured. For example, for a floor based antenna pair the response or disturbance may result from changes in coupling between the antennas as the person passes, whereas for a portal based antenna pair the changes may be largely resulting from attenuation as the person passes. We have previously described that the present invention is suitable for performing detection across wide areas such as entrances to large public venues. To achieve this multiple devices can be configured together. Previously we discussed how figure 8 shows a detection zone above a floor between a transmitter and receiver. Figure 9 shows three detection zones next to each other. Each detection zone would be formed by a transmitterreceiver pair or a plurality of transmitter-receiver pairs similar to those shown in figures 2. By arranging multiple detection zones adjacent to each a wider area can be monitored than using a single detection zone alone. Figure 10 is a schematic diagram illustrating how many pairs of antennas and detection zones may be arranged to cover an open space at the entrance to a large entertainment venue. In this figure each detection zone is formed by two pairs of antennas in a similar way to the arrangement of figure 2. In the following we shall describe the two pairs of antennas as an antenna group. In figure 10 thirty two antenna groups are provided which may cover an area of 10s of metres in width. Other numbers of antenna groups may be used to cover different areas such as 10s or 100s of antenna groups. In the embodiment of figure 10 the antenna pairs are embedded in the floor of the approach to the entertainment venue but other arrangements are possible, such as the portal of figure 3 or the floor to ceiling arrangement. For embodiments comprising multiple antenna groups the frequencies of adjacent antennas are preferably offset from each other to avoid interference. For example, for the embodiment of figure 9, the three antenna groups may be offset by 50 MHz each. Other offsets such as 20 MHz or 40 MHz may alternatively be used. Figure 11 is a schematic diagram showing how multiple adjacent systems such as those of figures 9 and 10 may be coupled together. Each pair or group of antennas is identified as a radio node in figure 11. The outputs from the radio nodes may be sent to a coordinating computer and then on to an external system which may be a system to which any alerts are sent. The coordinating computer may perform comparison of outputs from multiple radio nodes. For example, with the arrangement of antennas shown in figure 10 a person may cross one or more sets of antennas by walking transverse or diagonally to the line of antennas. The coordinating computer may be configured to analyse and combine outputs from adjacent antenna. For example, for the features described previously for feature matching some or all of the features may only provide a partial match. However, by combining results from adjacent radio nodes further matching may be performed. Although specific embodiments of the invention have been described with reference to the drawings, the skilled person will be aware that variations and modifications may be applied to these embodiments without departing from the scope of the invention as defined in the claims.

Claims

1. Apparatus for detection of a threat carried on the body of a person, the apparatus comprising:one or more pairs of antennas, each pair of antennas comprising: a transmit antenna configured to transmit RF at one or more frequencies to a respective detection zone between that pair of antennas, and a receive antenna configured to be spaced apart from the transmit antenna, the receive antenna configured to receive RF at the one or more frequencies from the respective detection zone;one or more transceivers for transmitting RF to and receiving RF from the one or more antennas at the one or more frequencies, the one or more frequencies comprising a first frequency for detecting a first material and a second frequency for detecting a second material; andone or more analysers configured to analyse signals from the receive antenna of each of the one or more antenna pairs, the one or more analysers configured to identify whether the first material is present in the respective detection zone based on characteristics of a disturbance to the RF at the first frequency and identify whether the second material is present in the respective detection zone based on characteristics of a disturbance to the RF at the second frequency, and based on determining the presence of at least the first material or the second material, providing an alert to a user or system indicative of the presence of a threat.

2. The apparatus of claim 1, wherein the detection zone comprises a first detection zone and a second detection zone, and the one or more pairs of antennas comprise:a first pair of antennas comprising a first transmit antenna configured to transmit RF at the first frequency to a first detection zone and a first receive antenna configured to be spaced apart from the first transmit antenna, the first receive antenna configured to receive first RF at the first frequency from the first detection zone; anda second pair of antennas comprising a second transmit antenna configured to transmit RF at the second frequency to a second detection zone and a second receive antenna configured to be spaced apart from the second transmit antenna, the second receive antenna configured to receive RF at the second frequency from the second detection zone.

3. The apparatus of claim 2, wherein the first detection zone comprises the second detection zone.

4. The apparatus of any preceding claim, wherein the antennas are configured to be disposed or mounted on or in a surface of a public space so as to form the detection zone(s) for the passing of persons there through.

5. The apparatus of any preceding claim, wherein the threat object is an explosive threat object.

6. The apparatus of any preceding claim, wherein the one or more analysers are configured to detect a person passing through the detection zone based on the disturbance to the RF propagated through the detection zone and identify a characteristic of the disturbance indicative of the threat object carried by the person.

7. The apparatus of claim 6, wherein the identification of a disturbance comprises analysing a time-based amplitude and / or phase disturbance of the received RF to determine a feature characteristic of the disturbance.

8. The apparatus of claim 7, wherein the feature characteristic is one or more of:an increased or reduced phase and / or amplitude;a number or timing of transients;the shape of a leading edge or a falling edge of the received signal;the amount of ringing;an inflexion point in the response; andany other distinct feature,in comparison to a received RF signal caused by the presence, or passing, of a person not carrying a threat in the detection zones.

9. The apparatus of claim 1, wherein the first material and the second material are different materials and respectively comprise any of: shrapnel such as metal, hard plastic and / or sharp stones, and explosives such as home-made explosive or commercial explosive.

10. The apparatus of any preceding claim, wherein the one or more pairs of antennas are configured to transmit and receive a plurality of frequencies for respectively detecting the presence of a plurality of materials, such as:three frequencies for respectively detecting shrapnel, home-made explosive and commercial explosive;four frequencies for respectively detecting metal shrapnel, plastic shrapnel, homemade explosive and commercial explosive; orfive frequencies for respectively detecting metal shrapnel, plastic shrapnel, sharp stones, home-made explosive and commercial explosive.

11. The apparatus of any preceding claim, wherein the one or more analysers are configured such that when the analysis of the signals from the one or more receive antennas is indicative of the presence of the first material and the first material is shrapnel such as one or more of: metal, hard plastic, sharp stones, and the analysis of the signals is indicative of the presence of the second material and the second material is one or more of: home-made explosive and commercial explosive, providing an alert to a user or system.

12. The apparatus of any of claims 1 to 10, wherein the one or more analysers are configured such that when the analysis of the signals from the one or more receive antennas is indicative of the presence of the second material and the second material is one or more of: home-made explosive and commercial explosive, providing an alert to a user or system.

13. The apparatus of any of claims 1 to 10, wherein the one or more analysers are configured such that when the analysis of the signals from the one or more receive antennas is indicative of the presence of the first material and the first material is one or more of: metal and hard plastic but the analysis of the signals does not indicate the presence of the second material and the second material is one or more of: home-made explosive and commercial explosive, providing no alert.

14. The apparatus of any preceding claim, wherein the one or more analysers are configured to:compare signals based on the transmitted RF and received RF at the first frequency to detect an amplitude and / or phase disturbance in the received signal; andcompare signals based on the transmitted RF and received RF at the second frequency to detect an amplitude and / or phase disturbance in the received signal.

15. The apparatus of claim 14, wherein the steps of comparing comprise using phase coherence of the transmitted and received signals to detect the amplitude and / or phase disturbance of the received signals.

16. The apparatus of any preceding claim, further comprising a first transceiver configured to generate RF at the first frequency, a splitter arranged to split the generated RF at the first frequency and direct a first portion of the RF at the first frequency to the first transmit antenna, and the first transceiver configured to receive a second portion of the RF at the first frequency and receive RF from the first receive antenna.

17. The apparatus of claim 16, wherein the analyser or the transceiver is arranged to compare the second portion of the RF at the first frequency and the received RF from the first receive antenna to determine the disturbance to the RF.

18. The apparatus of any of claims 1 to 15, further comprising a first transceiver configured to generate RF at the first frequency and direct the RF at the first frequency to the first transmit antenna, and the first transceiver is configured to receive RF from the first receive antenna.

19. The apparatus of claim 18, wherein the analyser or transceiver is arranged to compare the received RF from the first receive antenna against a baseline or quiescent state of received RF to determine the disturbance to the RF.

20. The apparatus of any of claims 15 to 19, further comprising a bandpass filter configured to pass the first frequency and filter out the second frequency, the bandpass filter arranged to receive the RF from the first receive antenna and after filtering send the received RF to the transceiver.

21. The apparatus of any preceding claim, wherein the first frequency and the second frequency are different frequencies and are between 30 MHz and 18 GHz or between 100 MHz and 10 GHz and preferably between 200 MHz and 6 GHz.

22. The apparatus of any preceding claim, wherein the antennas are substantially planar.

23. The apparatus of claim 22, wherein the antennas are inverted F, meandering F or other planar antennas.

24. The apparatus of any preceding claim, wherein the transmit antenna and the receive antennas are spaced apart by between 5cm and 5m or more, such as 10cm to 60cm or 2m to 3m.

25. The apparatus of any preceding claim, wherein one pair of antennas is nested between another pair of antennas.

26. The apparatus of any preceding claim, wherein the direction of propagation of RF from a first transmit antenna to a first receive antenna is parallel to, and in the same or the opposite direction to, the direction of propagation of RF from a second transmit antenna to a second receive antenna.

27. The apparatus of any preceding claim, comprising an array of antennas having multiple first antenna pairs and multiple second antenna pairs spaced transversely or parallel to the propagation direction of RF from the transmit antennas to the receive antennas.

28. The apparatus of any preceding claim, wherein the one or more pairs of antennas are printed on one or more flexible substrates and attached to, or embedded in, a foldable floor mat.

29. The apparatus of any of claims 1 to 28, wherein the one or more pairs of antennas are configured to be mounted in a floor of a building with the one or more detection zones in an area above the floor.

30. The apparatus of any of claims 1 to 28, wherein the one or more pairs of antennas are configured to be mounted in a ceiling of a building with the respective detection zones in an area below the ceiling.

31. The apparatus of any of claims 1 to 28, wherein one antenna of each pair of the one or more pairs is configured to be mounted on or in a first wall and the other antenna of each pair is configured to be mounted on or in a second, opposing wall, such that the respective detection zones are between the opposing walls.

32. The apparatus of any of claims 1 to 28, wherein one antenna of each pair of the one or more pairs is configured to be mounted on or in a ceiling, gantry or overhead structure and the other antenna of each pair is configured to be mounted on or in a floor below, such that the respective detection zones are between the floor and ceiling, gantry or overhead structure.

33. A building comprising the apparatus of any claims 29-32, wherein a plurality of adjacent detection zones are provided across the path of an entrance to the building.

34. Apparatus for detection of a threat carried on the body of a person, the apparatus comprising:a foldable floor mat having embedded therein one or more pairs of antennas, wherein:the one or more pairs of antennas comprise: a transmit antenna configured to transmit RF at a one or more frequencies to a respective detection zone between that pair of antennas, and a receive antenna configured to receive RF at the one or more frequencies from the detection zone, the mat configured such that when flat the transmit antenna and the receive antenna are spaced apart forming the detection zone there between;one or more transceivers for transmitting RF to and receiving RF from the antennas at the one or more frequencies, the one or more frequencies comprising a first frequency for detecting a first material and a second frequency for detecting a second material; and one or more analysers configured to analyse signals from the receive antenna of each of the one or more antenna pairs, the one or more analysers configured to identify whether the first material is present in the respective detection zone based on characteristics of a disturbance to the RF at the first frequency and identify whether the second material is present in the respective detection zone based on characteristics of a disturbance to the RF at the second frequency, and based on determining the presence of at least the first material or the second material, providing an alert to a user or system indicative of the presence of a threat.

35. The apparatus of claim 34, wherein the foldable floor mat comprises a plurality of pairs of antennas forming a plurality of spatially adjacent detection zones.

36. A building comprising apparatus for detection of a threat carried on the body of a person, the building having a concourse or entrance configured to simultaneously receive a plurality of persons there through, comprising:one or more pairs of antennas mounted in or to one or more surfaces of a wall, floor, overhead structure or ceiling of the building, wherein:each pair of the one or more pairs of antennas comprises a transmit antenna configured to transmit RF at one or more frequencies to a detection zone and a receive antenna configured to receive RF at the one or more frequencies from the detection zone, the antennas mounted in or to the one or more surfaces such that the transmit antenna and the receive antenna are spaced apart forming the detection zone there between in a space adjacent to or between the one or more surfaces;one or more transceivers for transmitting RF to and receiving RF from the one or more pairs of antennas at the one or more frequencies, the one or more frequencies comprising a first frequency for detecting a first material and a second frequency for detecting a second material; andone or more analysers configured to analyse signals from the receive antenna of each of the one or more antenna pairs, the one or more analysers configured to identify whether the first material is present in the respective detection zone based on characteristics of a disturbance to the RF at the first frequency and identify whether the second material is present in the respective detection zone based on characteristics of a disturbance to the RF at the second frequency, and based on determining the presence of at least the first material or the second material, providing an alert to a user or system indicative the presence of a threat.

37. The building of claim 36, further comprising a plurality of pairs of antennas forming a plurality of adjacent detection zones arranged for monitoring the passage of multiple persons through an entrance or concourse simultaneously.

38. A method of detecting a threat object carried on the body of a person, the method comprising:generating RF at one or more frequencies, the one or more frequencies comprising a first frequency for detecting a first material and a second frequency for detecting a second material;transmitting the RF at the one or more frequencies from one or more transmit5 antennas to a respective detection zone to one or more receive antennas spaced apart from a respective one or more transmit antenna;receiving at the receive antenna RF at the one or more frequencies from the respective detection zone; andanalysing signals from the receive antenna to identify whether the first material is 10 present in the respective detection zone based on characteristics of a disturbance to theRF at the first frequency and identify whether the second material is present in the respective detection zone based on characteristics of a disturbance to the RF at the second frequency, and based on determining the presence of at least the first material or the second material, providing an alert to a user or system indicative of the presence of a 15 threat.

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