GB2391710A - Antenna system - Google Patents

Abstract

An antenna system suitable for the detection of livestock carrying coding electromagnetic signal emitters, the system comprising first and second antenna loops for positioning adjacent a detection path, e.g. a livestock race, the first of the loops (1) lying along one side of the detection path, and the second of the loops (2,3) having arch portions (3) which traverse above or below the detection path, and side portions (2) lying alongside the detection path in the form of wings angled relative to the arch portions, such that a three-dimensional detection field is produced without the need for a third antenna, a phase shift between the loops, or loop multiplexing.

Description

ANTENNA SYSTEM

This invention relates to an antenna system for transmitting signals to energise microchips in transponders. The antenna proposed may also be used as a 5 receiver for the response signal emitted by transponders when they are energised. Preferred aspects relate to systems adapted for use in identifying livestock, and to their use for that purpose.

BACKGROUND

10 Antenna systems are used in a wide variety of situations. They are used in security systems in high street shops or for identifying animals in slaughterhouses or markets. The idea behind these systems is the same: the antenna transmits an 15 electromagnetic field over a certain area. Transponders

containing microchips are energised when their circuits lie across (i.e. at an angle to) field lines of the

electromagnetic field. Energising the transponder causes

it to emit a response signal. The same or another 20 antenna can detect the response signal and identify the transponder which has entered the area. In the case of shop security systems, electronic tags are attached to items of clothing. When these tags are detected by the antennas positioned at the shop exit, an alarm sounds.

25 Similarly, cattle can be given ear tags or carry stomach boluses with transponders that are energised by antennas mounted on the side of narrow passageways (races) through which they are herded. Thus the animals entering the

cattle market or slaughterhouse can be identified. This is important for the control of disease.

The present invention relates to an antenna for use in identifying cattle. In particular, the antenna; 5 provides an efficient and accurate means for detecting ear tags. A known method of detecting ear tags is to: mount a loop antenna on or in the side wall of a race such that the plane of the loop is parallel to the race.

The antenna is set up as a series resonant circuit; when 10 current flows through the antenna, an electromagnetic: field is set up whose field lines extend around the edges -

of the antenna giving a toroidal shape. As the loop is mounted on the side wall of the race, the field lines

extend generally across the race as shown in Fig. 10.

15 When a microchip in a transponder cuts those field lines

(i.e. lies across or move through them), a current is i induced in it, i. e. it is energised.: An alternative way of arranging the antenna is to have the plane of the loop perpendicular to the direction 20 of the race. The antenna loop is of sufficient size so i that the cattle can walk through it: this is known as a: portal antenna. In this arrangement the field lines t

extend in the direction of the race.

It is important in cattle identification that no 25 animals are missed. The systems described above have the disadvantage that there is a significant chance the transponder will not be energised because the microchip will be at the wrong angle. If the microchip is parallel

( to the field lines, i.e. it does not cut lines of

different electromagnetic strength, then no current will flow in it and it will not be energised. This is more of a problem with ear tags than with boluses, because the 5 ear tags are moved around when the cattle toss their heads. The advent of stricter controls on cattle movement following the BSE and foot and mouth scares, means that it is now vital that all the cattle passing through races are identified. If one is missed, then all lO of the cattle must be passed through the race again because there is no way of telling which animal was missed, which is time consuming and therefore costly.

A number of proposals have been made to provide an antenna which will detect a transponder whatever its 15 orientation. GB 1599120 shows a first example, where two loop antennas were used, mounted with their planes perpendicular to one another. Each loop produced its own field, nominally perpendicular to each other. However,

each field also affected the other due to mutual

20 inductance, with the result that significant 'dead zones' i where there was no or only a small field occurred.

US 4679046 teaches an alternative method, where a loop formed as a figure of eight is surrounded by a normal loop, the two loops being coplanar. This 25 arrangement achieved the same effect as two perpendicularly positioned antennas, but still suffered from the problem of 'dead zones' due to mutual inductance between the loops. US 4679046 offered two solutions to

the problem of 'dead zones': the first was to multiplex the two loops; the second was to introduce a phase shift between the currents in each loop to create a circularly polarised field. The latter arrangement is complicated

5 because finding the correct phase shift can be difficult, and the phase shift circuit requires more components, thus added expense. Moreover, US 4679046 uses two antennas to provide a field acting substantially in only

two dimensions, therefore there exists a particular 10 orientation which is only weakly covered. Thus a transponder in this orientation still stands a significant chance of not being detected.

Multiplexing involves supplying one loop at a time with power in a short cycle so that the net effect over 15 time is a field in both directions. This solves the

problem of mutual inductance, but adds further problems in terms of the complexity of the system circuit and lost' time as explained below. Thus, it is desirable to provide a single phase antenna that is capable of 20 providing full coverage for all orientations of i transponder. As mentioned above, a further limitation of known arrangements is that they only provide antennas whose fields are strong in two dimensions, i.e. the third

25 dimension is only weakly covered; transponders aligned perpendicularly to the other fields (i.e. along their

field lines) may not be detected. The solution proposed

to this in US 4679046 is the introduction of a third

antenna, with its plane perpendicular to the other antennas such that its field lines are in the weakly

covered dimension. This is undesirable because of the cost involved in adding another component, especially if 5 it too must either be multiplexed or given a phase shift, and because of the extra power required to give the system enough coverage.

Enough power needs to be provided to the antennas to give their fields enough range to cover the race. In

10 addition to the cost of providing power, the power available for antennas is limited by laws governing radio frequency radiation emission. The antennas divide the power between them; therefore more antennas need more power to achieve the same range. Thus, it is desirable 15 to use fewer antennas because of the saving in terms of components and costs.

SUMMARY OF THE INVENTION

The aim here is to provide an antenna which with two parts is capable of providing a field with components in

20 three dimensions, i.e. able to detect transponders at any orientation. A preferred feature of the invention is that it provides fields of comparable strength in three

dimensions with only two antenna parts. A novel arrangement of the two parts relative to one another 25 allows the antenna to provide the field without the need

for a phase shift to be introduced between the parts. In other words, the arrangement of the two parts is such that the effects of mutual inductance provides an overall

field with a very wide range of components. This means

it is effective for and efficient at detecting a transponder in any orientation.

We propose an antenna system for use in the 5 detection of objects (e.g. animals carrying transponders) moving on a detection path relative to the antenna system. The system comprises two antenna parts, a first part being a side loop located adjacent the detection path with its plane in the longitudinal direction of the 10 path; and a second part being a loop having longitudinally extending sections adjacent both sides of the detection path joined by arch portions which traverse the detection path.

The longitudinally extending parts may be in the 15 form of wings on each side of the detection path with two transverse sections spanning the detection path to join corresponding parts of the wings. The wings may be formed from at least one longitudinal portion connected to one or both of the transverse sections by at least one 20 riser portion.

The wings may be U-shaped. The longitudinal portion may be the base of the U. and the riser portion being the arms. Said riser portions may extend downwards relative to the detection path, and said transverse sections may 25 extend under said detection path. Alternatively, the U may be a smooth curve.

The detection path may be defined by a passageway for animals, and the second antenna part may be wrapped

( over the passageway with the longitudinally extending portions located in or adjacent side walls of said passageway. The second antenna part may be in the form of a saddle straddling the passageway.

5 Tile antenna parts are preferably coils of wire, preferably wound in opposing directions. In other words, the longitudinally extending part of the second antenna part adjacent the side loop is wound in the opposite direction to the bottom of the side loop.

10 An exemplary antenna system comprises two loops of wire, a first loop lying in a single plane, and a second loop having two e-shaped end sections and a middle section, the end sections being bent out of the plane of the middle section) wherein the two loops are located 15 adjacent each other, the plane of the first loop being substantially perpendicular to both the planes of the n's and the plane of the middle section. Preferably, the planes of the e-shaped end sections are parallel to each other. More preferably, the planes of the n's are 20 substantially perpendicular to the plane of the middle section. The height of at least one e-shaped end is preferably less than the height of the flat loop, more preferably between one quarter and three quarters of that height. Preferably, the part of the second loop adjacent 25 the first loop is in the plane of the first loop. This gives the field of the first loop maximal range across

the second loop.

( Preferably, the loops comprise a single cable wound in the shape of the loop several times. Preferably, a low resistance wire is used. More preferably, twin core cable e.g. speaker cable is used. When connected to an 5 AC power source, each loop may form part of a series resonant circuit, whereby the transmitted signal is produced when the current moves through the coils making up the antenna. i The antenna may be attached to a tuning module which 10 tunes the capacitance and/or inductance of the circuit so that its resonant frequency matches the frequency of the current. The connecting wires are preferably coaxial, e. g. ethernet cables. Coaxial cables have minimal field

emission; it is desirable in the antenna system to 15 concentrate the field to the loops - the connecting wires

should not lose power by emitting unnecessary fields. A

tuning module can be used to adjust the values of capacitance and/or inductance to compensate for the capacitance and inductance present in the wires 20 connecting the power source to the antenna. As with any arrangement where two field producing coils are placed in

proximity to each other, the fields are affected by

mutual inductance between the coils. We find that in antenna systems as described here, the mutual inductance 25 has little or no negative effect on the fields to produce

weak patches of field; in some single phase systems, e.g.

in US 4679046 when the phase shift is not used, mutual inductance causes the two fields to interfere with each

( other, leaving 'dead zones, where there is little field

strength. In these 'dead zones', there is little chance of energising a transponder. The present invention overcomes this problem by the particular shape of antenna 5 used. For this reason, it is also preferable to wind the wires making up the two loops in opposite directions.

Thus, viewing the antenna from one side, if the first loop is wound clockwise, then the second loop is wound such that the portion nearest the first loop (comprising 10 a leg of each n and a joining wire of the middle section) opposes it, i.e. it is wound anticlockwise. This is illustrated in Fig. 8.

The present invention uses two loop antennas to achieve a field acting in all three dimensions; previous

15 systems required an additional third antenna, but the configuration of the present system provides a three dimensional field without the need for phase shift. In

other words, we can provide a tri-axial single phase antenna comprising two loops which provide an 20 electromagnetic field capable of detecting all

orientations of transponder without needing to introduce a phase shift between the loops or multiplex them.

In livestock markets there is a need to move a large number of animals through an identification system.

25 Typically, e.g. cattle are identified by driving them through narrow races which allow only one animal through at a time. To speed up this process, several races may be used at once.

( Presently, there are two types of transponder: half duplex (HDX) and full duplex (FDX). HDX transponders require silence (i.e. no transmittance by antenna) to respond and be detected, whereas FDX transponders respond 5 by effectively reflecting the transmitted signal. These two requirements are obviously conflicting. However, it is desirable to have an antenna system capable of detecting both types. To do this efficiently whilst avoiding conflict requires an intelligent system that can 10 react to responses it receives. Such intelligent arrangements are known, but a problem arises when one tries to apply them to antenna systems which try to cover two or more dimensions using e.g. two separate antennas (e.g. US 1599120). In this situation, the antennas may 15 issue conflicting instructions, e.g. if the top antenna wanted to listen for an HDX signal but the side antenna wanted to measure an FDX signal. The advantage of the present system is that, because it can be considered as a single antenna detecting all orientations of 20 transponders, no conflicts like that outlined above occur, so the intelligent arrangements for efficiently detecting both HDX and FDX transponders are readily applied to it. The result is an antenna system which has a greatly improved accuracy and efficiency in detecting 25 transponders. Low frequency (e.g. RF) signals are preferably used in the detection of transponders, but the antenna system described herein would work equally well at all

frequencies. 10w frequencies are preferred because they travel through walls and animal tissue more easily. This is well-known.

Aspects of the invention include an antenna system 5 as described, optionally combined with one or more of corresponding tuning module, power source, transponder reader system (which may be converted, or which may adopt the special configuration described: this is an independent proposal herein). Also, a method of 10 detection in which the antenna is used to excite transponders on the detection path and the signals from the transponder are detected. This may be a method of monitoring livestock, for example, and the antenna system may be correspondingly adapted for mounting on/over the 15 barriers of a livestock passage or race.

An embodiment of the invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 shows a perspective view of a simple loop 20 antenna; Fig. 2 shows a plan view of the antenna of Fig. 1; Fig. 3 shows a perspective view of an antenna system which is an embodiment of the invention; Fig. 4 is a plan view of the antenna of Fig. 3; 25 Fig. 5 is an end view of the antenna of Fig. 3; Fig. 6 is a side view of the antenna of Fig. 3; Fig. 7 is a perspective view of the antenna positioned in a race;

Fig. 8 shows an example of how the loops are wound in the antenna of Fig. 3; Fig. 9 shows a schematic circuit diagram for the antenna of Fig. 3; 5 Fig. 10 is a perspective view of a loop antenna located in a side wall of a race.

DETAILED DESCRIPTION OF THE INVENTION

The antenna of the present invention use a particular arrangement and shape of wound coils to give 10 an electromagnetic field suitable for energising

transponders at any orientation. To illustrate the effect of field radiation from a current loop, Fig. 1

shows a simple loop antenna. The loop is made of several turns of cable and is connected in series to a tuning 15 module and a power supply (not shown). This is then a series resonant circuit capable of tuning by the tuning module (which can vary the inductance and/or capacitance of the circuit compensate for inductance and/or capacitance). When current flows through the antenna 20 (e.g. at resonance), an electromagnetic field is

produced. The field resembles a doughnut with the

antenna as its skeleton. Figs. 1 and 2 show typical field lines. The size of the current affects the size of

the doughnut, thus the curvature of the lines through the 25 loop can be reduced by increasing the current. Thus a loop aerial could be mounted on the side of a race with its plane parallel to the direction of the race having high enough current so that the field lines through its

( centre extend horizontally across the race. This was the idea behind known antennas used for cattle identification. The antenna shown in Fig. 3 is made up of two 5 rectangular loops: a flat loop 1 and a bent loop 2r3.

The bent loop has two e-shaped ends 3 bent perpendicular to a middle section comprising joining wires 2 which extend perpendicularly to the plane of both ends 3 to connect corresponding legs of the n's. Both loops are 10 made of turns of twin core speaker wire. Usually, three turns are used, but in principle, any number of turns may be used. The antenna is formed by positioning the loops adjacent each other so that the plane of the e-shaped ends 3 and the plane formed by the joining wires 2 are 15 both substantially perpendicular to the plane of flat loop 1. The plane formed by the legs of ends 3 and joining wire 2 closest to the flat loop 1 is actually in the plane of the flat loop 1. The loops are wound in opposite directions as shown in Fig. 8. If flat loop 1 20 is wound clockwise then the bent loop is wound such that the part of it closest to the plane of the loop is wound anticlockwise. In other words the joining wire closest to the flat loop 1 is wound in the opposite direction to the bottom part of the flat loop 1.

25 Fig. 9 shows a schematic circuit diagram for the antenna. The antenna is connected to AC power source via tuning module 5. In practice, the tuning module 5 is positioned as close to the antenna as possible to

minimise the power lost in the cables joining it to the antenna. The connecting wires 6 are ethernet coaxial cables. Flat loop 1 and bent loop 2,3 are connected in parallel over the power source 4, but each form a series 5 resonant circuit tuneable by the tuning module 5.

Figs. 3-6 shows schematic representations of the fields produced by the antennas when the antenna system

is powered. Similarly to the plain loop of Figs. 1 and 2, the flat loop 1 provides field lines 7 across the

10 antenna in a first direction from through its middle.

The bent loop 2,3 provides field lines in two mutually t

perpendicular directions 8,9 both of which are perpendicular to the first direction. The joining wires 2 act as if there is a loop in their plane, so produce 15 field lines 8 extending upwards through the antenna

system. Likewise, e-shaped ends 3 both act like loops to -

produce field lines 9 extending into the antenna system

in directions substantially perpendicular to the other field lines 7,8. Each field affects the other wires,

20 i.e. mutual inductance occurs, but the arrangement of the system means the fields cooperate to provide an overall i

field of complex shape having components in all

directions within the box-like frame formed by the bent loop 2,3.

25 Fig. 7 shows how the antenna would be used in a cattle market. A race 10 for guiding an animal 11 is shown. The antenna is positioned so that the flat loop 1 is located in a side wall of the race with its plane

( parallel with the direction of the race. Thus the field

lines 7 it produces laterally traverse the race from left to right as viewed in Fig. 7. The bent loop then spans the race, i.e. the joining wires 2 are located in the 5 side walls of the race running parallel to the direction of the race, and the tops of the e-shaped ends 3 traverse the race above the head of the animal 11. Thus, the antenna is positioned so that the animal 11 must pass through the box-like frame of the bent antenna 2,3 where 10 the complex electromagnetic field is present. Therefore,

an ear tag containing a transponder will be exposed to field lines in all directions, so will be energised and

emit a response signal whatever its orientation.

Claims (10)

- 1b Claims

1. An antenna arrangement suitable for the detection of livestock carrying coded electromagnetic signal emitters, the 5 system comprising first and second antenna loops for positioning adjacent a detection path, the first of the loops lying along one side of the detection path, and the second of the loops having arch portions which traverse above or below the detection path, and side portions lying alongside the 10 detection path in the form of wings angled relative to the arch portions.

2. An antenna system according to claim 1 in which the wings of the side portions of the second loop comprise respectively 15 at least one longitudinal portion, connected to one or both of the transverse arch portions by at least one riser portion.

3. An antenna system according to claim 2 in which each wing of the second loop has a riser portion at each end of the 20 longitudinal portion.

4. An antenna system according to any one of the preceding claims in which each of the first and second loops comprises a single cable with plural windings around the loop.

5. An antenna system according to any one of the preceding i claims in which the loops are of twin-core cable.

6. An antenna system according to any one of the preceding 30 claims, comprising means for providing an AC power source with which each loop forms part of a series resonant circuit.

7. An antenna system according to claim 6 comprising a tuning module to which the loops are connected by coaxial 35 cable.

( - ll

S. A method of detecting and/or identifying electromagnetic emitters, or objects associated with electromagnetic emitters, passing along a detection path, characterized by the use of an 5 antenna system as defined in any one of claims 1 to 7 adjacent that detection path.

9. A method according to claim 8 which is a method of detecting and/or identifying livestock.

10. A method according to claim g in which the animals pass one by one through a passageway having side walls.

ll. A method according to claim 9 or 10 in which the animals 15 carry coded transponders, e.g. in ear tags or in stomach boluses. Hi i 1 '