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(11) | EP 0 371 562 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Coil antenna device |
| (57) A loop antenna device (10) for a two-frequency electromagnetic detection system is
disclosed. The device serves to generate an electromagnetic interrogation field for
detecting responders comprised by the system. The device comprises a closed loop built
up of four adjoining sections having substantially equal impedances, the junction
between each pair of adjoining sections together (11, 12, 13, 14) with the junction
between the complementary pair of adjoining sections forming a connection port so
that two connection ports (31, 33, 32, 34) are provided which in operation are available
for two independent signals having different frequencies. |
[0001] This invention relates to a loop antenna device for a two-frequency electromagnetic detection system, such device serving to generate an electromagnetic interrogation field for detecting responders comprised by said system.
[0002] Loop antennas for electromagnetic detection systems, serving to generate an interrogation field for detecting responders associated with the system are known in the art. One known antenna of this kind is, for example the single loop antenna, the antenna coil of which has the shape of an O. Another known loop antenna is eight-shaped. An eight-shaped loop antenna has the advantage over and above the O-shaped antenna that it has a minimum sensitivity to interfering external radiofrequency signals. As a consequence, the risk of picking up spurious signals is quite small. Also, with an eight-shaped antenna coil, the radiation to the so-called distant field is minute, so that there is little chance of interference with other transmitting and/or receiving set-ups.
[0003] One disadvantage of the known loop antennas is that they are only suitable for one single pre-determined signal frequency or for a single pre-determined signal frequency range.
[0004] For two-frequency detection systems, however, it is desirable to have an antenna that is suitable for two different frequencies (or frequency ranges). Two-frequency systems are used, for example, to detect attempts at tampering with a responder. In such a system, the first signal frequency serves to detect the presence of a responder in an interrogation zone, and the second signal frequency serves to detect fraud with a responder. Such a system is described, for example, in US-A-4,742,341. It is also possible, to use a first frequency for detection purposes and a second frequency for the transfer of energy to a responder, for example, for charging a power accumulator of a responder.
[0005] According to the prior technique, antennas for two frequencies are produced by using for example two antenna coils which are alternately connected in a parallel and a non-parallel configuration for given periods of time. The field of operation of the antenna is changed periodically between two predetermined frequencies or frequency ranges.
[0006] When two frequencies are used and the system is alternately adjusted to one and the other frequency, each change in frequency (range) will cause a transitional period during which a responder cannot be detected, or fraud with a responder cannot be detected, or a power accumulator of a responder cannot be charged. Also, special switching means are required.
[0007] It is an object of the present invention to overcome the disadvantages outlined above and, in general, to provide an effective and simple loop antenna device for a two-frequency electromagnetic detection system.
[0008] For this purpose, according to the present invention, a loop antenna device of the above kind is characterized by comprising a closed loop built up of four adjoining sections having substantially equal impedances, the junction between each pair of adjoining sections together with the junction between the complementary pair of adjoining sections forming a connection port so that two connection ports are provided which in operation are available for two independent signals having different frequencies.
[0009] It is noted that the term "detection system" as used herein includes identification systems, which have responders detected similarly to the responders of detection systems. The only difference between responders of an identification system and those of a detection system is in fact that the responders of an identification system when detected can additionally be individually recognized by a coded signal.
[0010] One embodiment of the invention will now be described, by way of example, with reference to the accompanying schematic drawings. In said drawings,
Fig. 1 illustrates a known O-shaped antenna coil;
Fig. 2 shows a known eight-shaped antenna coil; and
Fig. 3 shows an embodiment of a loop antenna device according to the present invention.
[0011] Fig. 1 shows schematically an O-shaped antenna coil 1 with two terminals 2 and 3. Such an antenna coil, after being tuned with, for example, a suitable capacitor, is only suitable for a single frequency or a single frequency range. Also, an O-shaped antenna is sensitive to spurious signals.
[0012] Fig. 2 illustrates schematically an eight-shaped antenna 4 with two superimposed, substantially congruent loops 5 and 6 and two junctions 7,8 located on an axis of symmetry 9 of these loops. Such eight-shaped antennas are rather insensitive to interference by external radio signals and provide minimum radiation in the distant field. These eight-shaped antennas, however, are also only suitable, after being tuned, to function at a single pre-determined frequency or a single predetermined frequency range.
[0013] Fig. 3 shows schematically an embodiment of an antenna configuration according to the present invention. The loop antenna device shown comprises a loop 10, which is square in this example, and has four vertices A, B, C and D. From the vertices, branches 11,12,13 and 14 extend inwardly, coinciding with the diagonals of the square and terminating in two sets of connection points 31,33 and 32,34, each set of connection points forming a connection port.
[0014] From the square shown, an eight-shaped antenna can be constructed in two ways. Unlike the eight-shaped antenna of Fig. 2, there are now two axes of symmetry joining the vertices of the square. The axes of symmetry are perpendicular to each other. The two eight-shapes in the square are turned through 90° relative to each other. The first antenna consists of two halves, i.e., triangle ABC and triangle ADC, which halves are separated by axis of symmetry AC. Rotation through 90° gives the second antenna, consisting of halves BCD and BAD with axis of symmetry BD.
[0015] As will be described below, the eight-shaped antennas are not electrically coupled together. The operation of the antenna configuration is as follows. The first eight-shaped antenna is fed by connecting a source of A.C. voltage between terminals or connection points 31 and 33, which together form a connection port. Through the application of the A.C. voltage, a current I₁ will start to run. At the vertices A and C, this current will be equally distributed over branches ABC and ADC. As the impedances of the antenna sections AB, BC, AD and DC are all identical, the potentials in points B and D will be equal to each other. As a consequence, there can be no difference in voltage between connection points 32 and 34. Also, no voltage can be generated through induction. Normally, in the antenna sections formed by the conductor between connection points 32 and vertex B (branch 13) and the conductor between connection point 34 and vertex D (branch 14), a voltage would be induced by the magnetic fields which, in turn, are generated through the voltage applied between connection points 31 and 33 and the currents flowing as a consequence. However, by reason of the fact that branches 13 and 14 are located in the plane of symmetry, no voltage is induced in the antenna sections referred to.
[0016] If, similarly, a voltage is applied between connection points 32 and 34, then, as a result of the symmetrical construction, no voltage differential is formed between connection points 31 and 33. Therefore, if a voltage is applied to the four connection points, two antennas are active which, electrically, are fully uncoupled from each other.
[0017] The two antennas can generate different resonant frequencies by means, for example, of capacitors. Thus, for example, a capacitor not shown can be applied across connection point 31 and 33, as a result of which the resonance frequency of the first eight-shaped antenna will assume a value of 2 MHz, while across connection points 32 and 34 a capacitor can be provided which leads to a resonance frequency of, for example, 8 MHz of the second eight-shaped antenna. As the antennas are not electrically coupled together, the resonance frequencies can be freely selected.
[0018] Accordingly, the antenna device illustrated in Fig. 3 forms two electrically independent antennas which, however, mechanically are virtually fully integrated and essentially use the same conductors.
[0019] It is noted that, in principle, connection points 31,33 and 32,34 could be closer to vertices A.C and B.D respectively, or even coincide with them. In that case, except for the location of the connection points, the two antennas are physically fully identical.
[0020] Furthermore, sections AB, BC, CD and DA may have any curved and/or bent shape. The important feature is that the impedances of the sections should be equal, and that the magnetic fields caused by the A.C. currents which in operation flow through two successive sections (e.g. AD and DC) eliminate each other at an intermediate branch (e.g. 14,13).
1. A loop antenna device for a two-frequency electromagnetic detection system, said
device serving to generate an electromagnetic interrogation field for detecting responders
comprised by said system, characterized by comprising a closed loop built up of four
adjoining sections having substantially equal impedances, the junction between each
pair of adjoining sections together with the junction between the complementary pair
of adjoining sections forming a connection port so that two connection ports are provided
which in operation are available for two independent signals having different frequencies.
2. A loop antenna device as claimed in claim 1, characterized in that the pair of
junctions associated with at least one connection port are located in one and the
same plane of symmetry of the closed loop.
3. A loop antenna device as claimed in claim 2, characterized in that the junctions
of at least one pair of junctions located in one and the same plane of symmetry link
up with antenna branches located in said plane of symmetry and forming connection
points at their ends.
4. A loop antenna device as claimed in claim 3, characterized in that the antenna
branches extend from the junctions to a point near the centre of the loop.
5. A loop antenna device as claimed in any of claims 2-4, characterized in that the
closed loop has two planes of symmetry each containing two antenna branches extending
from junctions located within the plane of symmetry to a point near the centre of
the loop.
6. A loop antenna device as claimed in any of the preceding claims, characterized
in that the closed loop has the shape of a square and has antenna branches extending
along the diagonals to a point near the centre of the square, where the ends of the
antenna branches form two connection ports.
7. A loop antenna device as claimed in any of the preceding claims, characterized
in that each connection port is connected to a tuning capacitor.
8. A two-frequency electromagnetic detection system, characterized by a loop antenna
device as claimed in any of claims 1-7.