Passive entry x-y-z transponder antenna

Abstract: In a transponder unit adapted to communicate with an interrogation unit by transmitting RF signals to and receiving RF signals from the interrogation unit, a transponder antenna comprising an elongated tubular first ferrite antenna coil (12), oriented to transmit and receive RF signals a first direction parallel to the barrel of said coil; an elongated tubular second ferrite antenna coil (14), oriented at a different angle from said first coil and adapted to transmit and receive RF signals in a second direction parallel to the barrel of said coil; and a substantially flat, planar air coil (10) adapted to transmit and receive RF signals in a direction perpendicular to the plane of said coil, and oriented at a different angle from said first and said second coils.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to a transponder unit adapted to communicate with an interrogation unit by transmitting RF signals to and receiving RF signals from the interrogation unit. BACKGROUND OF THE INVENTION Such transponder units in combination with interrogation units are appropriate for many applications, including many in which either the transponder or communication unit is mobile, as where either the transponder unit or the reader unit, or both, are carried by a person, in an automobile or other mobile vehicle, or attached to an animal. When used in these types of applications, the transponder unit should be able to effectively receive and transmit RF carrier signals when it is oriented in any of a variety of directions and distances relative to the interrogation unit. In EP-A-0 689 301 a transponder unit is described which is provided with two antennas having a dipole-type pattern and being oriented in two orthogonal directions with respect to each other as far as the direction of their pattern maxima are concerned. In this transponder unit a switch is provided for connecting the receiving part of the unit to one or the other of the two antennas depending on the level of the received RF signals. This means that only one of the two antennas is active so that no true omnidirectional pattern exists. The transponder unit "scans" the environment by switching from one of the antennas to the other. The switching requires additional control circuitry for evaluating the level of the received signals and for generating a control signal by which switching from one antenna to the other can be effected. The transponder unit disclosed in the before-mentioned document is not provided with any means for generating its own supply voltage from the received RF signals. In DE-A-29 48 033 an antenna system is disclosed which includes two elongated ferrite antennas oriented with respect to each other in two orthogonal directions so that an omnidirectional pattern is achieved. However, the antenna coils are coupled with each other so as to provide a critically coupled band pass filter. This antenna system is intended for use in a receiver for receiving time signals use for synchronizing radio controlled clocks. SUMMARY OF THE INVENTION There is a need for a transponder unit which can effectively transmit and receive RF signals from a remote interrogation unit when these units are oriented in many different directions and distances relative to each other. In accordance with the invention a transponder unit of the type described above is provided which is characterized by the features of the characterizing part of claim 1. Due to the use of three antenna coils the transponder unit can effectively receive RF signals irrespective of the orientation of the transponder unit with respect to the interrogation unit. Moreover, by use of the resonant tuning circuits and the diodes DC voltages can be derived from the received RF signals, which DC voltages are superimposed and added in order to construct a single supply voltage for the transponder unit. BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is a graph showing one orientation of an antenna according to the present invention; FIG. 2 depicts an antenna arrangement according to the present invention; FIG. 2A depicts an alternative embodiment of an antenna arrangement; FIG. 2B depicts another alternative embodiment of an antenna arrangement; FIG. 3 depicts antenna output circuits connected in series; FIG. 3A is a block diagram of the circuit connected in series; FIG. 3B is a block diagram of an alternative embodiment having output circuits connected in series; FIG. 4 depicts antenna output circuits connected in parallel; FIG. 4A is a block diagram of the circuit connected in parallel; FIG. 4B is a block diagram of an alternative embodiment having output circuits connected in parallel; FIG. 5 depicts a reactance modulator circuit for use withe present invention; and FIG. 6 depicts an active oscillator circuit for use with the present invention. DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a graphical representation of one arrangement of antennas according to the present invention. As can be seen from the graph of FIG. 1, one embodiment of the invention comprises an air coil 10, shown here oriented in the Y direction, and one or more ferrite coil antennas. FIG. 1 shows two such ferrite coil antennas 12 and 14, oriented in the X and Z directions, respectively. The ferrite antennas are therefore oriented approximately 90 degrees relative to each other, and from the air coil 10. The ferrite antennas 12 and 14 may be of any conventional construction, such as antennas used in conventional automobile-type transponders. It will be understood that although the invention will be described below with reference to an embodiment in which the antennas are oriented approximately ninety degrees relative to each other, the precise orientation and alignment of the antennas may be adjusted or modified as needed to accommodate differing conditions and circumstances. It will be appreciated by those in the art that certain applications may exist in which satisfactory performance may be achieved with the antennas oriented at angles other than ninety degrees relative to each other. The individual air coil 10 and the ferrite antenna coils 12 and 14 have a dipole-type pattern with maxima and minima. It is a technical advantage of the present invention that, by orienting the antennas in this arrangement, and by coupling the antennas as described hereinbelow, a transponder unit is able to effectively receive and reconstruct RF carrier signals which are oriented in any direction relative to the transponder with minimal degradation. In conventional single antenna arrangements having dipole-type patterns with maxima and minima projected in a single plane or axis, the strength of a received RF signal would vary according to whether the antenna was properly aligned in the plane of the signal. Even if the antenna were located adjacent a maxima of the carrier signal, the antenna's ability to receive the signal accurately could be reduced significantly if the antenna were improperly oriented relative to the plane of the signal. Accordingly, a transponder antenna constructed according to the present invention is suitable for a wide range of applications for which conventional antenna arrangements are inadequate, such as applications in which either the transponder or reader unit, or both, are moveable relative to each other, including, for example, when the transponder or reader units are mounted on vehicles or carried by human beings. In an alternative embodiment, one air coil antenna may be combined with one ferrite antenna coil in order to provide directionality in two planes, e.g., X + Y, X + Z, or Z + Y. Alternatively, more than three antennas may be used as necessary for improved directionality. FIG. 2 illustrates an example of three antennas 10, 12 and 14 mounted within a transponder unit in order to achieve the arrangement depicted in the graphical representation of FIG. 1. As shown, ferrite coil antennas 12 and 14 are oriented with the longitudinal axis of the coils oriented in the X and Z directions, respectively. The direction of flux or transmission for ferrite coil antennas lies along this longitudinal axis (in a direction relative to the coil indicated by the "right-hand rule", or parallel to the "barrel" of the generally tubular coil), so that orienting the coils 12 and 14 along these axes will cause the coils 12 and 14 to transmit and receive RF carrier signals along the X and Z axes, respectively. The air coil 10 is positioned within the transponder unit as shown in FIG. 2. Air coil 10 is a generally flat planar coil which operates such that the direction of flux (the direction in which signals are transmitted from and received by the coil) is perpendicular to the plane of the coil. As can be seen from FIG. 2, it is a technical advantage of the shown implementation of the present invention that the generally flat planar shape of air coil 10 permits antennas 10, 12 and 14 to be arranged so that the ferrite coils 12 and 14 lie in generally the same plane as air coil 10, so that the antenna arrangement occupies very little space. Since the arrangement depicted in FIG. 2 does not occupy a significant amount of space in the Y-direction, the coil arrangement of the present invention therefore may be used in smaller and thinner (for example, hand-held) transponders than can conventional antenna arrangements, while still enabling the transponder to effectively transmit and receive omnidirectionally. FIGs. 2A and 2B illustrate alternative embodiments of the present invention in which only one ferrite coil antenna is provided in combination with the air coil 10. FIG. 2A shows the air coil 10 aligned in the Z-direction, in combination with one ferrite coil antenna aligned in the Y-direction. FIG. 2B shows an air coil 10 aligned in the Z-direction in combination with a ferrite air coil aligned in the X-direction. Of course, the alignment of the antennas shown in FIGs. 2, 2A and 2B could be altered without departing from the scope of the present invention. FIGS. 3 and 4 illustrate arrangements of resonant tuning circuits that may be used with the antennas depicted in FIG. 2. In general, each resonant tuning circuit comprises an inductor, a capacitor, and a diode. Each of the antennas 10, 12 and 14, are coupled independently to a separate resonant tuning circuit, and the three resonant tuning circuits are coupled together either in series, as shown in FIG. 3, or in parallel, as shown in FIG. 4. FIG. 3 shows three resonant tuning circuits coupled in series. Each resonant tuning circuit comprises an inductor and a capacitor, and the three resonant tuning circuits are coupled together with diodes. Points 20, 22 and 24 indicate where an oscillator and/or modulator circuit (described below) may be coupled when the transponder antenna is adapted to transmit signals. FIG. 3A is a block diagram depicting the antenna arrangement when the circuits are coupled in series. Alternatively, if only one ferrite antenna coil is used in combination with an air coil, as in FIGs. 2A or 2B, the circuit of FIG. 3 could be easily altered in a manner apparent to those in the art by omitting one of the resonant tuning circuits. FIG. 3B is a block diagram depicting the antenna arrangement when two circuits are coupled in series. FIG. 4 shows three resonant tuning circuits coupled in parallel. As is the case with FIG. 3, each resonant tuning circuit comprises an inductor and a capacitor, and the three resonant tuning circuits are coupled together with diodes. Points 20, 22 and 24 indicate where an oscillator and/or modulator circuit (described below) may be coupled when the transponder antenna is adapted to transmit signals. FIG. 4A is a block diagram of an antenna arrangement when the circuits are coupled in parallel. Alternatively, if only one ferrite antenna coil is used in combination with an air coil, as in FIGs. 2A or 2B, the circuit of FIG. 4 could be easily altered in a manner apparent to those in the art by omitting one of the resonant tuning circuits. FIG. 4B is a block diagram depicting the antenna arrangement when two circuits are coupled in parallel. A similar antenna arrangement may also be provided in a reader unit, in order ro provide omnidirectionality at a remote location. In operation, when the transponder is used to receive RF carrier signals transmitted from a remote location, the three antennas 10, 12 and 14 operate to independently receive components of the carrier signal oriented in the Y, X and Z directions, respectively. The received signals are passed to the resonant tuning circuit connected to each antenna. The series arrangement of the resonant tuning circuits shown in FIG. 3 act to superimpose and add the DC voltage components of the received carrier signal, to reconstruct the signal components into a single DC voltage signal representative of the signal which was transmitted from the remote location. The parallel arrangement of the resonant tuning circuits shown in FIG. 4 operates such that the highest induced voltage either supplies all RF circuits, or supplies only the RF circuit having the highest individual voltage. In this manner, an RF carrier signal transmitted from a remote location may be accurately reconstructed even if the signal is oriented in a peculiar manner relative to the transponder. In addition, the use of an air coil 10 and two ferrite coils 12 and 14 oriented in the manner described above enables the entire arrangement to be positioned inside a very small space, so that the antenna arrangement herein described may be used in a small, hand-held or pocket-sized transponder. When the transponder is used to transmit RF signals, each of the antennas 10, 12 and 14 may be independently coupled either to a modulated oscillator, or to a passive reactance modulator. FIG. 5 shows a reactance modulator circuit configured in a full-duplex, back scatter mode, that may be coupled to each of the antennas 10, 12 or 14, and FIG. 6 shows a circuit for a modulated active oscillator which may also be used. In one arrangement of the present invention, a similar antenna arrangement may be provided in a reader unit which transmits an omnidirectional energy field to the transponder unit. The transponder unit uses the received energy to generate a data signal, either by modulating a RF signal onto the received field via the passive reactance modulator of FIG. 5, or by receiving the energy and operating an active oscillator as shown in FIG. 6, and utilizing the received energy to project its own field. Alternatively, since reader units typically require more power than transponder units, the reader unit may be provided with two or three air coil antennas oriented in different directions, instead of a single air coil in combination with one or two ferrite coil antennas.

Claims: A transponder unit adapted to communicate with an interrogation unit by transmitting RF signals to and receiving RF signals from the interrogation unit, characterized by an elongated tubular first ferrite antenna coil (12) having a dipole-type pattern oriented to transmit and receive RF signals in a first direction, said first direction being in line with the direction of the maxima of the dipole-type pattern, an elongated tubular second ferrite antenna coil (14) having a dipole-type pattern oriented at a 90° angle from said first coil and adapted to transmit and receive RF signals in a second direction, said second direction being in line with the direction of the maxima of the dipole-type pattern, a substantially flat, planar air coil (10) adapted to transmit and receive RF signals in a direction perpendicular to the plane of said coil and oriented at a 90° angle from said first and second directions, each of said antenna coils being coupled to a circuit comprising a resonant tuning circuit and a diode for generation a DC voltage representative of the RF signal received by each said antenna coils and being used for constructing a transponder supply voltage, said tuning circuits being coupled together. The transponder unit of claim 1, wherein said resonant tuning circuits are connected in series and coupled together with the diodes. The transponder unit of claim 1, wherein the resonant tuning circuits are connected in parallel and are coupled together with the diodes. The transponder unit of any of claims 1 to 3, further comprising a first output circuit coupled to said first ferrite antenna coil and a second output circuit coupled to said air coil. The transponder unit of claim 4, wherein said first and second output circuits comprise passive reactance modulators. The transponder unit of claim 4, wherein said first and second output circuits comprise active oscillators.

Inventors: SCHUERMANN JOSEF H, SCHUERMANN JOSEF H., SCHüRMANN JOSEF H.
Applicants: TEXAS INSTRUMENTS DEUTSCHLAND, TEXAS INSTRUMENTS DEUTSCHLAND GMBH
Application year: 1997
Main classification: H01Q21
IPC codes: H01Q7, G07C9, H01Q21