MXPA97000953A - Transmitter and recept tie antenna - Google Patents

Abstract

The present invention relates to a far field canceling antenna consisting of: an electrical circuit element, and a first antenna structure, the first antenna structure consists of: a first loop element electrically coupled to the circuit element to generate fields, and a branch loop element surrounding the first loop element so that the voltages are induced in the branch loop element from the fields generated by the first loop element, the branch loop element consists of a conductive loop continuous to maximize the current from the voltages induced in it, further characterized because the current in the derivative delayed element generates fields that largely cancel in the far field, the fields generated by the first element of the

Description

LRZO ANTENNA TRANSMITTER AND RECEIVER CRYPE OF THE INVENTION The present invention generally relates to antennas and more particularly to loop antennas that generate fields that are generally cancellers at distances of a wavelength or more from the antenna.

BACKGROUND OF THE INVENTION In certain known types of electronic systems, particularly those designed for electronic article surveillance (EAS), provision is made for a mixed antenna consisting of two or more antennas coupled together and to which signals are supplied from a transmitter to produce a surveillance zone consisting of an induction field adjacent to the mixed antenna, which is strong enough to detect the presence of predetermined types of objects near the antenna. In general, EAS systems include both a transmitting antenna and a receiving antenna, which collectively establish a surveillance zone, and labels that are affixed to items that are protected. The transmitting antenna generates an electromagnetic field within a scale of a predetermined frequency. The labels each include a resonant circuit having a predetermined resonant frequency generally equal to the first frequency. When one of the tags is present in the surveillance zone, the field generated by the transmitting antenna induces a voltage in the resonant circuit on the tag, which causes the resonant circuit to generate an electromagnetic field, causing a disturbance in the field inside. of the surveillance zone. The receiving antenna detects the disorder in the electromagnetic field and generates a signal indicating the presence of the label (and in this way, the protected article attached to the label) in the surveillance zone, to avoid the production of electromagnetic fields. Given that they are relatively strong that can interfere with the operation of another electronic device, it is desirable to design such EAS systems so that the network effect of the radiated fields at remote antenna positions (generally thirty, .__. eters) is substantially zero, or at least insufficient to cause any serious problems To provide the desired far field cancellation, the construction of the mixed antenna with a plurality of loops is known, the planes of which are substantially parallel and adjacent, but displaced from each other , and in which the current flow of the transmitter is adjusted in phase and amplitude in different loops, so that the fields produced by the ties are essentially added to zero. By using such a mixed antenna it has been discovered that it is possible to provide a far-field cancellation by + e the appropriate choice of the cross-sectional areas and number of turns in several loops. Although such mixed antennas provide far-field cancellation, the orientation of the magnetic fields due to the respective loop antennas are essentially constant at any point in the space close to the antennas. For example, in the case where two ties of an equal area are ^ lean + re yes on the plane of the loops so that a current path in figure 8 is described (ie, a loop having a right-hand current direction and the other having a left-hand current direction) and where the number of turns in the loops is equal, there will be a zone in the second plane perpendicular to the plane of the loops and pass through a midpoint between the loops where the field orientation is perpendicular to the second plane. However, essentially no field will exist in any direction within the second plane. In an EAS system, this substantially reduces the coupling probabilities and substantially inhibits the coverage of the surveillance zone. The present invention provides an antenna that includes at least one active loop to generate and respond to localized fields close to, and preferably within a longer passive loop, which cancels eubstantially between the far fields generated by the active azo or those remote fields to which the active loop is responsive. By providing a repaired loop to reduce the far field coupling, the active loop can be conducted by transmitting a circuit with a relatively high current, while satisfying regulatory requirements for far field radiation. This allows an antenna with relatively long fields in more than one orientation within close proximity to the antenna. It also allows a separate receiving antenna, which is highly sensitive to externally emitted signals, to be placed close to passive and active loops. An antenna of the present invention can be used to improve systems where simultaneous transmission and reception occurs, or where it is desired to trace different elements of the antenna in different phases and / or frequencies. Providing different phases allows the antenna to be tightly coupled in more than one direction at a point. The antenna can be used in an EAS system and provides larger fields in all possible orientations within the 'closest possible proximity to the antenna, thus causing the tag to respond, and simultaneously providing an antenna lifting pattern that is also uniformly sensitive to the radio frequency (RF) so that the responses of the tag can be felt.

BRIEF DESCRIPTION OF THE INVENTION Briefly described, the present invention is a "far field canceling antenna consisting of an electrical circuit and a first antenna structure." The first antenna structure consists of a first loop element electrically coupled to the circuit element. that, when the circuit element is a transmitter, it can be used to generate fields and a branch loop element surrounding the first loop element so that the voltages are induced in the branch loop element from the fields generated by The first loop element The branch loop element consists of a continuous conductor loop to maximize the currents from the voltage induced in it The currents in the branch loop element generate fields that cancel out largely in the far field, the fields generated by the first loop element The present invention also provides a field cancellation antenna an anode consisting of an electrical circuit element, a first loop element electrically coupled to the circuit element, a branch loop element surrounding the first loop element and second loop element located next to both the first loop element and to the branch loop element. The branch loop element largely cancels the fields generated by the first loop element in the far field when the circuit element is a transmitter. The second loop element can be positioned so that a coupling between the second loop element and both the first loop element as the branch loop element is minimized. The branch loop element consists of a continuous-conductive loop to maximize currents from a voltage induced therein.

BRIEF DESCRIPTION OF THE DRAWINGS The above description, as well as the following detailed description of the preferred embodiments of the present invention, will be better understood when read in conjunction with the accompanying drawings. For the purpose of illustrating the present invention, embodiments which are presently preferred are shown in the drawings. It should be understood, however, that the present invention is not limited to the particular arrangements and instrumentalities shown in the drawings. Figure 1 is a schematic diagram of a far field canceling antenna in accordance with a first mode of the present invention; Figure 2A is a schematic diagram of a branch loop element in accordance with the present invention; Figure 2B is a schematic diagram of a loop element in figure 8 in accordance with a second embodiment of the present invention; Figure 2C is a schematic diagram of a loop element in figure of 0 in accordance with the present invention; Figure 2D is a schematic diagram of a far field canceling antenna in accordance with a second embodiment of the present invention; Figure 3 is a schematic diagram of a far field canceling antenna in accordance with a third embodiment of the present invention; and Figure 4 is a schematic diagram of a far field canceling antenna in accordance with a fourth embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Some terminology is used in the following description solely for convenience and is not limiting. The words "upper part", "lower part", "lower part" and "upper part" designate directions in the drawings to which reference is made. The terminology includes the words mentioned above specifically, derived from them and words of similar importance. The present invention is directed to an antenna that can transmit and simultaneously receive electromagnetic energy at one or more frequencies within a predetermined frequency range., wherein the size of the antenna may be less than the wavelength of the electromagnetic energy transmitted and received by the antenna. The antenna e may also be used primarily as a transmitting antenna or primarily as a receiving antenna to create a rotating field. The antenna of the present invention is well suited for use in systems where it is desirable to transmit and / or simultaneously receive electromagnetic fields in close proximity (i.e., less than half a wavelength) to the antenna. An example of such a system is an electronic article surveillance (EAS) system in which the antenna is used to establish a surveillance zone. Referring now to the drawings in detail, where similar numbers indicate similar elements therethrough, an electrical schematic diagram of a far field canceling antenna 10 according to a first preferred embodiment of the invention is shown in FIG. This invention is for generating and / or coupling electromagnetic fields. Preferably, the antenna system 10 is operative at radio frequencies, which preferably include frequencies above 1,000 Hz, and most preferably include frequencies above 5,000 Hz, and even rnuy preferably include frequencies above 10,000 Hz. However, it should be understood that the Antenna system 10 can be operated at lower frequencies without departing from the scope of the present invention. The antenna 10 includes at least a first active antenna loop element 12, an electrical circuit element 14 and a branch loop element 16. The first antenna loop 12 consists of a loop generally rectangular in shape, which is electrically coupled to, and driven by * a transmitting circuit in the case of a transmitting antenna, and which is electrically coupled to, and conducts a receiving circuit in the case of a receiving antenna. For illustrative purposes, the first loop element 12 is shown generally rectangular in shape and with one stream flowing through the first loop element 12 in a clockwise direction as indicated by the stream flow arrow 18. Other sizes, Loop shapes or configurations can be employed and the current will flow in the opposite direction if desired. The first loop element 12 consists of a configuration of cable or conductors to carry current and? generate fields so that the generated fields are not canceled in the far field. The far field of an antenna is an area of one or more wavelengths outside the antenna. For an antenna to operate at 8.2 NHz, the Federal Communications Commission (FCC) defines the far field as an area 30 meters or slightly smaller than a wavelength from the antenna. In the present invention, the first loop element 12 generates electromagnetic fields in the far field. Although the first loop element 12 is shown to consist of a one-loop loop, it will be apparent to those skilled in the art 10 that the first loop element 12 may consist of a multiple loop element, such as a field canceling configuration. far from a figure of 8 of three loops (not shown) which is the functional equivalent of a one-loop loop that generates far fields. A current flowing through the first loop element 12 establishes a magnetic field with a magnetic flux extending concentrically from at least a portion of the first loop element 12 and generally perpendicular to the current flow 18 as is well known in the art. In the presently preferred embodiment, the dimensions of the first loop element 12 are generally smaller than the wavelength of the electromagnetic field generated by the current flowing through the first loop element 12 divided by 2? R, so that the electric fields generated are largely independent of the magnetic fields generated. The electrical circuit element 14 may consist of a current source electrically coupled to the first loop element 12 to supply current to the first loop element 12 and which is capable of supplying sufficient current to the first loop element 12 to develop the fields above mentioned electromagnetic energy. The electrical circuit element 14 may be a conventional transmitter consisting of a signal oscillator (not shown) and a suitable amplifier / filtering network (not shown) of a type capable of driving the load impedance. presented by the first loop element 12. As it will be appreciated, the frequency at which the first loop element 12 radiates electromagnetic fields depends substantially on the oscillation speed of the transmitter. In this way, the frequency can be set and adjusted, appropriately fitting the transmitter in a well-known manner. Alternatively, the circuit element 14 may consist of receiving circuits electrically coupled to the first loop element 12 to receive electromagnetic energy from a transmitting antenna and / or the resonant circuit of a tag (not shown) to generate an indicator signal whether or not a label is present in the vicinity of the first loop element 12. Electrical circuit elements of the type used in the present invention for transmission and / or reception are generally known. Such circuit elements are , describe for example, in the patent of E.U.A. No. 5,373,301, designated to Chec point Systems, Inc., of Thorofare, New Jersey, the description of which is incorporated herein by reference. A more detailed description of the electrical circuit element 14 is not required to understand the present invention.

The branch loop element 16 consists of a loop of generally continuous rectangular shape. The branch loop element 16, although shown as generally rectangular in shape, may consist of other shapes and folds, such as the circular one. The branch loop element 16 is not electrically connected to either a transmitter circuit or a receiver circuit. Instead, the branch loop element 16 is a "passive" loop. The first loop element 12 and the branch loop element 16, in the present embodiment, are generally in the same plane, the first loop element 12 preferably being located within or surrounded by the branch loop element 16. The first Tie element 12 interacts with the Tie-in element 16 mainly through a mutual magnetic coupling between the elements 12 and 16. The current flowing through the first loop element 12 in the direction indicated by the arrow 18 induces a flow of current in the branch loop element 16 in a direction opposite to the direction of current flow in the first loop element 12, as indicated by the current flow arrow 19. As will be appreciated, the magnitude of the flow The magnetic radiation radiated by the antenna loop corresponds to the current flowing through the antenna loop, multiplied by the area of the antenna loop. The branch loop element 16 is a continuous-conductive loop (i.e. a short circuit) to maximize the opposite phase current induced in the branch loop element 16 from the first loop element 12. In this way, co o it will be understood by those skilled in the art, the size (area) of the shunt loop element 16 is determined based on the required magnitude of the current induced in the shunt loop element 16 so that the electromagnetic fields generated by the element of branch on branch 16 cancel substantially the electromagnetic elec fields generated by the first loop element 12 in the far field. Note that by providing the lead loop element 12 where an area of the lead loop element 16 is larger than an area of the first loop element 12, it provides a good far field cancellation. • _. Together, the first loop element 12 and the branch loop element 16 consist of a first antenna structure in which the currents in the branch loop element 16 generate fields that largely cancel out the fields generated by the first loop element 12 in the far field. It has been found that such a structure, ie, the first loop element 12 essentially located within the longest passive loop or passive loop element 16 consists of a far field canceling antenna. It has also been found that other far field canceling antennas, co or the antenna in figure 8 can be incorporated within this structure so that a mutual coupling between the structure and the antenna in figure 8 is minimized or essentially zero. The first loop element 12 and the branch loop element 16 each preferably consist of a conductor or cable of any suitable type. However, it will be appreciated that other conductive elements, such as a multi-conductor cable, may be used, if desired, without departing from the scope of the present invention. For example, it may be desirable to use mechanically functional structural elements to fabricate the first loop element 12 and the branch loop element 16. Alternatively, electrically conductive decorative elements may be used. Referring now to Figures 2a to 2d, a second mode of a far-field canceling antenna 20 is r, • shown (Figure 2D) together with the component parts of the antenna 20 (Figures 2A-2C). Fig. 2A is a schematic diagram of a branch loop element 16 for use in the antenna 20. Fig. 2C is a schematic diagram of a first active loop element 12 for use in antenna 20 and Fig. 2B is a schematic diagram of a second loop element 22 for use in the antenna 20. Together, the first loop element 12 and the branch loop element 16 comprise a first antenna structure, as shown in Fig. 1, wherein the fields generated by the voltages induced in the branch loop element 16 cancel the fields generated by the first loop element 12 in the far field. Again, although the first loop element 12 is shown as being one loop or conductor loop, it will be understood by those skilled in the art that the first loop element 12 may have more than one loop. Referring now to Figure 2B, the second loop element 22 is essentially a figure loop of 8 having an upper loop 24 and a lower loop 26. The second loop element 22, by itself, consists of an antenna canceller far field, as described in the aforementioned US patent »No. 5,373,301. As shown, the shape of the second loop element 22 has been modified so that the area of the upper loop 24 and the area of the lower loop 26 are smaller than what would normally be for a loop in conventional figure 8, and the loops 24, 26 are spaced apart or offset from each other. This configuration is solely for convenience, since it will be understood that the second loop element 22 may also consist of a conventional Figure 8 loop or a differently modified Figure 8 loop. For example, the second loop element 22 can have more than two turns or loops 24 and 26 and the second loop element 22 can be either symmetric or asymmetric. That is, the upper and lower loops 24 and 26 need not necessarily be of an equal area, nor should the lower and upper loops 24 and 26 be connected in series. Alternatively, the upper and lower loops 24 and 26 can be connected in parallel, or through an electrical network to introduce phase deviations between the two loops 24 and 26. However, an important aspect of the second loop element 22 is , in general, minimize the fields generated in the far field when configured as a transmitter. The second loop element 22 also includes connection points 28a and 28b for connecting the second loop element 22 to an electrical circuit element (not shown). For example, the second loop element 22 can be connected to a transmitter circuit and / or a receiver circuit. The connection points 28a and 28b are shown as being close to the geometric center of the second loop element 22, and this location is generally optimal. However, it will be understood that connections can be made at other points along the - * • second element of Loop 22 .. Referring now to Figure 2D, a schematic diagram of the far field cancellation antenna 20 is shown in accordance with the second embodiment of the present invention, including each of the component parts shown in Figures 2A-2C. The first loop element 12 is shown connected to a transmitter 30 to generate a current flowing through the first loop element .12. He ; first loop element 12 is located within the branch loop element 16 and the first loop element 12 and the branch loop element 16 has a size and are arranged so that a coupling between the first loop element 12 and the second branch loop element 16 is suitable for obtaining the far-field cancellation effect described above. The second loop element 22 is positioned with respect to the first loop element 12 and the branch loop element 16 so that a magnetic coupling between the second loop element 22 and the loop elements 12 and 16 is minimized. In effect, the first loop element 12 generates fields that are coupled to the branch loop element 1.6, but operates independently of the second loop element 22. In the presently preferred embodiment, the first loop element 12 is located near the center of the loop. bypass element 16 and is generally in the same plane as the branch loop element 16, and the second loop element 22 is located next to and parallel to the plane of the first loop element 12 and the loop element in derivation 16, but is spaced from these elements so that none of the loop elements 12, 16, 22 is in direct electrical contact. Also, the first loop element 12 and the branch loop element 16 are substantially magnetically decoupled from the second loop element 22. The relationship * specific spatial is one in which the loop elements overlap or partially overlap each other to the point that the network flow generated from the spiral of one of the loops is substantially zero within the area of the spiral of the other loop and vice versa. The decoupling occurs when the second loop element 22 is superimposed on the first loop element 12 and is spaced from the branch loop element 16. By overlap, no attempt is made to say that the loops are in contact or touch each other, but only that the bonds are in a particular spatial relationship with each other. That is, the 'loops are in parallel planes and have a common area so that a plane perpendicular to the loops in the common area intersects each of the loops that are in "superimposed" relationship .. As will be appreciated by those experts in the technique,? n coupling coefficient exists between the first loop element 12 and the second loop element 22. In the presently preferred embodiment, the coupling coefficient is less than 0.5 and most preferably, the coupling coefficient is less than 0.1. It has been found to be very advantageous to allow the first loop element 12 and the second loop element 16 to operate essentially independently of the second loop element 22. For example, the first loop element 12 can be driven by a transmitter circuit. with a relatively high current, compared to a loop in figure of 0 isolated, satisfying at the same time the requirements of the FCC or another regulatory agency for far-field radiation. At the same time, the second loop element 22 can be connected to a receiver circuit 32 at connection points 28a and 28b. This allows the antenna 20 to provide simultaneous transmission and reception in a single coupling or structure. Alternatively, the first loop element 12 may be connected to an electrical network 34 to introduce phase deviations between the current of the first loop element 12 and the second loop element 22. Preferably, the currents are conducted 90 ° out of the phase with each other. By introducing phase deviations between the current drawn through the first and second loop elements 12 and 22, a rotating field is generated close to the antenna 20. A one-turn loop, like the first loop element 12, generates a perpendicular field and a loop in figure 8, as the second loop element 22 generates a field parallel to the antenna 20 to a far point from the antenna 20. If the phase deviation between the conducted current in the first loop element 12 and the second loop element 22 is 90 ° apart, the orientation of the fields generated by the loops 12 and 22 is horizontal to vertical, and in this way, a rotating field in the regions near the center of the antenna 20, approximately in a plane perpendicular to the place of the antenna 20. creating a rotating field increases the detection in a monitoring system that incorporates the antenna 20 (ie, increasing the number of orientations where a good coupling can be achieved).

In this way, the antenna 20 of Figure 2D is desirable for use in an EAS system because the sensitivity of the antenna 20 provides improved tag detection. In an EAS system, the first loop element 12 can be electrically coupled to the transmitter circuit 30 and the electrical network 34 to generate fields that create a surveillance zone. At the same time, the branch loop element 16 generates fields that cancel the fields generated by the first loop element 12 in the far field. The second loop element 22 can be electrically coupled to the receiver circuit 32. When a label (not shown) or an article attached to the label enters the surveillance zone, the label is irradiated by the fields generated by the first loop element. 12. The tag, in turn, resonates at a predetermined frequency, which is detected by the second loop element 22. The receiver circuit 32 then generates a signal indicating the presence of the tag in the surveillance zone. Typically, the separation in an EA system between the transmitting antenna and the receiving antenna is in the range of from 0.610 to 1.83 meters, depending on the EAS system in particular and the particular application in which the system is employed. In the present invention, the first antenna structure (i.e., the first loop element 12 and the branch loop element 16) and the second loop element 22 are generally co-located, or located within a single structure, and establish a monitored area or surveillance zone close to them. In general, the surveillance zone is located at, or near, an exit or entrance to an installation (not shown) but can be at any other location, either on each side or within an exit check corridor. It will be appreciated by those skilled in the art that although in the illustrated embodiment, the antenna 20 includes a transmitter circuit 30 connected to the first loop element 12 and a receiver circuit 32 connected to the second loop element 22, which are generally co-operative. located, ie on the same side of the surveillance zone, there are other EOS systems well known to those skilled in the art in which the transmitter and the receiver are separated by a predetermined distance to establish the surveillance zone. Accordingly, the particular antenna 20 and / or the configuration described in Figure 2D can be suitably modified. For example, a first receiver antenna (consisting of the first loop element 12, the branch loop element 16 and the second loop element 22) and a second transmitter antenna, generally identical to the receiving antenna, but spaced apart from the antenna. It can be used in an EOS system to establish a surveillance zone. Figure 3 is a schematic diagram of a third embodiment of a far field canceling antenna system 50. The antenna 50 includes a first electrically active loop element 12 connected to a transmitting circuit 30. The transmitting circuit 30 generates a flowing current through the first * loop element 12 to generate electromagnetic fields. A branch loop element 16, which is of a larger area than the first loop element 12, is positioned proximate to, and preferably surrounding the first loop element 12 to cancel the fields generated by the first loop element 12 to a far distance from the first loop element 12 (ie, far fields). As previously described, the current flowing in a betting direction to the flow of the current in the first loop element 12 is induced in the branch loop element 16. The branch loop element 16 has a size and is positioned in such a way that the induced current in the lead time element 16 generates fields that substantially cancel out the fields generated by the first loop element 12 in the far field. A monitoring zone is created because the first loop element 12 and the loop element in., which is close to the first loop element 12 and the branch loop element .16. The antenna system 50 also includes a loop element in figure 8 52, electrically coupled to a receiver circuit 32. The loop element in figure 8 52 includes? N upper loop 54 connected to a lower loop element 56. In the presently preferred embodiment, the upper loop element 54 and the lower loop element 56 are of a generally equal area and are spaced or offset from each other. The loop element in figure 8 52 detects the presence of a tag (not shown), as described in relation to figure 2D, which enters the surveillance zone. The tag responds to nearby magnetic fields generated by the first loop element 12. The loop element in figure 8 52 receives the response of the tag from which an alarm signal can be generated indicating the presence of a tag in the surveillance zone. The loop element in figure 852, itself, is a far field canceling antenna, as a current flowing in the upper loop 54 is equal in magnitude to the current flowing in the lower loop. , but it flows in an opposite direction. Referring now to Figure 4, there is shown a schematic diagram of a fourth embodiment of a far field canceling antenna 36 in accordance with the present invention. In the fourth embodiment, a loop element of a figure 8 8 with a cross bar 40 divides the loop element 38 into two loops, an upper loop of a turn 42 and a lower loop of a turn 44. The crossbar 40 includes connection points 46a, 46b for connecting an electrical circuit element thereto. Either a transmitter circuit or a receiver circuit, as previously described, can be connected between the connection points 46a and 45b. In the modality presently - preferred, the cross bar 40 is at a relative angle to the sides of the loop element 38. By orienting the cross bar 40 at a relative angle to the sides of the loop element 38 it helps to create a coupling at vertical angles. It will be understood by those skilled in the art that the actual angle of the cross bar 40 can be adjusted to various degrees depending on the desired performance and the requirements for the application of the antenna 36. The upper and lower loops 42 and 44 of the loop element in figure 8, 38 are shown as generally the same in area. As is well known, providing equal-area loops helps cancel far-field coupling. However, the upper and lower loops 42 and 44 do not need to be of the same area as other geometries that provide the desired deletion properties in the far field can be used. The perimeter of the loop element 38 consists of a rectangular loop that can be used as a branch loop, to be inductively coupled to an active loop, such as the first loop element 12. The first loop element 12 is connected to an electrical circuit element, such as the transmitting circuit 30 previously described for developing a current in the first loop element 12. In the presently preferred embodiment, the first loop element 12 is superimposed on, but not in electrical or physical contact with the loop element in form of 8 38. That is to say, that the first loop element 12 / - has a size and is positioned so that when driven by a voltage or current source, a current flowing through the first element 12 induces a voltage at the perimeter of the loop in the figure of 8 38, and causes a corresponding current flow in the loop perimeter in figure 8 38, with the objective that a sum of the fields generated by the first loop element 12 and the perimeter of the loop in figure of 8 38 are in opposite directions or phases and Widely cancel each one in the far field. As will be appreciated, higher or lower cancellation can be achieved depending on? * > relative size and positioning of the first loop element 12 with respect to the loop in figure of 8 30. Although particular embodiments of the present invention have been described, it will be apparent that the present invention may be altered or modified, and still provide cancellation of desired far field. For example, although the first loop element 12 has been described as a single loop, the first loop element 12 may consist of a configuration of "" - three loops having poor far field canceling properties due to asymmetry reasons or phase of the currents, however, when located next to or within the loop element in branch 16, so as to have a coupling between the components of the near field of the active loop 12 and the loop in branch 16, a field cancellation occurs far away, although the antenna of the present invention is described < - in the present with reference to the EAS systems, it will be appreciated that such reference to the EAS systems is provided for illustrative purposes only and is not limiting. The antenna of the present invention is well suited for use in many other types of applications, and more particularly, without application in any area in which the electromagnetic energy and radiated by the antenna is used to carry out a communication or a identification function. For example, the antenna of the present invention can be used in conjunction with a sensor (which is energized by the electromagnetic energy transmitted by the antenna) in an environment where it is difficult to energize or otherwise communicate with the sensor to through cables connected to the sensor. In this environment, the antenna can be used to power and receive information remotely from the sensor. For example, the antenna of the present invention can be used in conjunction with a sensor that measures the blood sugar level of a patient, in which the blood sugar sensor is subcutaneously implanted in the tissue of a patient . As will be appreciated, it is highly desirable that the patient's skin not be pierced with wires to connect the sensor. It is also highly desirable to remove the batteries from the sensor. With the present invention, it is possible to use the electromagnetic energy generated by the antenna to power the sensor located under the skin of the patient and to simultaneously use the antenna to receive the electromagnetic energy transmitted by the sensor, when the electromagnetic energy transmitted by the sensor refers to the blood sugar level of the patient. Another application is related to communicating with a passive transponder that identifies its owner for access control. Other useful applications of the present invention will also be made apparent to those skilled in the art. Accordingly, it will be appreciated that changes and modifications may be made to the embodiments described above without departing from the inventive concept thereof. Therefore, it is to be understood that the present invention is not limited to the particular embodiments described, but is designed to include all modifications and changes that are within the scope and spirit of the invention, as defined by the appended claims.

Claims (19)

NOVELTY OF THE INVENTION CLAIMS

1. - A far field canceling antenna consisting of: an electrical circuit element; and a first antenna structure, the first antenna structure consists of: a first loop element electrically coupled to the antenna 'circuit element to generate fields; and a branch loop element surrounding the first loop element for the voltages to be induced in the branch loop element from the fields generated by the first loop element, the branch loop element consists of a continuous conductor loop for maxirnize the current from the voltages induced in it, further characterized because the current in the branch loop element generates cancellable fields 'Widely in the far field, the fields generated by the first loop element.

2. The antenna according to claim 1, further characterized in that the size of the antenna is substantially less than an operating wavelength of the antenna so that the antenna mainly generates magnetic fields.

3. The antenna according to claim 1, further characterized in that the branch loop element and the first loop element are partially and magnetically coupled together.

4. The antenna according to claim 1, further characterized in that the circuit element consists of a transmitter.

5. The antenna according to claim 1, further characterized in that the circuit element consists of a receiver.

6. The antenna according to claim 1, '- further comprising a second loop element located next to the branch loop element so that the coupling between the second loop element and the first antenna structure is minimized.

7. The antenna according to claim 6, further characterized in that the second loop element consists of? N antenna loop in figure of 8 generally flat, the antenna loop in figure of 8 has an upper loop ^ and a lower tie.

8. The antenna according to claim 7, further characterized in that the upper loop and the lower loop are connected in series.

9. The antenna according to claim 7, further characterized in that the upper loop and the lower loop are connected in parallel.

10. The antenna according to claim 6, further comprising an electrical network, further characterized in that the first loop element and the second loop element are connected through the electrical network, the electrical network introduces phase deviations between the first loop element and the second loop element, thus causing a rotating field to exist near the antenna.

11.- The antenna in accordance with the claim 7, further characterized in that the second loop element is electrically connected to a receiver circuit and the electrical circuit connected to the first loop element consists of a transmitter circuit, so that the antenna provides a simultaneous transmission and reception.

12. The antenna according to claim 7, further characterized in that the upper loop and the lower loop are offset from each other.

13. The antenna according to claim 7, further characterized in that the upper loop and the lower loop are of an equal area.

14.- The antenna in accordance with the claim 7, characterized in that the upper loop and the lower loop are symmetrical.

15.- The antenna in accordance with the claim 1, further characterized in that the bypass loop element includes a transverse bar, the transverse bar divides the bypass loop element into two loops, the two loops being connected in the transverse bar.

16.- The antenna in accordance with the claim 15, further characterized in that the first loop element is centered within the branch loop element.

17.- A far-field canceling antenna consisting of: an electrical circuit element; a first loop element electrically coupled to the circuit element for generating fields; a branch loop element surrounding the first loop element for the voltages to be induced in the branch loop element from the fields generated by the first loop element, the loop element -'- in derivation consists of a loop continuous conduit for maximizing the current from the voltages induced therein, further characterized in that the current in the branch loop element generates fields canceling widely in the far field, the fields generated by the first loop element; a second loop element located proximate the branch loop element so that the coupling between the second loop element and the first loop element and the branch loop element is minimized.

18. The antenna according to claim 17, further characterized in that the electrical circuit connected to the first loop element consists of a transmitting circuit, and the second loop element consists of an antenna loop in a generally flat figure 8, the The loop in figure 8 has an upper loop and a lower loop, and further characterized in that the second loop element is electrically connected to a receiver circuit so that the antenna provides a transmission in simultaneous reception.

19. - In an electronic article surveillance system, a far-field canceling antenna, the antenna consists of: a first loop element; a transmitting circuit electrically coupled to the first loop element to generate an electrical current flowing through the first loop element in a first direction, the current generates electromagnetic fields; a bypass loop element positioned proximate the first loop element so that an electrical current is induced in the bypass loop element from the fields generated by the first loop element, the electrical current in the bypass loop element it flows in a second direction opposite to the first direction, further characterized in that the current in the branch loop element generates fields that largely cancel out in the far field, the fields generated by the first loop element so that a surveillance zone is created in a nearby field; and a generally flat Figure 8 loop element located proximal to the Bypass loop element so that the coupling between the Figure 8 loop element and the first loop element and the bypass loop element is minimized; and a receiver circuit electrically coupled to the loop element in figure 8 to detect magnetic resonance in the surveillance zone at a predetermined frequency and generate an alarm signal in the same indicator of the presence of a protected article in the surveillance zone .