DE20314836U1 - Reader antenna for Radio Frequency Identification system consists of two conductors connected to oscillation circuit with inductance, capacitance and resistor - Google Patents

Abstract

The inductance (L) is variable and has a fixed and a moving connection to a loop antenna with two conductors separated by dielectric material. A fixed resistor (R) is connected in parallel with the inductance and its resistance is chosen to match the characteristics of the antenna. A variable capacitor (C) in parallel with the resistor is used to set the oscillation frequency.

Description

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Patent Attorney

Dipl.-Phys. Cordula Knefel

Wertherstrasse 16, 35578 Wetzlar

PO Box 1924, 35529 Wetzlar

Telephone 06441/46330 - Fax 06441/48256

F G 1121

FEIG ELECTRONIC GmbH

Lange Strasse 4 35781 Weilburg-Waldhausen

RFID reader antenna

Description

The invention relates to a structure of an RFID antenna that can transmit energy and data to RFID transponders and receive data from the RFID transponder. The antenna has an antenna coil that enables energy and data transmission through the magnetic field.

According to the state of the art, an antenna of an RFID system is constructed from an inductance formed from one or more turns and a capacitor that tunes the antenna to the operating frequency.

The antenna of an RFID system has two tasks: On the one hand, the transmission of energy to the transponder and on the other hand, the transmission of data to and from the

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Transponder. The energy and data transmission is based on the magnetic coupling of the alternating fields of the reader and the transponder in close range. A transmission of energy and data through the alternating electric field does not take place.

Requirements for a reader antenna of an RFID system are:

The antenna should be tuned as closely as possible to the operating frequency of the RFID system in order to achieve good coupling and thus good energy and data transmission. If the resonance frequency of the reader antenna is tuned to the operating frequency of the RFID system, a high quality of the reader antenna will achieve high energy transmission. However, too high a quality can impair the data transmission of the RFID system. The reason for this is that the data to be transmitted is modulated onto the operating frequency of the RFID system and thus at least two useful signal sidebands occur that are symmetrical to the operating frequency of the RFID system. In order to be able to transmit these sidebands well, the quality of an RFID reader antenna must be optimized so that the energy range and the data range of the system are the same. This means that reader antennas of different RFID systems have different levels of quality, even if the operating frequencies of the antennas are the same.

Another requirement for an RFID reader antenna arises from the fact that the reader antenna transmits energy to the transponder. This means that the reader antenna is in turn supplied with energy by a transmitter. Depending on the range of the RFID system, this can be milliwatts up to several watts. In order to optimally transmit the power from the end stage of the reader to the antenna,

The reader power amplifier and the reader antenna have the same input and output resistance. An RFID reader antenna therefore requires a certain input impedance so that the energy from the reader power amplifier is optimally transmitted to the antenna. A typical input impedance of an RFID reader antenna is 50 ohms, since 50 ohm cables are often used to connect the reader and antenna. However, other antenna impedances are also possible, for example if the antenna is integrated directly on the reader and the antenna is adapted to the output impedance of the reader power amplifier.

Another requirement for an RFID reader antenna is to compensate as much as possible for the currents that flow through the parasitic capacitances between the antenna and ground potential. This means that no or as little interference current as possible should flow between the antenna and the RFID reader due to these capacitances.

As described above, the parameters of operating frequency, quality, input impedance and compensation of the antenna currents against ground potential are important when designing an RFID reader antenna. From an electrical point of view, these parameters are influenced by the inductance, capacitance and resistance of the antenna. A reader antenna according to the state of the art is designed in such a way that the inductance is formed by one or more turns. The required antenna capacitance is achieved by fitting capacitors and the quality is achieved by fitting a resistor. This means that a typical RFID reader antenna has the following components: One or more turns, which are formed, for example, in the form of conductor tracks on a circuit board or

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Wires wound into a coil or formed into a frame made of metal strip or tubing.

The antenna also has one or more capacitors that are connected to the antenna coil according to a specific circuit. A typical connection is to solder the capacitors onto a circuit board. If resistors are required, these are usually connected to the antenna in a similar way to the capacitors.

Due to the different components and the required connection technology, several production steps are necessary in the manufacture of such antennas. Production steps for an antenna that is attached to a circuit board include, for example, production of the circuit board and subsequent placement of the components on the circuit board. If you want to produce such an RFID antenna in large quantities at very low cost, a certain cost threshold arises that cannot be undercut. The reason for the cost threshold is the use of different components and the sequence of different production steps.

The technical problem underlying the invention is to construct an antenna in such a way that few components and manufacturing steps are necessary so that it can be manufactured cost-effectively.

This technical problem is solved by an RFID reader antenna having the features according to claim 1.

The technical solution for the RFID reader antenna described here is that all parameters or components of the RFID antenna are defined by the form

and the distance between two conductor track structures can be achieved.

In particular, in contrast to known transponder antennas, the impedance matching parameters are achieved through the conductor track structure of the impedance-matching conductor track. The symmetrical structure of the resonant circuit conductor track structures in relation to the overlapping surfaces and the connection and symmetry point of the antennas achieves electrical symmetry of the antennas to the ground potential, so that the currents flowing to and from the ground potential can compensate each other and no parasitic ground currents occur.

According to a preferred embodiment, the conductor track structures overlap only in the area in which the capacitance is formed. Further overlaps in the area of the one or more turns are avoided in order to avoid undesirable couplings and additional capacitances.-

According to another particularly preferred embodiment, the length of the conductor tracks between the connection and symmetry point of the antenna and the capacitance of the antenna is the same length. This compensates for parasitic currents. If the conductor tracks are the same length up to the capacitance for setting the operating frequency, the parasitic capacitances between the conductor tracks and the ground potential and thus the currents between the conductor track and the ground potential are also almost the same. However, these currents have different polarities, so that they cancel each other out. If this is not the case, an interference current flows through the RFID reader.

A further advantageous embodiment provides that the impedance is adjusted by the position of at least one connection of a conductor track for impedance matching with the conductor tracks of the antenna loop.

A simple and common matching circuit is the "gamma matching" (Rothammels Antennenbuch (ISBN 3-88692-033-4)).

The connector for connecting the antenna is advantageously designed in such a way that it forms an electrical contact between the conductor tracks on the two different layers. It is also possible to make a through-hole connection through the carrier material, but this is associated with greater costs in terms of production effort.

According to a particularly preferred embodiment, the antenna according to the invention has two conductor layers. However, it is also possible to provide more than two conductor layers depending on requirements.

Further features and advantages of the invention will become apparent from the accompanying drawing, in which embodiments of an RFID reader antenna according to the invention are shown only by way of example, namely:

Fig. 1 an equivalent circuit diagram of the "gamma adaptation";

Fig. 2 is a plan view of an embodiment of an RFID reader antenna according to the invention with one winding;

Fig. 3 is a plan view of the upper conductor track;

Fig. 4 is a plan view of the lower conductor track;

Fig. 5 is a plan view of an embodiment of an RFID reader antenna according to the invention with three turns;

Fig. 6 is a plan view of the modified surface of the conductor tracks in the overlap region;

Fig. 7 is a plan view of the modified surface of the conductor tracks in the overlap region;

Fig. 8 is a plan view of the adjustment of the tap (5);

Fig. 9 is a side view of the contacting of the conductor tracks via a connector.

In addition to an antenna conductor, an RFID antenna also consists of a matching circuit. A large number of different circuits for adjusting antennas can be found in the literature. The function will be explained using an example:

Fig. 1 shows the principle of "gamma adaptation" using an equivalent circuit diagram.

The resonant frequency of the antenna is set via the capacitance (C), i.e. the reactive component of the antenna impedance is compensated by this capacitance. By tapping the inductance (L), i.e. at the

antenna, the impedance is adjusted by transforming the impedance of the resonant circuit using a transmission factor. The size of the damping resistor (R) is responsible for the quality of the antenna. It results from the shape of the antenna, the thickness of the conductor tracks (skin effect) and the position of the conductor tracks relative to each other.

Fig. 2 shows a plan view of an embodiment of an RFID reader antenna according to the invention with one turn. The conductor track (la) is the conductor track on the top side, and the conductor track (Ib) is the conductor track on the bottom side. The conductor tracks (la) and (Ib) essentially only overlap in a defined area (3), which forms the capacitance of the antenna resonant circuit, and are connected to one another at the connection and symmetry point (6). The conductor tracks are applied to a dielectric layer (4).

The antenna impedance is adjusted via the connection (5) according to the tap on the inductance (L) in Fig. 1. A conductor track (2) is provided for impedance adjustment. The connection to the RFID reader is made via the connection and symmetry point (6).

Fig. 3 shows the conductor track (la) on the top side of the dielectric layer with the connection (5) and the contact (6a).

Fig. 4 shows the conductor track (Ib) on the underside of the dielectric layer with the contact (6b).

Fine adjustment of the capacity is possible by varying the area of the overlap (3).

Reducing the area causes a reduction in the capacity (C).

Another example according to Fig. 5 shows the possibility of an antenna with several turns. Since the inductance of the antenna with one turn is also reduced when the antenna area is reduced, the capacitance to compensate for the reactive component must be increased by increasing the overlap area of the conductor tracks. By using several turns, the inductance increases and thus the required capacitance decreases.

Fig. 5 shows an antenna with three turns. The conductor track (la) on the top and the conductor track (Ib) on the bottom do not lie on top of each other. The overlap for adjusting the capacitance is essentially only in the defined area (3). The connection (5) to the conductor track (2) is used to tap the impedance, as with the antenna with one turn (Fig. 2). The connection to the other RFID components is made via the connection (6, 7).

It is possible to adjust the operating frequency of the antenna by changing the capacitance (C). To do this, the overlap area (3) is simply reduced by scratching or lasering, as shown in Fig. 6, or by punching, as shown in Fig. , or other methods. Reducing the overlap area (3) simultaneously leads to a reduction in the capacitance (C) and thus to a change in the antenna matching. The impedance is adjusted by changing the position of the tap (5), as shown in Fig. 8.

Fig. 9 shows a possibility of connecting the conductor tracks in the area of the symmetry point (6) via a connector (10).

The power that is converted into heat is dissipated to the environment over a large area via the conductor tracks.

Reference numbers

L Inductance of the antenna loop

C Capacity for setting the operating frequency

R Parallel resistor for adjusting the quality

la Antenna loop conductor on top

Ib Conductor track of the antenna loop on bottom side

2 Conductor track for impedance matching

3 Overlap area of conductor tracks (la) and (Ib) 3a Conductor track in overlap area (3) on top side 3b Conductor track in overlap area (3) on bottom side

4 dielectric carrier material

5 Tap the conductor track (2) on the antenna loop

6 Connection and symmetry point of the antenna loop 6a Conductor track at the connection point (6) on the top side

6b Conductor track at connection point (6) on bottom side

7 Connection point of the conductor track impedance matching (2)

8 connectors at the connection and symmetry point (6)

Claims (13)

1. Antenna of an RFID reader, consisting of at least one inductance and one capacitance, which is supplied with an alternating voltage by the output stage of the RFID reader and which serves for energy and data transmission with the transponder, wherein the parameters operating frequency, quality, impedance and symmetry to earth determine the antenna characteristics, characterized in that - the antenna is formed by the shape of at least two conductor track structures, - the two conductor track structures are arranged at a certain distance, - at least one carrier material having dielectric properties is arranged between the at least two conductor track structures, - the at least two conductor track structures are positioned relative to each other such that they overlap in at least one area, and - the at least two conductor track structures are positioned relative to each other in such a way that at least one turn results from the at least two conductor track structures. 2. Antenna according to claim 1, characterized in that the conductor track structures overlap only in the region in which the capacitance is formed. 3. Antenna according to claim 1 or 2, characterized in that the length of the conductor tracks between the connection and symmetry point ( 6 ) of the antenna and the capacitance of the antenna is at least approximately the same length. 4. Antenna according to one of the preceding claims, characterized in that the impedance can be tuned by the position of at least one connection ( 5 ) of a conductor track ( 2 ) for impedance matching with the conductor tracks ( 1a ) and ( 1b ) of the antenna loop. 5. Antenna according to one of the preceding claims, characterized in that parameters of the antenna are determined by the conductor track thickness. 6. Antenna according to one of the preceding claims, characterized in that the parameters of the antenna can be changed by subsequently separating and/or removing parts of the conductor track structure. 7. Antenna according to one of the preceding claims, characterized in that parts of the conductor track structure are designed as a supply line of the antenna. 8. Antenna according to one of the preceding claims, characterized in that the conductor track structure is designed such that a connection of the conductor tracks of the antenna loop ( 1 a) and ( 1 b) is formed at the connection and symmetry point of the antenna loop ( 6 ). 9. Antenna according to one of the preceding claims, characterized in that the conductor track structure is constructed in such a way that electrical losses converted into heat can be easily dissipated. 10. Antenna according to one of the preceding claims, characterized in that the individual conductor track structures of a layer have no crossing points with the conductor track structures of the same layer. 11. Antenna according to one of the preceding claims, characterized in that the carrier material is rigid. 12. Antenna according to one of claims 1 to 10, characterized in that the carrier material is flexible. 13. RFID reader, characterized in that the RFID reader has at least one antenna with the features of one or more of the preceding claims.