MϋLTIFKEQUENCY ANTENNA FOR RFID APPLICATIONS FIELD OF THE INVENTION

The invention relates to identification util- izing radio frequency signals.

BACKGROUND OF THE INVENTION

An RFID system (RFID, Radio Frequency Identi- fication) can be used to attach information to items to be tagged and for reading the information in question using a specific reader device. Conventionally, these functions have been implemented using a bar code attached to an article that is read by a reader de- vice. The RFID system can be used, for example, in stores at the checkout counter, or in a bus in the form of an electronic travel ticket system. In addition, important applications can be found in industrial production and logistics. An RFID system in- eludes a reader device and several transponders, that is, identification tags that are incorporated into objects to be identified. When the transponder reaches the coverage area of the reader device (where there is a sufficiently powerful field transmitted by the reader device) , the transponder sends the information saved to it to the reader device. In this sense, an RFID system functions like those transponders to be used in airplanes, in conjunction with which a land station sends query data to an airplane, and the transponder within the airplane responds to the query by sending its location and speed data to the land station.

A transponder consists of a microchip and of an antenna coupled to it. The antenna of the trans- ponder acts as the receiving element of a signal transmitted by the reader device, and also as the transmitting element of a signal modulated by the mi-

crochip. In a passive system, the transponder does not have a separate power supply, but the power required by the microchip is obtained from the field transmitted by the reader device. In that case, in the cou- pling between the microchip and the antenna, impedance matching is emphasized, whereby the power received by the antenna can be made to effectively couple to the microchip and vice versa.

The communication between the reader device and the transponder in high-frequency identification is performed by means of electromagnetic waves . For the use of a UHF-RFID system such as this, the operating frequencies of 869 MHz in Europe, 915 MHz in the USA, and 955 MHz in Japan have been allocated. In ad- dition, frequencies 2.45 GHz and 5.8 GHz have been allocated to RFID. In sensor applications, it is also useful to gather energy, for example, from the transmission frequencies of GSM base stations. In other words, multi-frequency operation of an antenna is a considerable advantage. Typically, multi-frequency operation is achieved by means of several radiation elements, or by utilizing harmonic frequencies. Addition of several radiation elements, however, considerably increases the size of the antenna, and the mutual cou- pling between the elements can considerably worsen the performance of the antenna. Instead, by using harmonic frequencies one can only achieve the multiples of the fundamental frequency.

Important criteria for an antenna designed for RFID applications include a small size, affordable manufacturing costs, adaptability directly to the inputting circuit using ideal impedance matching, keeping of properties irrespective of the substrate, an omnidirectional radiation pattern, high efficiency, as well as structural sturdiness. One of the most important properties is the size because from the usability standpoint, the transponder should be up to a couple

of millimeters thick. Because the microchip to be attached to the antenna is of the order of just about a millimeter in diameter, the size of the antenna determines the size of the whole transponder. For antennas designed for RFID use, one must take into consideration that a transponder typically is attached to some kind of surface. The material of the attachment area can considerably affect the radiation properties of the transponder. From the usability standpoint, it is preferable that the antenna acts in an approximately similar way irrespective of the substrate material of the transponder; that is, the same type of radiation pattern, impedance matching and an approximately equal reading distance can be achieved irrespective of the substrate material. It is, however, typically difficult to design an antenna so as to be totally immune to the effects of the substrate.

For RFID antennas it is substantial that the input of the antenna is obtained from a microchip. To avoid power losses, the antenna shall be fitted into a microcircuit, and at its simplest, this is performed without a separate adapter block. Typically, in applications of radio engineering, an impedance level of 50 ω is used, but in the case of an RFID antenna, the im- pedance of the microchip resembles an RC series circuit having in practice a big capacitance and a small real part.

One prior-art antenna solution of a UHF-RFID transponder is a dipole to be placed onto a circuit board. One antenna solution containing dipole antennas for RFID applications has been disclosed in publication US 5572226. A dipole antenna usually is quite wideband and easy to manufacture. Also, the multi- frequency operation can be achieved using several resonance elements. A straight (conjugate) impedance matching in a dipole antenna can be achieved for various impedances of the supply circuit by shifting the

place of supply on the dipole. However, if the antenna is placed near the surface, it does not radiate. Hence, dipoles shall be placed into a free space. This makes dipole antennas useless in the majority of RFID applications .

As a prior-art solution, one has presented a planar microstrip antenna for RFID applications . This kind of antenna solution has been described e.g. in publication WO 9716865. Microstrip antennas have the advantage of applicability to mass production and easy connectivity of the semiconductor components to a microstrip structure. A problem with the microstrip antenna is its big size. The size can be reduced by using a substrate with a high dielectric constant, but in that case, the antenna will become more narrowband. In that case, also the surface-wave power is maximized, resulting in increased antenna losses and back lobe.

One known antenna solution is PIFA (Planar Inverted F Antenna) . One example of this type of antenna is disclosed in publication EP1096602. PIFA is a small-sized antenna structure, but generally speaking, PIFA structures mainly radiate in a vertical current mode, which makes them rather susceptible to the prop- erties of the materials in the vicinity, a disadvantage in the RFID applications being examined. A PIFA structure can also be made to operate at several frequencies by using several resonance elements or harmonic frequencies . One problem with the prior art is that the properties of the antenna solutions presented change when placed onto various surfaces . Another main problem is the too big size of the prior-art multiple element multi-frequency antennas for RFID applications.

OBJECTIVE OF THE INVENTION

It is an objective of the present invention to disclose a multi-frequency antenna solution for RFID solutions that solves the disadvantages of the prior art referred to above.

SUMt-ARY OF THE INVENTION

As for the features characteristic of the invention, reference is made to the claims.

The present invention discloses an RFID system and a new type of two-frequency antenna solution applicable to the transponders to be used therein. If necessary, the solution can be expanded to a multi- frequency solution by adding more resonance elements to the antenna, like in the prior art technology.

The antenna has a so-called PIFA structure (PIFA, Planar Inverted F Antenna) containing a rectangular ground plane and a rectangular main conductor plate placed in parallel on top thereof and disposed at a little distance from the ground plane. In the structure, the main conductor plate is coupled to the ground plane at one side thereof by means of a short- circuit plate. The short-circuit plate is, in turn, placed perpendicularly to the ground plane and the main conductor plate. The structure is flat because the ground plane is near to the conductor plane. In the present invention, the short-circuit plate has a length corresponding to about the length of the side of the main conductor plate. On the other hand, the rear of the short circuit necessitates no ground plane, which is a clear advantage in respect of the manufactory technology. In other words, the first long edge of the short-circuit plate is attached to one

edge of the ground plane, and the second long edge is attached to one edge of the main conductor plate.

In the PIFA structure presented in the invention one important factor is the current distribution wherein a horizontal current component is dominating. In most cases, PIFA structures operate with a dominating vertical current component. A PIFA structure operating with a dominating vertical current component is more insusceptible to the effects of the substrate be- cause the surface currents induced by a horizontal source are concentrated more in the ground plane of the antenna .

Generally speaking, the antenna power feed can be directed to any point of the main conductor plate, but in a preferred embodiment of the invention, the supply point is disposed at the edge of the main conductor plate. As the supply circuit of the antenna of the transponder functions a microchip whose impedance is typically capacitive and the real part small. To operate, the microchip naturally necessitates a ground terminal. In the invention, the ground terminal is achieved by means of an open microstrip line, i.e. a stub. The open end of the stub is bent towards the open edge of the main conductor plate i.e. the oppo- site edge of the short circuit, whereby capacitive coupling is achieved between them. In other words, the stub consists of at least two mutually differently oriented parts that are at a right angle with respect to one another and in parallel to the sides of the main conductor plate. The first part of the stub is coupled to the supply site of power and is in parallel to the side of the main conductor plate which includes the supply site of power. The second part of the stub is, in turn, in parallel to the side of the main con- ductor plate disposed opposite to the short-circuit plate, the second part including an open end.

The feed inductance induced by the stub, as well as the series capacitance induced by the coupling together form a structure that can be used to achieve two-frequency behavior utilizing one resonance ele- ment. Thus, the two-frequency behavior is based on certain kind of impedance matching which is made possible by the impedance level to be used in the RFID. In other words, when reviewed using the Smith diagram, the beginning and end parts of one resonance circle can be placed near the desired impedance level, enabling one to achieve two-frequency behavior. A one- element two-frequency antenna has the advantage of a small size compared to conventional multiple element solutions. On the other hand, using the antenna in question, the frequency bands can be freely selected, unlike in solutions based on harmonic frequencies. Moreover, with respect to the manufacturing technology, the use of a stub as a substitute for a short circuit is useful. On the other hand, in addition to two bands, additional bands can be attached to the antenna using known multiple element technique. In that case, one or more additional conductor plates are attached to the antenna structure. In that case, the ground plane and the short-circuit plate are shared by the main conductor plate and additional conductor plates. If there is one additional conductor plate, then the first part of the stub is disposed between the main conductor plate and the additional conductor plate. As the antenna substrate (the material to be placed between the main/additional conductor plate and the ground plane) , one can use either air or such a material that has a small dieletric constant, such as polyethylene. One example of the invention has the di- mensions 6.3 * 5.1 * 0.3 cm when filled with polyethylene. As the frequency bands of the antenna, one can select any of the two bands associated with UHF-RFID.

More frequency bands can be obtained by attaching more resonance elements to the antenna.

In short, the present invention has the advantages of a small size, two-frequency behavior (ad- dition of bands made possible) , insusceptibility to the substrate and easy manufacture.

LIST OF FIGURES

In the following section, the invention will be described in detail with reference to the examples of its embodiments, in which:

Fig. 1 represents one example of a two- frequency antenna of the invention as seen from the top and the side; and

Fig. 2 represents one example of an antenna of the invention as seen from the top and the side, wherein added to the two-frequency antenna is a third frequency band by means of the prior-art technology.

DETAILED DESCRIPTION OF THE INVENTION

The present invention shows an antenna that can be used in transponders i.e. identifications tags of RFID applications. The transponder with its antennas communicates with a reader device having another antenna coupled thereto.

The structure of the antenna presented above is a so-called PIFA structure (PIFA, Planar Inverted F Antenna) in which the main conductor plate is placed on top of a parallel ground plane. The main conductor plate is connected to the ground plane by means of a short-circuit plate which in perpendicular to the aforementioned plates. In a PIFA structure, the length of the short-circuit plate can vary in practice, but in the present invention, the length is selected to be the same as the length of the side of the main conduc-

tor plate. In this manner, the current distribution of the antenna can be made horizontally dominating.

One example of an antenna solution of the invention is presented in Fig. 1. The PIFA structure shown in the example includes a ground plane 10, a short-circuit plate 11 and a main conductor plate 12. A signal is input to the main conductor plate 12 of the antenna from a supply point i.e. a microchip 13, which is disposed at the edge of the main conductor plate 12 of the antenna. The ground terminal of the microchip is achieved by means of a short-circuit stub 14 whose end is bent towards the open edge disposed opposite to the short circuit of the main conductor plate 12. In practice, the short-circuit stub is a mi- crostrip line whose one end is connected to the supply point 13 of the antenna and the other end is open. The antenna can also have other resonance elements 15 to achieve more than two frequency bands, as is shown in Fig. 2. Figs. 1 and 2 show, by way of example, possi- ble dimensions of the antenna, but the invention is not limited merely to antenna parts in accordance with these dimensions. The dimensions of the example are for a polyethylene-filled antenna whose lowest frequency is 869 MHz. Specific to the antenna of Fig. 1 is that the length of the short-circuit plate 11 is about the length of the side of the main conductor plate 12. In addition, specific to the antenna is that both the main conductor plate 12 and the ground plane 10 are rectangular and mutually parallel. The rear of the short circuit 11 necessitates no ground plane at all, which, in other words, means that at its one long side, the short circuit board 11 is connected to the edge of the ground plane, and at its other long side connected to the edge of the main conductor plate 12. As is known, in practice, in the manufacture of circuit boards, the short-circuit plate 11 can be re-

placed by using several through holes that effectively form a short-circuit plate.

The ground plane 10 of the antenna, the short-circuit plate 11, the main conductor plate 12, and the stub 14 shall naturally consist of a conducting material. From the functionality standpoint, the conductivity shall be at least of the order of brass. For example copper is a usable antenna material in the present invention. The antenna structure has been so optimized that the dominating component of the antenna ' s current distribution is horizontal, making the antenna insusceptible to the effects of different substrates. In addition, by utilizing feed inductance (stub 14) and capacitive coupling (in Figs. 1 and 2, between the vertical part of the stub 14 and the open edge disposed opposite to the short circuit of the main conductor plate 12), the antenna has been optimized to achieve two-frequency behavior with one radiation ele- ment. It is a question about a novel optimization of the impedance circle of an antenna on the Smith chart.

The ground plane 10 included in the antenna is optimized to be as small as possible. It must also be noted that the rear of the short-circuit plate 11 (in Figs. 1 and 2, the left-hand side of the short circuit) necessitates no ground plane at all, making the antenna suitable for mass production. A small antenna (main conductor plate 12) provided with a small ground plane 10 is an advantageous solution, because by adding a big ground plane to any PIFA or microstrip solution, the antenna is made insusceptible to the effects of various substrates. In addition, achieving of two-frequency behavior does not increase the size of the antenna at all. Substantial in the present invention is also the fact that the structure is low, that is, the distance between the ground plane 10 and the main conduc-

tor plate 12 shall be considerably smaller than the length of the side of the main conductor plate 12. From the usability standpoint, this is advantageous, but furthermore in the embodiment of the present in- vention, a height of about 0.009A (at 869 MHz) strengthens the horizontal current.

With a PIFA structure the radiation pattern of the antenna is broad, and like in microchip antennas, its directivity is rather small. As the substrate one can use a material having a low dielectric constant, such as air, polyethylene (ε _r = 2.33), or some circuit board material.

The lower usable frequency of a two-frequency antenna in accordance with the dimensions of the exam- pie of Fig. 1 is 869 MHz, and the upper usable frequency can be e.g. 915, 940 or 955 MHz. The bandwidth of a half of a power is of the order of about 14 MHz for both frequency bands. In the example of Fig. 2, besides the two frequencies, there is a freely select- able third frequency.

The invention is not limited merely to the examples of its embodiment referred to above; instead many variations are possible within the scope of the inventive idea defined by the claims .