DE102006003717A1 - Antenna for a backscatter-based RFID transponder - Google Patents

Abstract

The invention relates to an antenna for a backscatter-based RFID transponder with an integrated receiving circuit having a capacitive input impedance for receiving a radio signal which is spectrally in an operating frequency range, the antenna having two antenna branches that extend in a spiral from a central area in which the antenna branches with the integrated receiving circuit are connectable, extend outward. According to the invention, each antenna branch has a branch length along the branch which is selected such that one of the series resonance frequencies of the antenna is below the operating frequency range and the next higher parallel resonance frequency of the antenna is above the operating frequency range. The invention also relates to a backscatter-based RFID transponder with such an antenna.

Description

The The present invention relates to an antenna for a backscatter-based RFID transponder (radio frequency identification) as well as a backscatter-based RFID transponder with such an antenna.

The Invention is in the field of wire and contactless communication. It is particularly in the field of radio-based communication for the purpose of identification of objects, animals, persons etc. as well as the one for this used transponders and "remote Sensors ".

Although in principle applicable to any contactless communication systems, The present invention as well as the underlying Issue below in relation to RFID communication systems and their applications explained. RFID stands for "Radio Frequency Identification".

at RFID systems are used between a stationary or mobile base station, often referred to as a reader, "reader" or read / write device and one or more transponders attached to the one to be identified Objects, animals or persons are attached, data using high-frequency Transmit radio signals bidirectionally.

Of the Transponder, which is also called a "tag" or "label", has one regularly Antenna for receiving the radio signal radiated by the base station and an integrated circuit (IC) connected to the antenna on. The integrated circuit includes a receiving circuit for receiving and demodulating the radio signal as well as for detecting and process the sent data. In addition, the integrated Circuit a memory for storing the for the identification of the corresponding Subject required data. Furthermore, the transponder a sensor e.g. for temperature measurement, e.g. also part the integrated circuit is included. Such transponders will be also as "remote Sensors "designated.

RFID transponder can be everywhere be used to advantage where automatic marking, Detection, interrogation or monitoring should be done. With the help of such transponders are objects like e.g. Container, Pallets, vehicles, machines, luggage, but also animals or persons individually markable and contactless as well as without line of sight identifiable.

At "Remote In addition, sensors can physical properties or sizes are detected and queried.

On the field of logistics Containers, pallets and the like can be identified, for example to determine the current whereabouts in the course of their transport. At "Remote Sensors "may e.g. the temperature of the transported goods or goods regularly measured and stored and at a later time Time to be read out. In the field of plagiarism protection, items such as e.g. integrated circuits are provided with a transponder, to prevent unauthorized replicas. In the trading area, RFID transponders replace the barcodes frequently applied to products. Further Applications exist e.g. in the automotive sector at immobilizers or systems for monitoring air pressure in tires and personal access control systems.

passive Have transponders not over one independent Energy supply and remove the required for their operation Energy emitted by the base station electromagnetic field. Although semi-passive transponders have their own power supply but do not use the energy provided by them to send / receive data, but for example to operate a sensor.

RFID systems with passive and / or semi-passive transponders whose maximum Distance from the base station is well over a meter operated in frequency ranges, especially in the UHF or microwave range lie.

In Such passive / semi-passive RFID systems with relatively long range comes to data transfer From a transponder to the base station in general, a backscattering ("backscattering") is used, in Part of the incoming from the base station at the transponder Energy reflected (re-radiated) becomes. At this time, the carrier signal radiated from the base station becomes in the integrated circuit of the transponder according to the to be transmitted to the base station Data modulated and reflected by the transponder antenna. Such transponders are called backscatter-based Transponder called.

In order to achieve the largest possible range in backscatter-based transponders, it is necessary to supply the highest possible proportion of the on-base station on the transponder incoming energy of the integrated receiving circuit of the transponder. Power losses of any kind are to be minimized here. For this purpose, on the one hand transponder antennas with a relatively wide Reception frequency range required. Such relatively broadband antennas may also provide the benefit of meeting the requirements of multiple national or regional regulatory authorities with only one type of antenna. On the other hand, the energy absorbed by the transponder antenna is to be supplied as undiminished as possible to the integrated receiving circuit, which usually has a capacitive input impedance, ie an impedance with a negative imaginary part.

From the DE 103 93 263 T5 For example, an antenna for an RFID system is known that has a planar branching structure with two branches. Starting from a central region, the two branches each extend helically in a full turn to the outside. The input impedance of this antenna is also capacitive.

adversely Here is that the Impedance of this antenna greatly depends on the conjugate complex value of Impedance of chip input circuit deviates and therefore between Antenna and chip an additional, separate matching circuit with a coil and a capacitor is required. Due to parasitic resistances of these components comes it transponder side to power losses, the disadvantageous reduce the range. Furthermore, the separate matching circuit limits the freedom in the placement of the chip and causes more complex and therefore more expensive implementations of the transponder.

In front In this background, the invention is based on the object Antenna for a backscatter-based RFID transponder with an integrated receiving circuit (IC) for Receiving a spectrally located in an operating frequency range radio signal specify the larger ranges as well simpler implementations of the transponder allows and broadband reception of high-frequency radio signals allowed. It is also the object of the invention to provide an easy to implement backscatter-based To specify RFID transponders the greater range with a broadband reception of high-frequency radio signals having.

According to the invention this Task solved by an antenna and a backscatter-based RFID transponder with the features of claims 1 and 15 respectively.

The antenna according to the invention has two antenna branches spiraling from a central region, in which the antenna branches with the integrated receiving circuit are connectable to the outside extend, each antenna branch along a branch length of the branch that has been chosen is that one the series resonance frequencies of the antenna below the operating frequency range and the next higher parallel resonance frequency the antenna is above the operating frequency range.

Of the inventive RFID transponder has an integrated receiving circuit with a capacitive input impedance and an antenna according to the invention connected to the integrated receiving circuit on.

The Essence of the invention is to the length of the antenna branches so choose, that the desired operating frequency range between one of the series resonance frequencies and the next higher (adjacent) Parallel resonance frequency of the antenna is located. This will ensure that the antenna has inductive reactance values in the operating frequency range. this makes possible it, the input impedance of the antenna in the operating frequency range to the conjugate complex values of the input impedance of the integrated receiving circuit approximate that no separate matching circuit between antenna and receiving circuit is required. To this Way transponder side power losses are reduced, so that yourself reach high ranges and a broadband reception of high frequency Radio signals possible is. Furthermore This makes simpler and cheaper implementations allows the transponder.

advantageous Refinements and developments of the invention are the dependent claims and the description with reference to the drawing.

In a preferred embodiment the antenna according to the invention is the branch length chosen so that the Antenna has values of inductive input impedance that are in Operating frequency range such to the complex conjugate values the capacitive input impedance are approximated that between Antenna and integrated receiving circuit no circuitry is required for impedance matching. This allows the IC without restrictions by separate components for impedance matching directly in the central Area of the antenna branches are placed so that particularly simple and inexpensive, but nevertheless powerful transponder realizations with large ranges allows become.

In an advantageous embodiment, the branch length is selected such that the one of the series resonance frequencies lies below the operating frequency range, which results in the antenna having values of inductive input impedance which in the operating frequency range are in such a way connected to the complex conjugate values of the capacitive input impedance Danz are approximated that no circuitry for impedance matching is required between the antenna and integrated receiving circuit. From those frequency ranges in which the antenna has inductive reactance values, the appropriate determination of the branch length advantageously selects the one which enables very good impedance matching and thus very high ranges without separate components for impedance matching.

Advantageous corresponds to the series resonance frequency of the lowest series resonance frequency fs1 of the antenna - and so that the parallel resonance frequency of the lowest parallel resonance frequency fp1 of the antenna. By choosing the branch length in this way, that the desired Operating frequency range between the lowest series resonant frequency and the lowest parallel resonance frequency of the antenna, can the antenna impedance advantageous even with relatively small effective resistances of the integrated Receive circuit to the complex conjugate values of the input impedance matched to the receiving circuit.

Preferably each antenna branch is configured, at least one full turn, in particular at least 1.5 full revolutions around the central area to describe. As a result, the antenna impedance can be advantageous be easily adapted in the UHF frequency band.

Preferably each antenna branch has a branch width across the branch, the changing along the branch, the branch width starting from the central area towards the outside preferably increases. This is advantageous a very broadband Reception allows.

In a further preferred embodiment each antenna branch forms an inner radial spiral and an outer radial spiral, these radial helices preferably have a logarithmic function consequences. Such antennas advantageously have particularly low Reflections on.

In further advantageous embodiments are the antenna branches polygonal or piecewise straight. This can be at a given square or rectangular area one better space utilization reach through the antenna.

Preferably the antenna branches are planar and lie in one common level. Preferably, each antenna branch comprises one thin conductive layer, which is formed on a substrate. This allows the antenna particularly easy to implement.

In a preferred embodiment the RFID transponder according to the invention the integrated receiving circuit in the central region of the antenna branches arranged. this makes possible very simple implementations of the transponder.

In a further preferred embodiment comprises each antenna branch a thin one conductive Layer that is formed on a substrate, and the integrated Receiving circuit is formed on the substrate. This is especially possible simple implementations of the transponder.

The Invention will be described below with reference to the schematic figures The drawings specified embodiments explained in more detail. in this connection shows

1 an RFID system with a transponder according to the invention;

2 a frequency response of the input impedance of an antenna with two spiral branches;

3 three embodiments of an antenna according to the invention; and

4 A fourth embodiment of an antenna according to the invention.

1 schematically shows an example of an RFID system. The RFID system 10 has a base station 11 and at least one transponder according to the invention 15 on. The base station exchanges with the aid of high-frequency radio signals 11 contactless and bidirectional data with the transponder (s) 15 out.

The base station 11 has at least one antenna 12 for transmitting and receiving radio signals in an operating frequency range fB, a transceiver unit connected to the antenna (s) 13 for transmitting and receiving data and a control unit connected to the transmitting / receiving unit 14 for controlling the transmitting / receiving unit 13 on.

The backscatter-based, passive or semi-passive transponder 15 has an antenna 16 for receiving the radio signal lying spectrally in the operating frequency range fB and a receiving circuit connected to the antenna 17 for demodulating the received radio signal and for detecting the data contained therein. The receiving circuit 17 is part of a 1 not shown integrated circuit (IC), for example, an ASIC (application specific integrated circuit) or ASSP (application specific standard product), which also regularly a memory for Having stored the data required for an identification of the corresponding items. If necessary, the transponder contains 15 or the integrated circuit further in 1 not shown components such as a sensor for temperature determination. Such transponders are also referred to as "remote sensors".

following It is assumed that the Operating frequency range fB is in the UHF frequency band, in a frequency range between about 840 MHz and about 960 MHz. alternative The operating frequency range can also be found in the almost worldwide available ISM band (industrial, scientific, medical) range between 2.4 and 2.5 GHz. Further alternative operating frequency ranges are 315 MHz, 433 MHz or 5.8 GHz.

by virtue of of different current requirements of the regulatory authorities regarding the maximum permissible transmission power in the frequency range between 840 and 960 MHz be in reading operation ranges of about 5m for the European market (500 mW ERP) and about 11m for the USA (4 W EIRP).

The integrated receiving circuit 17 has a complex-valued input impedance Z1 with a real part (effective resistance) R1 and an imaginary part (reactance) X1. The effective resistance R1 is hereby preferably relatively small in order to minimize power losses. Since integrated inductors would take up relatively large chip areas, the reactance X1 is regularly capacitive (X1 <0) and, in particular for small values of the effective resistance R1, greater in magnitude than the effective resistance: | X1 | > | R1 |.

Applicant developed integrated receive circuits 17 have input impedances Z1 with effective resistors R1 in the range of approximately 4 ... 35 ohms and capacitive reactances X1 whose absolute values are above approximately 150 ohms. The amount of the imaginary part (| X1 |) clearly exceeds the real part (R1) (| X1 |> 4 · R1). As the manufacturing technology of integrated circuits continues to increase and the size of the structure decreases, the magnitude of the capacitive reactances X1 increases further.

The antenna 16 of the transponder 15 According to the invention comprises two antenna branches, which spiral from a central region in which the antenna branches with the integrated receiving circuit 17 are connectable, extend to the outside. The input impedance of the antenna 16 is hereinafter denoted by Z2 = R2 + j × X2, where R2 indicates the effective resistance and X2 the reactance of the antenna. Embodiments of the antenna according to the invention are described below with reference to the 3 and 4 described.

2 schematically shows the frequency response of the input impedance Z2 of an antenna with two spiral branches. The frequency response of the input impedance Z2 is shown here over a frequency range which is significantly wider than the above-mentioned range between approximately 840 and 960 MHz. In 2a is the effective resistance R2, ie the real part of Z2, and in 2 B the reactance X2, ie the imaginary part of Z2, is plotted against the frequency f.

From the in 2 B shown waveform of the imaginary part of Z2 can be seen that the reactance X2 of the antenna at low frequencies f is initially capacitive (X2 <0), with increasing frequency after a zero crossing at the frequency f = fs1 but inductive (X2> 0). After passing through a strongly inductive maximum value follows a steep drop, in the course of which it comes after another zero crossing at the frequency f = fp1 again significantly capacitive reactances. This sequence of transitions with a first, relatively slow transition from capacitive to inductive reactances, followed by a second, faster transition from inductive to capacitive reactances is qualitatively repeated even at higher frequency values.

Those Frequencies at which the reactance disappears (X2 = 0), are called resonance frequencies. Zero crossings with positive slope, i. Transitions from capacitive to inductive reactances, are referred to as so-called. Series resonance frequencies fs1, fs2, fs3, ..., zero crossings with negative slope, i. Transitions from Inductive to capacitive values, however, as so-called parallel resonance frequencies fp1, fp2, ... are designated. The lowest series resonance frequency is also as the "first" series resonant frequency fs1 and the lowest parallel resonance frequency as the "first" parallel resonance frequency fp1 denotes.

From the in 2a shown curve profile of the real part of Z2 can be seen that the effective resistance R2 of the antenna is initially weak at lower frequencies f, then slowly increases with increasing frequency, then rapidly up to a maximum value of this first strong, then weak except for one Minimum value drops. This wave or U-shaped course of the resistance R2 over the frequency f repeats qualitatively at higher frequency values. How out 2a 1, the maximum values of the effective resistance R2 occur at the parallel resonance frequencies fp1, fp2, fp3,... and the minimum values at the series resonance frequencies fs1, fs2,.

The invention is based on the idea that in 2 shown curves of the active and reactive resistance of the antenna in the horizontal direction, ie in the direction of the frequency axis to stretch or compress by the (path) length L of the two spiral antenna branches is varied. The longer the antenna branches are selected in this case, the more the curves are compressed to the ordinate. The shorter the antenna branches are selected, the more the curves are stretched to the right, ie to larger frequency values. The variation of the branch length L advantageously does not take place (only) in integer multiples of complete (360 degrees) revolutions of the branches around the central region, but continuously or in steps with a small increment.

This opens up the possibility, by means of an appropriate choice (definition) of the branch length L, the input impedance Z2 of the antenna 16 ( 1 ) in the desired operating frequency range to the complex conjugate values of the input impedance Z1 of the receiving circuit 17 to match and so to achieve a complete, but at least a partial impedance matching without separate components.

According to the branch length L is chosen so that one of the series resonance frequencies fs1, fs2, fs3, ... of the antenna below the operating frequency range fB and the next higher of the parallel resonance frequencies fp1, fp2, ... the antenna is above the operating frequency range. By "next higher" parallel resonant frequency is meant the lowest of those parallel resonant frequencies which are greater than the one below the operating frequency range, the series resonant frequency Thus, the branch length L is chosen such that the desired operating frequency range fB is between a series resonant frequency fsk with k = 1 , 2, 3, ... and the next higher parallel resonance frequency fpk (having the same value of index k) is determined according to 2 B ensures that the antenna has inductive reactance values X2> 0 in the operating frequency range. Without separate components for impedance matching between antenna 16 and receiving circuit 17 Thus, due to the selection of the branch length L according to the invention, the input impedance Z2 of the antenna approaches the conjugate complex value Z1 '= R1-j * X1 of the input capacitive impedance Z1 = R1 + j * X1 (with X1 <0) of the receiving circuit, thus reducing power losses and therefore result in higher ranges.

How close in this way the inductive input impedance Z2 of the antenna can be brought to the likewise inductive impedance Z1 'depends on many, but in particular the following boundary conditions: a) the frequency position and width of the desired operating frequency range fB, b) the value of capacitive input impedance Z1 of the receiving circuit 17 and its course in the operating frequency range, and c) the exact configuration of the antenna according to the invention (shape of the antenna branches, width of the branches, distances between the branches, implementation of the antenna, etc.).

In a preferred embodiment of the invention, the branch length L is chosen such that the inductive input impedance Z2 of the antenna has values approximating in the operating frequency range fB to the impedance Z1 'or matching Z1' between the antenna 16 and integrated receiving circuit 17 no separate circuitry for impedance matching is required. This is possible in particular in the case of operating frequency ranges fB, which are significantly less wide than the differences fp1-fs1, fp2-fs2, etc. or in the case of flat progressions of Z1 and Z2 in the operating frequency range, but also in wider operating frequency ranges, provided the values of Z1 are not too wide are unfavorable (unfavorable values here are very high or extremely low effective resistances R1, as well as very high reactances | X1 |). Since no separate impedance matching circuitry is required in these cases, the IC can advantageously be placed directly in the central region of the antenna branches without restriction by impedance matching components, thus enabling particularly simple and inexpensive, yet powerful transponder realizations with long ranges.

For example, among the above with reference to 1 explained boundary conditions, the length L of the two spiral antenna branches can be chosen so that between antenna 16 and integrated receiving circuit 17 no separate circuitry for impedance matching is required and yet higher ranges and a broadband reception can be achieved. Embodiments of antennas according to the invention are described below with reference to FIGS 3 and 4 described.

The 3 and 4 show embodiments of inventive antennas for a backscatter-based RFID transponder according to the above description of 1 ,

at all illustrated embodiments are planar antennas whose branches are each in one common plane.

The both antenna branches of each embodiment differ only by a rotation of 180 degrees from each other. They are in their outer form identically designed.

Preferably, the two antenna branches each comprise a thin conductive layer, for example of copper, silver, etc., which is formed on a common substrate, for example of polyimide, or on a printed circuit board. Preferably, the integrated receiving circuit is also on this substrate 17 ( 1 ) of the transponder, which is advantageously arranged in a central region of the respective antenna. Alternatively, the thin conductive layer may be applied to a foil on which the integrated receiving circuit is arranged by means of flip-chip technology. The consisting of antenna and integrated receiving circuit transponder is finally attached to the object to be identified.

The branch length L is in the illustrated embodiments each chosen so that the frequency range from about 840 MHz to about 960 MHz between the lowest Series resonance frequency fs1 and the lowest parallel resonance frequency fp1 of the antenna lies, which in each case results in antenna branches, which is essentially two full turns (360 degrees) around the central one Describe area.

to increase the broadband have all illustrated embodiments Antenna branches on whose branch width W across the branch along of the branch changes. This change The branch width can be continuous along the branch or else jump in steps. Starting from the central area takes the branch width W to the outside in general too.

3 shows a plan view of a first, a second and a preferred third embodiment.

In these embodiments, each antenna has 20 two branches 21 . 22 which are configured identically except for a rotation through 180 degrees and spirally in oval turns of a central area 23 extend outwardly, each branch essentially describing two rotations of 360 degrees each.

Each of the antenna branches 21 and 22 forms an inner radial spiral 21a respectively. 22a and an outer radial coil 21b respectively. 22b that limit the respective branch. The radial helices 21a . 21b . 22a . 22b obey this a logarithmic function, which is why this type of antenna is also referred to as a logarithmic spiral antenna.

Starting from the central area 23 points each antenna branch 21 . 22 a branch length L along the branch and a branch width W across the branch, the branch length L being selected according to the invention as described above and the branch width W continuously changing along the branch.

As in 3 from the in the central antenna area 23 provided contact surfaces can be seen, the antenna branches 21 . 22 at these pads directly from the integrated receive circuit 17 of the transponder 15 be contacted. The integrated receiving circuit 17 is in the central area 23 arranged and preferably formed on the same substrate, on which also the antenna branches 21 . 22 are formed. This advantageously simplifies the implementation of the transponder.

This in 3a illustrated first embodiment is characterized by relatively wide antenna branches 21 . 22 whose width starting from the central area 23 generally increases towards the outside. Along each branch, the width increases and decreases incrementally in each revolution, resulting in a "periodic" increase in width, each branch describing exactly two full 360 degree revolutions around the central region 23 , In the x-direction, this antenna has an extension of about 8.3 cm, in the y-direction about 3.6 cm.

In the frequency range from about 840 MHz to about 960 MHz, the first exemplary embodiment has inductive input impedances Z2 with values of the effective resistance R2 between about 4 and about 37 ohms and values of the reactance X2 between about 160 and about 370 ohms. Thus, the input impedance Z2 is sufficient to the conjugate complex values of the input impedance Z1 of the receiving circuit 17 of the transponder 15 with reference to above 1 is described. A separate circuit arrangement for impedance matching is advantageously not required.

This in 3b illustrated second embodiment is characterized by relatively narrow antenna branches 21 . 22 from, which are arranged at a relatively large distance from each other. The width of each branch increases from the central area 23 in turn, generally towards the outside, while along the branch there again results a "periodic increase." At the outer end of the branch, the width decreases continuously, each branch describing about 2.1 full 360 degree revolutions around the central one Area 23 , In the x-direction, this antenna has an extension of about 6.8 cm, in the y-direction about 3.3 cm, so that the surface occupied by the antenna is advantageously smaller by about 25% than in the first embodiment.

In the above-mentioned frequency range, the second exemplary embodiment has inductive input impedances Z2 with values of the effective resistance R2 between approximately 4 and approximately 16 ohms and values of the reactive impedance X2 stands between about 180 and about 370 ohms. A separate circuit arrangement for impedance matching is advantageously also not required here.

This in 3c illustrated third embodiment is characterized in comparison to the first embodiment 3a by an extension in the direction of the x-axis and a compression in the direction of the y-axis. The width of each branch in turn generally increases towards the outside and periodically increases and decreases along the branch. Each branch describes exactly two full 360-degree turns around the central area 23 , In the x-direction, this antenna has an extension of about 10 cm, in the y-direction about 1.6 cm, so that this antenna is particularly suitable for production on a belt and / or for applications in which an elongated surface is available for the antenna. The area occupied by this antenna is advantageously about 45% smaller than in the first embodiment.

in the above-mentioned Frequency range, the third embodiment inductive input impedances Z2 with values of effective resistance R2 between about 4 and about 35 ohms and values of reactance X2 between about 170 and about 400 Ohm up. A separate circuit arrangement for impedance matching is advantageous also not required here.

Due to the low steepness of the characteristics of the impedance over the frequency, the in 3 shown antennas on a high bandwidth. The bandwidth of the overall system (transponder) depends greatly on the impedance of the integrated receiving circuit, the antenna substrate carrier and the substrate on which the transponder is mounted. Applicant's investigations have revealed overall system bandwidths in excess of 30 MHz.

Instead of referring to 3 also described spiral antennas with oval turns can be provided with circular turns, if, for example, a square or circular surface is available for the antenna. In this case, the width of each branch increases from the central area 23 continuous and monotonous along the branch - possibly with the exception of a slowly expiring branch end analogous to 3b ,

4 shows a perspective view of a fourth embodiment of an antenna according to the invention.

In this embodiment, the antenna 30 two branches 31 . 32 which are identical except for a rotation through 180 degrees and spiral in angular windings from a central area 33 extend outward, each branch 2 . 25 Describes rotations by 360 degrees.

Each of the antenna branches 31 and 32 in this case has a plurality of straight branch sections, which are arranged at angles of 90 degrees to each other. This type of antenna is also called a polygonal spiral antenna. In addition to right angles, other angles between the branch sections can be provided so that almost any number of corners per full turn of a branch can be realized. Furthermore, the turns can also be rectangular instead of square.

Starting from the central area 33 points each antenna branch 31 . 32 a branch length L along the branch and a branch width W across the branch, the branch length L having been selected according to the invention as described above, and the branch width W changing along the branch.

The antenna branches 31 . 32 are in the central area 33 directly with the integrated receiving circuit 17 of the transponder 15 connected. The integrated receiving circuit 17 is in the central area 33 arranged and preferably formed on the same substrate on which the antenna branches are formed. This simplifies the implementation of the transponder.

The width W of the antenna branches preferably remains constant in each straight branch portion, but changes "leaps" in the corners, starting from the central region 33 For example, the first straight section may have a first width, the next straight section may have a second, larger width, and the third section may have a third (in comparison to the second width, in turn) greater width, etc. As an alternative to such piecewise constant width along the antenna branches increase the branch width of all or only certain antenna branches linearly starting from the central region along the branch.

In the 4 shown antenna has an x / y extent of about 7cm × 7cm. In the above-mentioned frequency range, the fourth exemplary embodiment has inductive input impedances Z2 with values of the effective resistance R2 between approximately 7 and approximately 30 ohms and values of the reactance X2 between approximately 100 and approximately 240 ohms. Depending on the location of the operating frequency range, a separate circuit arrangement for impedance matching is not required.

Although the present invention has been described above by means of embodiments, it is not limited thereto, but modifiable in a variety of ways. For example, the invention is not limited to passive or semi-passive transponders, nor to the specified frequency bands, the specified impedance values of the integrated receiving circuit or the illustrated forms of the turns of the antenna branches, etc. The inven tion can rather be used advantageously in a variety of contactless communication systems.

10

RFID system

11

Base station, Read / write device, reader, reader

12

antenna the base station

13

Transmit / receive unit the base station

14

control unit the base station

15

transponder or remote sensor

16

antenna of the transponder

17

integrated Reception circuit of the transponder

20

antenna of the transponder

21 22

first or second antenna branch

21a, 22a

inner Radial spiral of the first and second antenna branch

21b, 22b

outer radial spiral of the first and second antenna branches

23

centrally Area of the antenna

30

antenna of the transponder

31 32

first or second antenna branch

33

centrally Area of the antenna

EIRP

Emitted isotropic radiated power

ERP

Emitted radiated power

ISM

industrial, scientific, medical (frequency band at 2.4 GHz)

RFID

radio frequency identification

fB

Operating frequency range

fp1

lowest Parallel resonance frequency of the antenna

fp2

second lowest Parallel resonance frequency of the antenna

fs1

lowest Series resonance frequency of the antenna

fs2

second lowest Series resonance frequency of the antenna

L

branch length

R1, R2

effective resistance of Z1 or Z2, real part of Z1 or Z2

W

branch width

X1, X2

reactance of Z1 or Z2, imaginary part from Z1 or Z2

Z1 = R1 + j × X1

input impedance the integrated receiving circuit

Z2 = R2 + j × X2

input impedance the antenna

Claims (20)

Antenna ( 16 ; 20 ; 30 ) for a backscatter-based RFID transponder ( 15 ) having a capacitive input impedance (Z1) having an integrated receiving circuit ( 17 ) for receiving a radio signal spectrally in an operating frequency range (fB), comprising: a) two antenna branches ( 21 . 22 ; 31 . 32 ) spiraling from a central area ( 23 ; 33 ), in which the antenna branches with the integrated receiving circuit ( 17 ) are connected to the outside, characterized in that b) each antenna branch has a branch length (L) along the branch ( 21 . 22 ; 31 . 32 ) which is selected such that one of the series resonance frequencies (fs1, fs2) of the antenna is below the operating frequency range (fB) and the next higher parallel resonance frequency (fp1, fp2) of the antenna is above the operating frequency range (fB). Antenna according to Claim 1, characterized in that the branch length (L) is chosen such that the antenna has values of an inductive input impedance (Z2) which in the operating frequency range (fB) approximate to the complex conjugate values of the input capacitive impedance (Z1) in that between antenna and integrated receiving circuit ( 17 ) no circuitry for impedance matching is required. Antenna according to Claim 1 or 2, characterized in that the branch length (U) is chosen such that the one of the series resonance frequencies (fs1, fs2) lies below the operating frequency range (fB), which results in the antenna receiving values of an inductive input impedance (Z2). which in the operating frequency range (fB) approximates to the complex conjugate values of the capacitive input impedance (Z1) in such a way that between the antenna and the integrated receiving circuit ( 17 ) no circuitry for impedance matching is required. Antenna according to one of the preceding claims, characterized characterized in that Series resonance frequency (fs1, fs2) of the lowest series resonance frequency (fs1) corresponds to the antenna. Antenna according to one of the preceding claims, characterized in that each antenna branch is designed to rotate at least one complete revolution, in particular at least 1.5 full revolutions, around the central region ( 23 ; 33 ) to describe. Antenna according to one of the preceding claims, characterized in that each antenna branch has a branch width (W) transverse to the branch and the branch width (W) changes along the branch. Antenna according to Claim 6, characterized in that the branch width (W) originates from the central region ( 23 ; 33 ) increases towards the outside. Antenna ( 16 ; 20 ) according to one of the preceding claims, characterized in that each antenna branch ( 21 . 22 ) an inner radial spiral ( 21a . 22a ) and an outer radial spiral ( 21b . 22b ). Antenna according to Claim 8, characterized that the inner radial spiral and the outer radial spiral follow a logarithmic function. Antenna ( 16 ; 30 ) according to one of claims 1 to 7, characterized in that the antenna branches ( 31 . 32 ) are polygonal or piecewise straight. Antenna according to claim 10, characterized in that that the branch width is piecewise constant along the branch. Antenna according to one of the preceding claims, characterized characterized in that Antenna branches in their outer form are configured identically. Antenna according to one of the preceding claims, characterized characterized in that Antenna branches are designed planar and in a common Lie flat. Antenna according to one of the preceding claims, characterized characterized in that each Antenna branch a thin conductive Layer includes, the is formed on a substrate. Backscatter-based RFID transponder ( 15 ), comprising: a) an integrated receiving circuit ( 17 ) with a capacitive input impedance (Z1), b) one with the integrated receiving circuit ( 17 ) connected antenna ( 16 ; 20 ; 30 ) according to any one of the preceding claims. Backscatter-based RFID transponder according to claim 15, characterized in that the integrated receiving circuit ( 17 ) in the central area ( 23 ; 33 ) of the antenna branches is arranged. A backscatter-based RFID transponder according to claim 15 or 16, characterized in that each antenna branch comprises a thin conductive layer formed on a substrate and the integrated receiving circuit (10). 17 ) is formed on the substrate. Backscatter-based RFID transponder according to one of Claims 15 to 17, characterized that the Capacitive input impedance (Z1) a resistance (R1) and a Reactive resistance (X1), wherein the value of the reactance (X1) is greater in terms of amount as the value of the effective resistance (R1). Backscatter-based RFID transponder according to one of Claims 15 to 18, characterized that the Transponder is configured passive or semi-passive. Backscatter-based RFID transponder according to one of Claims 15 to 19, characterized that the Operating frequency range (fB) in the UHF or microwave frequency range lies.