WO2010025516A1

WO2010025516A1 - Impedance compensation in an rf signal system

Info

Publication number: WO2010025516A1
Application number: PCT/AU2009/001157
Authority: WO
Inventors: Graham Alexander Munro Murdoch
Original assignee: Magellan Technology Pty Ltd.
Priority date: 2008-09-05
Filing date: 2009-09-04
Publication date: 2010-03-11
Also published as: AU2009290143A1

Abstract

The present invention relates to the field of RF signal transmission. In one form, the invention relates to the use of impedance modules to enhance the transmission of signals at relatively high data rates, such as, for example, transmission of signals between a first device, such as an antenna and second device, such as an associated device, like an interrogator, driver and/or reader.

Description

IMPEDANCE COMPENSATION IN AN RF SIGNAL SYSTEM FIELD OF INVENTION

The present invention relates to the field of RF signal transmission.

In one form, the invention relates to transmission of signals at relatively high data rates, such as, for example, transmission of signals between a first device, such as an antenna and second device, such as an associated device, like an interrogator, driver, power amplifier, transmitter and/or reader.

It will be convenient to hereinafter describe the invention in relation to transmissions between an antenna and a second device; however it should be appreciated that the present invention is not limited to that use only. BACKGROUND ART

The discussion throughout this specification comes about due to the realisation of the inventors and/or the identification of certain prior art problems by the inventors.

An RFID system consists of a reader, antenna and a remote tag. The reader is connected to the antenna which transmits a radio signal to the tag. The tag transmits a reply which is received by the antenna and decoded by the reader. In most RFiD systems, the antenna is formed integrally with its corresponding reader. However, in some RFID system applications, there are physical constraints which do not allow for the placement of an antenna and an integral reader. In such applications, there is a need to provide an antenna separately from the reader. If the antenna is coupled to the reader via a wire or cable, the coupling provides further problems of signal degradation, bandwidth constraint and/or signal attenuation.

In the prior art, it is usual to try to properly match the impedance of antenna to the characteristic impedance of the cable. When the impedance is matched, the length of the cable may be relatively long^'. It has been found, however that this impedance matching leads to a disadvantage. When the impedance of the antenna is matched to the impedance of the cable, the antenna will have a relatively narrow bandwidth, and thus the matched antenna is not considered suitable for high speed data rates. High speed data rates need to have a wide bandwidth antenna. Despite the disadvantage of a relatively narrow bandwidth matching is done because when an antenna is not matched, the cable itself serves to transform the antenna impedance as- a function of cable length and operating frequency. For example, at around 13.56 MHz, a cable length of 1m or more has been found to significantly transform the impedance by a factor of around 3. A reader will be designed to operate with a specific antenna impedance. Thus there is a need to control the impedance of cable and antenna as 'seen' by a reader in an operating system. The prior art method of controlling this impedance is to match the antenna to the cable impedance.

In practical situations, it is not uncommon to have antenna(s) and associated devices, such as reader(s) spaced a considerable distance apart. Thus there is also a need to provide a relatively lengthy cable to connect antennas spaced apart a distance of many meters. Equally there is a need to enable a single reader to be connect (by cable) to multiple antennas.

Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.

An object of the present invention is to alleviate problems associated with coupling an antenna and an associated device.

A further object of the present invention is to alleviate at least one disadvantage associated with the prior art. SUMMARY OF INVENTION

In a first aspect of embodiments described herein there is provided an impedance module adapted to provide a first impedance value at a first part of the module, and a second impedance value at a second part of the module, the module comprising a portion of electrical cable, and an impedance compensation arrangement.

In a second aspect of embodiments described herein there is provided a compensation module comprising impedance elements adapted to transform a first impedance at a first portion of the module to a second impedance at a second portion of the module. In a third aspect of embodiments described herein there is provided a method of and/or system for providing a wideband transmission medium adapted for relatively high data rate transmission, comprising providing an antenna having a first impedance, and providing at least a portion of an electrical cable, having a second impedance, the first impedance being different than the second impedance.

In a fourth aspect of embodiments described herein there is provided a method of and/or system for coupling a first device to a second device, comprising providing at least one impedance module intermediate the first device and the second device, the impedance module serving to substantially maintain the impedance of the first device as seen by the second device.

In a fifth aspect of embodiments described herein there is provided a method of and/or system for connecting an antenna to reader using a cable of any length without the need to match the antenna impedance to the cable impedance.

Other aspects and preferred aspects are disclosed in the specification and/or defined in the appended claims, forming a part of the description of the invention.

In essence, the present invention comes about due to the realisation that where there is an impedance mismatch between the first device, such as an antenna and a cable (coupling the antenna to a second device, such as an associated device, like an interrogator, driver, and/or reader) and that the impedance transforming effect of the cable can be substantially cancelled out restoring the original impedance of the first device at the far end of the cable. It is therefore possible to keep a relatively broad bandwidth in the antenna as well as relatively high data rate transmission via the antenna and cable combination. In effect, an antenna is provided mismatched to a cable, and the impedance transforming effect of the cable is cancelled out by a 'compensation module' for example, at the reader end of the cable, or at the antenna end, to re-establish the original antenna mismatched impedance at the reader end of the cable. The antenna mismatched impedance is transformed over the length of the cable, and then re-adjusted back to the mismatched impedance of the antenna by a compensation module. Alternatively the antenna impedance is pre-compensated before being transformed over the cable length back to the mismatched impedance of the antenna. As a further alternative a combination of pre- compensation before the cable and re-adjustment after the cable is used to transform the antenna impedance back to the mismatch impedance. A 'compensation module¹ serves to provide the substantial pre-adjustment and/or re-adjustment necessary in terms of impedance and/or reactance. In accordance with one aspect of the present invention, the compensation module is coupled to the cable at a position away from where an antenna is or is likely to be coupled to the cable. In accordance with another aspect of the present invention, the compensation module is coupled to the cable at a position proximate to the where the antenna is or is likely to be coupled to the cable. In accordance with yet another aspect of the present invention, compensation modules are coupled to the cable at positions both away from where an antenna is or is likely to be coupled to the cable and at positions proximate to the where the antenna is or is likely to be coupled to the cable.

In accordance with another aspect of the present invention, this configuration of cable and compensation module and or modules is called an 'impedance module'.

In one embodiment, each impedance module includes a length of cable, and a compensation module and or modules. The compensation module and or modules in effect 'compensates', pre-compensates, cancels, pre-adjusts or 'readjusts' for the transformation of the cable. The compensation module and or modules deal with compensating the impedance and/or reactance transformation caused by the cable. In this way, the antenna impedance can be 'carried forward' by each impedance module. Then, if necessary, and to cover relatively large distances, a number of impedance modules can be coupled end to end.

Where a length of cable is substantially half a wavelength the magnitude of the impedance transformation of the cable Is substantially 1.0. The impedance at the reader end of the cable is relatively the same as the antenna impedance at the far end of the cable. Where a cable length consisting of multiples of half a wavelength is used then the impedance transformation of the total cable length will accordingly be substantially 1.0. The impedance modules are advantageously used by connecting a number of modules together till their combined length is half a wavelength where upon a cable of half a wavelength is substituted. Further impedance modules may be added if further length is required until a further half wavelength is added where upon another half wavelength cable is substituted. The process of extension is then carried on in a similar fashion as required.

In accordance with a further aspect of invention, it has been found that the principles associated with the utilisation of a Smith Chart can be used to design a suitable compensation module, for example a compensation module suitable for RFID systems.

The present invention has been found to result in a number of advantages, such as:

• Antennas do not require matching to the cable impedance

• Wide band antenna operation

• High speed data transmission to/from antennas

• Distance between the antenna and the reader can be large

• Many antennas can be connected to one reader at a central location Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, white indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

Further disclosure, objects, advantages and aspects of the present application may be better understood by those skilled in the relevant art by reference to the following description of preferred embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and in which:

Figure 1A illustrates a prior art arrangement of coupling a reader and an antenna; Figure 1 B illustrates another prior art arrangement of coupling a reader and an antenna;

Figures 1C and 1D illustrate the electrical relationships of a prior art arrangement of coupling a reader and an antenna;

Figure 2A shows an antenna connected to a length of transmission line cable.

Figure 2B shows with a Smith Chart the transformation of the antenna impedance by the length of transmission line cable shown in Figure 2A.

Figure 2C illustrate various embodiments in accordance with the present invention showing the compensation module located at one end of the cable, located at the other end of the cable and located at both ends of the cable;

Figure 2D illustrates an embodiment of a compensation module in accordance with the present invention and the operation of the compensation module using a Smith Chart;

Figure 2E illustrates another embodiment of a compensation module in accordance with the present invention and the operation of the compensation module using a Smith Chart;

Figure 2F illustrates another embodiment of a compensation module in accordance with the present invention and the operation of the compensation module using a Smith Chart;

Figure 3 illustrates one embodiment of an impedance module in accordance with the present invention and the operation of the invention using a Smith Chart;

Figure 4A illustrates another embodiment of a compensation module in accordance with the present invention;

Figures 4B shows the measured operation of the circuit shown in Figure 4A using a Smith Chart;

Figure 4C shows the measured operation of the circuit shown in Figure 4A using a transmission plot;

Figure 4D shows the operation of the circuit shown in Figure 4A using a Smith Chart;

Figure 4E illustrates another embodiment of a compensation module in accordance with the present invention; Figure 4F shows the operation of the compensation module shown in Figure 4E using a Smith Chart;

Figure 5 shows a number of impedance transforming circuits that may be used by the invention;

Figure 6A illustrates an embodiment in which a number of impedance modules are used to couple a driver and a remote antenna;

Figure 6B illustrates graphically the relative impedance associated with the embodiment of Figure.6A;

Figure 7A illustrates an embodiment in which a cable an integer number of half wavelengths long is used to couple a driver and a remote antenna;

Figure 7B illustrates graphically the relative impedance associated with the embodiment of Figure 7A;

Figure 7C illustrates an embodiment in which an impedance module and a half wavelength cable are used to couple a driver and a remote antenna;

Figure 7D illustrates graphically the relative impedance associated with the embodiment of Figure 7D;

. Figure 8 illustrates a plurality of remote antennas coupled to one reader in accordance with an embodiment of the present invention, and

Figure 9 illustrates an alternative arrangement for connecting multiple antennas to a reader. DETAILED DESCRIPTION

Figure 1A illustrates a prior art arrangement in which a reader 100 is coupled directly to an antenna A1. The reader may be designed to operate with a wide bandwidth when connected directly to the antenna. The antenna may be a tuned circuit. Figure 1B illustrates a prior art arrangement in which a reader 100 is coupled via a coupling means 101, such as a cable, to an antenna A1. The antenna may be a tuned circuit. Typically, the cable coupling the reader and the antenna is a transmission line with a characteristic impedance of 50 Ohms. It has been found that the impedance value of many antenna circuit designs suitable for RFID systems do not match to the impedance of transmission line cable. Typically matching the antenna impedance to the cable impedance is considered very desirable and is routinely done because the matched antenna impedance at any point along the cable length is then well controlled and equals the cable's characteristic impedance. The antenna impedance is matched to the cable impedance using an impedance transforming circuit. This results in very serious limitations to the data rate transmission caused by narrowing of the circuit bandwidth as will be explained below. Figures 1C and 1D illustrate the electrical relationships of the prior art arrangements shown in Figures 1A and 1 B. Typically the reader/driver is matched to the cable and will have an output impedance of 50 ohms and the antenna is a tuned coil. A typical RFID antenna coil may have an inductance of 3uH and a coil resistance of 5 ohms. In Figure 1 C the reader is connected directly to the tuned antenna. The total series resistance seen by the coil is 55 ohms. Assuming an operating frequency of 13.56 MHz then the Q of the antenna is 4.6 and the bandwidth is 2.9 MHz. In Figure 1D the antenna is matched to the cable impedance by a matching transformer. The 50 ohm cable impedance is transformed down to 5 ohms and the total series resistance seen by the coil is now 10 ohms. Assuming an operating frequency of 13.56 MHz then the Q of the antenna is 26 and the bandwidth is only 530 kHz. The practice of matching the antenna impedance to the cable impedance results in a significantly reduced antenna bandwidth.

In summary the invention is to not match the antenna to the cable and the mismatch between the antenna and the cable is used to maintain a wide bandwidth suitable for high data rate transmissions. The impedance at the reader end of the cable is adjusted to the antenna impedance value or another suitable impedance value by a combination of cable length and an impedance compensation module or modules. If the cable length fortuitously correctly transforms the antenna impedance to the desired impedance value then the impedance compensation module is a trivial direct connection. If the cable length does not transform the antenna impedance to the desired impedance value then the impedance compensation module consists of an impedance transforming network that transforms the impedance to the desired value. The impedance transforming network may be located at the antenna end of the cable, the reader end of the cable, at intermediate polnt(s) along the cable, at both ends of the cable or at any combination of either end(s) and / or intermediate point(s).

The operation of the invention may be further explained with reference to a Smith Chart, the Smith Chart being considered known to those skilled in the art. A Smith Chart can be used to display a sequence of normalized impedance, admittance or reflection coefficients in a circle of unity radius for impedance or admittance involving a transmission-line of characteristic impedance Z₀. The mapping of the complex reflection coefficient T vs. the complex impedance (Z) or complex admittance (Y) is given by

where the normalized impedance/admittance are defined as z^s !' y^{≡ γz}o

The Smith Chart can be used as a tool to visualize complex-valued quantities

and calculate the mapping between them. The Smith Chart is a well understood electrical tool and references can be found in any engineering text book dealing with radio frequency design ('Fields and Waves in Communication Electronics' Ramo, Whinnery and Van Duzer, Wiley Press).

Figure 2A shows an antenna of impedance ZA connected directly to a length of transmission line cable. The antenna impedance of the antenna is transformed to ZB at the end of the cable. The impedance transforming effect of the cable is shown using the Smith Chart shown in Figure 2B. If ZA - 5 + jθ then the impedance ZB at the end of a cable of electrical length of 45 degrees is ZB = 10 + j50. The series resistance of the cable will add some additional series impedance however for the purposes of understanding the invention this can be ignored.

Figure 2C shows various representations of the invention, however it is to be noted that many other embodiments based on variations are also possible in accordance with the present invention, such as; • an antenna of impedance ZA connected unmatched to a cable of length t and a compensation module located at the other end of the cable, .

• an antenna of ZA connected to a compensation module and then connected unmatched to a cable of length I

• an antenna of impedance ZA connected to a compensation module and then connected unmatched to a cable of length t and a compensation module is also located at the other end of the cable

Where an antenna of impedance ZA is connected unmatched to a cable of length I and a compensation module is located at the other end of the cable.the impedance ZA is transformed along the length I to an impedance ZB. The degree of transformation can be readily calculated analytically or graphically with a Smith Chart. An impedance compensation module transforms the impedance ZB to ZC. Depending upon the length of the transmission line cable the compensation module type and component values can be chosen to return the transformed impedance to the original antenna impedance ZA. The compensation module may include suitable impedance components to provide the readjustment of ^• impedance and frequency response as is necessary in the particular situation to which the present invention is applied. This is dependent, at least in part, on the impedance of the antenna, the impedance of the cable used to couple the antenna to the remote second device, and the electrical length i of the coupling cable and the desired transformed impedance value. In certain circumstances it may be preferable that ZC equals ZA, the original load impedance, however under other circumstances it may be preferable that ZC is different from ZA. Example values for ZA, ZB and ZC for a cable of 45 degrees electrical length would be:

ZA = 5 + jO

ZB = 10 + j50 for a cable of 45 degrees electrical length and

ZC = 10 + jθ where the compensation module subtracts j50 from the impedance ZB. Where an antenna of impedance ZA is connected to a compensation module and then connected unmatched to a cable of length I, the impedance ZA is transformed by the compensation module to ZD. The impedance ZD is transformed along the length i to an impedance ZC. .The degree of transformation can be readily calculated analytically or graphically, for example, with a Smith Chart. Depending upon the length of the transmission line cable the compensation module type and component values can be chosen to return the transformed impedance to substantially the original antenna impedance ZA. The compensation module may include suitable impedance components to provide the readjustment of impedance and frequency response as is necessary in the particular situation to which the present invention Is applied. Example values for ZA₁ ZD and ZC for a cable of 45 degrees electrical length would be: . ZA = 5 + jθ

ZB = 5 - j50 where the compensation module subtracts j50 from the impedance ZA and

ZC = 2.5 + jθ for a cable of 45 degrees electrical length

Where an antenna of impedance ZA is connected to a compensation module and then connected unmatched to a cable of length t and a compensation module is also located at the other end of the cable, the impedance ZA is transformed by the compensation module to ZE. The impedance ZE is ^• transformed along the cable length I to an impedance ZF. The impedance ZF is transformed to ZC by the compensation module located at the other end of the cable. The degree of transformation can be readily calculated analytically or graphically, for example, with a Smith Chart. Depending upon the length of the transmission line cable the compensation module types and component values can be chosen to return the transformed impedance to the original antenna impedance ZA. The compensation module may include suitable impedance components to provide the readjustment of impedance and frequency response as is necessary in the particular situation to which the present invention is applied. Example values for ZA₁ ZE₁ ZF and ZC for a cable of 90 degrees electrical length would be: ZA = 5 + JO

ZE = 5 - j50 where the compensation module subtracts j50 from the impedance ZA

ZF = 5 +j50 for a cable of 90 degrees electrical length and

ZC = 2.5 + jO where the compensation module subtracts j50 from the impedance ZF

Figure 2D illustrates one embodiment of a compensation module applicable to provide a substantial impedance transformation between an antenna and a reader. The impedance ZB presented to the right hand side of the circuit will be transformed to impedance ZC at the left hand side of the circuit. For example if ZB has a complex impedance of 10 + j50 ohms, L1 has a complex impedance of +J121 ohms and C1 has a complex impedance of-j35.7 ohms then the impedance ZC seen looking into the circuit will be 5 ohms. The impedance at intermediate points through the circuit can be calculated using simple circuit theory and can be shown to be:

ZA = 5 + jO

ZB = Z11 = 10 + j50

Z12 = 5 + j35.7

ZC = Z13= 5+ JO For operation at 13.56MHz, the values of L1 and C1 are 1.42uH and 329pF.

Figure 2D also illustrates the impedance transformations of the circuit shown using a Smith Chart. The circuit shown in Figure 2D only has two components. The circuit of Figure 2D does not have the feature of bi-directional operation.

Figure 2E shows an alternate circuit that may be used for impedance transformations. The circuit shown in Figure 2E may have the feature of bidirectional operation. That is for Figure 2E the impedance ZB when placed on either side of the circuit may undergo the same impedance transformation. This bi-directional feature is an advantage during installation as the orientation of the circuit is not important. Figure 2E also illustrates the impedance transformations using a Smith Chart. Figure 2F illustrates another embodiment of a compensation module applicable to provide a substantial impedance transformation between an antenna and a reader. The impedance ZB presented to the right hand side of the circuit will be transformed to impedance ZC at the left hand side of the circuit. For example if ZB has a complex impedance of 10 + J50 ohms, C1 has a complex impedance of -j50 ohms and transformer T1 has an impedance transformation ratio of 1 :2 then the impedance ZC seen looking into the circuit will be 5 ohms. The impedance at intermediate points through the circuit can be calculated using simple circuit theory and can be shown to be:

ZA = 5 + jθ

ZB = Z11 = 10 + j50

Z12 = 1O + JO

ZC = Z13 = 5+ jO

For operation at 13.56MHz the value of C1 is 235pF and the impedance transformation ratio of T1 is 1:2. Figure 2F also illustrates the impedance transformations using a Smith Chart.

Figure 3 illustrates one embodiment of an impedance module in accordance with the present invention. The impedance module comprises a cable 31 and a compensation module 32. The length of the cable 31 and the component values used for the compensation module 32 are relatively closely related. The electrical length of the cable is a fixed value for a particular compensation module design. For example a device such as an antenna 35 having an impedance of ZA = 5Ω + jθ is coupled to the impedance module with a length of cable 31 , where the electrical length I of the cable is 45 degrees. The impedance shown at one end of the cable 31, at point 36, will be 5Ω + jθ. The impedance at the other of the cable, at point 33, will be Z1 = 10Ω + j50. When the impedance module includes a compensation module as described in Figures 2D₁ 2E or 2F above, the impedance at point 34 is again Z2 = 5Ω + jθ. Thus if there is a second device 37 such as a driver coupled to point 34, the second device 37 will be mismatched to the antenna in accordance with the present invention leading to a relatively broad bandwidth and relatively high data rate for transmission of signals between the second device 37 and the antenna 35. Figure 3 also illustrates these impedance transformations using a Smith Chart.

Whilst the circuits shown in Figures 2D₁ 2E or 2F and 3 above provide the correct impedance transformation the resulting frequency response may not match the original antennas response closely enough. Figure 4A shows an alternate circuit for the impedance module where the component values have been chosen to give both the correct impedance transformation whilst also maintaining the identical frequency response of the antenna when directly connected to the reader. The inclusion of parallel (or series) resonant circuits can be used to shape the impedance transformation and frequency response across a broad band of frequencies. Figures 4B and 4C respectively show the measured impedance and the frequency response of the antenna with the impedance module shown in Figure 4A. The frequency response of the antenna only is shown on Figure 4C for comparison purposes and shows that both are nearly identical.

The impedance ZB presented to the right hand side of the circuit will be transformed to impedance ZC at the left hand side of the circuit. For example if ZB has a complex impedance of 10 + j50 ohms, and L1 , L2, C1 , C2 and C3 have complex impedances of +J63.9, +J42.6, -J137, -j35.8 and -j66 respectively then the impedance ZC seen looking into the circuit will be 5 ohms. The impedance at intermediate points through the circuit can be calculated using simple circuit theory and can be shown to be:

ZB = 10 +j50

Z11 = 5 +j35.6,

Z12 = 5 - j0.2

ZC = 5 + jO

For operation at 13.56MHz the value of L1. L2, C1 , C2 and C3 are750nH, 50OnH, 86pF, 327pF and 178pF respectively. Figure 4D also illustrates the impedance transformations of the associated circuit using a Smith Chart.

Figure 4E shows an alternate circuit for the impedance module where the component values may be been chosen to give both the correct impedance transformation whilst also maintaining the identical frequency response of the antenna when directly connected to the reader. The impedance ZB presented to the right hand side of the circuit will be transformed to impedance ZC at the left hand side of the circuit. For example if ZB has a complex impedance of 10 + j50 ohms, and at the operating frequency L1 and C1 has a parallel complex impedance of -jA = -j30, L2 and C2 a series complex impedance of +jB = +J50, L3 and C3 a parallel complex impedance of -JC = -j15 respectively then the impedance ZC seen looking into the circuit will be 5 ohms. The impedance at intermediate points through the circuit can be calculated using simple circuit theory and can be shown to be:

ZB = 10 + j50

Z11 = 10 + J20

Z12 = 5 + j15

ZC = 5 + jO

For operation at 13.56MHz the value of L1 , L2, C1 , C2 and C3 a skilled person can calculate the values of these components.

Figure 4F also illustrates the impedance transformations of the compensation module shown in Figure 4E using a Smith Chart.

The impedance converting circuit of the impedance module can take on different forms depending upon the required transformation ratio and frequency response characteristics. Different forms of transforming circuits are well known and an example can be found in engineering text book dealing with radio frequency design ("Fields and Waves in Communication Electronics" Ramo, Whinnery and Van Duzer, Wiley Press). Examples of various circuits are shown in Figure 5. These include Pi networks, Tee networks and transformers. The inclusion of series or parallel connected circuit components or series or parallel connected resonant circuits can be used to shape the impedance transformation and frequency response across a broad band of frequencies. The examples given in Figures 2, 3 and 4 are for the purpose of example and in no way limit the scope of the invention.

While the example shown in Figures 2, 3 and 4 shows a cable with 45 or 90 degrees electrical length this length is chosen for illustrative convenience only, a shorter, or longer, cable may be used.

Figure 6A illustrates an embodiment in which a number of impedance modules (module 1 , module 2 to module n) are used to couple a driver 37 and its load remote antenna 35. The driver is designed to operate with an antenna that has an impedance ZA. The various modules (1 to n) are preferably identical, and comprise a compensation module 32, and a length of cable 31 , as described above. Figure 6B illustrates graphically the relative impedance associated with the embodiment of Figure 6A. Figure 6B shows schematically the change or readjustment of impedance between the values of ZA and ZB at various points along the circuit configuration as shown in Figure 6A.

Where a length of cable is exactly half a wavelength the magnitude of the impedance transformation of the cable is 1.0. The impedance at the reader end of the cable is the same as the antenna impedance at the far end of the cable. Where a cable length consisting of multiples of half a wavelength is used then the impedance transformation of the total cable length will accordingly be 1.0. The series resistance of the cable will add some additional series impedance however for the purposes of understanding the invention this can be ignored. ^• ..

Figure 7A shows an embodiment where the length of cable is an integer multiple of half wavelengths and the compensation module consists of a direct connection. The antenna mismatch is maintained along the length of the cable and at every half wavelength along the cable length the impedance of the load 35 equals the mismatched value of ZA. Figure 7B shows schematically the change of impedance at various points along the circuit as shown in Figure 7A.

The compensation modules are advantageously used by connecting a number of modules together till their combined length is half a wavelength where upon a cable of half a wavelength is substituted. Further compensation modules may be added if further length is required until a further half wavelength is added where upon another half wavelength cable is substituted. The process of extension is then carried on in a similar fashion as required.

Figure 7C shows a cable longer than half a wavelength where the extra length additional to half a wavelength is compensated for with a compensation module. The half wavelength section can also be replaced with a length consisting of a multiple of half wavelength lengths. Figure 7D shows schematically the change of impedance at various points along the circuit as shown in Figure 7C. Figure 8 illustrates a plurality of remote antennas coupled to one reader in accordance with an embodiment of the present invention. There are a number of antennas A1 , A2 An and they are coupled via corresponding impedance modules M1 to Mm and/or cables consisting of an integer multiple of half wavelengths to a second device, such as reader 100. This embodiment has been found to be particularly advantageous as a single reader may be coupled to a number of remote antennas. ^• Further processing 104 of the signals received by the reader may be performed by suitable circuitry (not shown).

The arrangement shown in Figure 8 can be extended in a hierarchical way by multiplexing remote antennas using a 'rnux' as shown in Figure 9. Figure 9 illustrates a plurality of remote antennas A1 , A2 A_n coupled to one reader in accordance with an embodiment of the present invention where multiple antennas are connected through mux box(s) X into a single cable. The mux box is an RF switch that connects through only one antenna to the reader 100 at a time. By sequentially switching between antennas, many antennae can be sequentially operated from a single reader. The muxs may be cascaded in a hierarchical fashion as shown. The mux's are coupled via corresponding impedance modules M consisting of compensation modules and cables and/or cables consisting of an integer multiple of half wavelengths. The mux boxes may include impedance modules to compensate for the impedance transformation of the connected cable. The switching of the. mux boxes may be controlled by a separate data control cable (not shown) or the control signals can be passed down the cable used for passing the RF signals. For clarity, Figure 9 only shows muxs with 3 ports however the mux boxes may have any number of ports from two to many tens or hundreds of output ports. This embodiment has been found to be particularly advantageous as a single reader may be coupled to a number of remote antennas. Further processing 104 of the signals received by the reader may be performed by suitable circuitry (not shown).

The present invention has many applications. For example, the present invention may be used in or in connection with various RFID systems, such as those disclosed in

• Active shelving for warehousing or inventory management • RFID enabled displays for stock security and stock control

• Gaming tables, cashiers and vaults

• RFID enabled documentation management systems

• Access control systems

• Livestock identification systems

• Warehousing systems

• Health Science Application systems

• US5153583

• US5302954

• US5258766 AU524377

• AU785098 US7259654

• US publication number 2006-0226955-A1

• PCT/AU2006/001318

• PCT/AU2007/000331

• PCT/AU2006/001321

While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures. For example, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface to secure wooden parts together, in the environment of fastening wooden parts, a nail and a screw are equivalent structures.

"Comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof." Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise¹, 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Claims

1. An impedance module adapted to provide a first impedance value at a first part of the module, and a second impedance value at a second part of the module, the module comprising: a portion of electrical cable, and an impedance compensation arrangement.

2. An impedance module as claimed in claim 1 , wherein the arrangement is a compensation module as herein disclosed.

3. An impedance module as claimed in claim 1 or 2, wherein the portion of cable is half a wavelength in length.

4. An impedance module as claimed in claim 1 or 2, wherein the portion of cable is a multiple of half a wavelength in length.

5. An impedance module as claimed in any one of claims 1 to 4, the module being adapted to couple a first device to a second device.

6. An impedance module as claimed in claim 5, wherein the first device is any one or any combination of antenna, transmitter and / or an electrical device.

7. An impedance module as claimed in claim 5 or 6, wherein the second device is any one or any combination of an interrogator, driver, reader, antenna, receiver and / or an electrical device.

8. An impedance module as claimed in any one of claims 1 to 7, the module being adapted to operate in a RFID system.

9. A compensation module comprising impedance elements adapted to transform a first impedance at a first portion of the module to a second impedance at a second portion of the module.

10. A compensation module as herein disclosed.

11. An impedance module including the compensation module as claimed in claim 9 or 10.

12. A RFID system including the impedance module as claimed in any one of claims 1 to 8 or 11.

13. A method of providing a wideband transmission medium adapted for relatively high data rate transmission, the method comprising the steps of: providing an antenna having a first impedance, and providing at least a portion of an electrical cable, having a second impedance, the first impedance being different than the second impedance.

14. A method as claimed in claim 13, wherein the first impedance is not matched to the second impedance.

15. A method as claimed in claim 13 or 14, further comprising the step of connecting an impedance compensation device.

16. A method as claimed in claim 15, wherein the impedance device serves to compensate for the impedance transforming effects of the cable.

17. A method as claimed in any one of claims 15 or 16, wherein the impedance device is connected between an antenna and a reader.

18. A method as claimed in claim 15, 16 or 17, wherein the device comprises a module as claimed in any one of claims 1 to 10.

19. A method of coupling a first device to a second device, the method comprising providing at least one impedance module intermediate the first device and the second device, the impedance module serving to substantially maintain the impedance of the first device as seen by the second device.

20. A method as claimed in claim 19, wherein the impedance module also substantially maintains the frequency response.

21. A method as claimed in claim 19 or 20, wherein more than one impedance module is provided between the first and second devices.

22. A method as claimed in claim 19, 20 or 21 , wherein the first device is antenna and the second device is any one or any combination of an interrogator, driver and/or reader.

23. A method as claimed in any one of claims 19 to 22, wherein the first and/or second devices form a part of a RFID system,

24. A method as claimed in claim 23, wherein the first and/or second device is adapted to operate at substantially 13.56 MHz.

25. A method as herein disclosed.

26. An apparatus, system and/or device as herein disclosed.